diff --git a/app/game/Main.hs b/app/game/Main.hs
index 5904a0578..921e2a379 100644
--- a/app/game/Main.hs
+++ b/app/game/Main.hs
@@ -94,7 +94,7 @@ cliParser =
   widthOpt = option auto (long "width" <> metavar "COLUMNS" <> help "Use layout with maximum width")
 
   langVer :: Parser LanguageVersion
-  langVer = flag SwarmLangLatest SwarmLang0_5 (long "v0.5" <> help "Read (& convert) code from Swarm version 0.5")
+  langVer = flag SwarmLangLatest SwarmLang0_6 (long "v0.6" <> help "Read (& convert) code from Swarm version 0.6")
 
   printKeyMode :: Parser KeybindingPrint
   printKeyMode =
diff --git a/data/entities.yaml b/data/entities.yaml
index 728856d75..bf570bb9a 100644
--- a/data/entities.yaml
+++ b/data/entities.yaml
@@ -1180,14 +1180,14 @@
     - "A rubber band can tie multiple commands together so that other robots can't execute commands in between them.  It can be used via the `atomic` command. For example, suppose robot A executes the following code:"
     - |
       ```
-      b <- ishere "rock"; if b {grab; return ()} {}
+      b <- ishere "rock"; if b {grab; pure ()} {}
       ```
     - "This seems like a safe way to execute `grab` only when there is a rock to grab.  However, it is actually possible for the `grab` to fail, if some other robot B snatches the rock right after robot A sensed it and before robot A got around to grabbing it on the next game tick."
     - "This will make robot A very sad and it will crash."
     - "To prevent this situation, robot A can wrap the commands in `atomic`, like so:"
     - |
       ```
-      atomic (b <- ishere "rock"; if b {grab; return ()} {})
+      atomic (b <- ishere "rock"; if b {grab; pure ()} {})
       ```
   properties: [pickable]
   capabilities: [atomic]
diff --git a/data/scenarios/Challenges/2048.yaml b/data/scenarios/Challenges/2048.yaml
index 592c88e5b..53cf59976 100644
--- a/data/scenarios/Challenges/2048.yaml
+++ b/data/scenarios/Challenges/2048.yaml
@@ -9,11 +9,11 @@ objectives:
     condition: |
       try {
         as base {has "2048"}
-      } { return false }
+      } { pure false }
 solution: |
   def makeN : Int -> Cmd Unit = \n.
     if (n == 1)
-      {harvest; return ()}
+      {harvest; pure ()}
       {makeN (n/2); makeN (n/2); make (format n)}
   end;
   place "1";
diff --git a/data/scenarios/Challenges/Mazes/easy_cave_maze.yaml b/data/scenarios/Challenges/Mazes/easy_cave_maze.yaml
index 7cf6c4300..9a86e425b 100644
--- a/data/scenarios/Challenges/Mazes/easy_cave_maze.yaml
+++ b/data/scenarios/Challenges/Mazes/easy_cave_maze.yaml
@@ -46,7 +46,7 @@ robots:
     system: true
     program: |
       def until = \c. b <- c; if b {} {until c} end;
-      until (d <- scan down; return (d == inl ()));
+      until (d <- scan down; pure (d == inl ()));
       create "goal"
 known: [water]
 entities:
diff --git a/data/scenarios/Challenges/Mazes/easy_spiral_maze.yaml b/data/scenarios/Challenges/Mazes/easy_spiral_maze.yaml
index 954cbf542..d7d6974b7 100644
--- a/data/scenarios/Challenges/Mazes/easy_spiral_maze.yaml
+++ b/data/scenarios/Challenges/Mazes/easy_spiral_maze.yaml
@@ -46,7 +46,7 @@ robots:
     system: true
     program: |
       def until = \c. b <- c; if b {} {until c} end;
-      until (d <- scan down; return (d == inl ()));
+      until (d <- scan down; pure (d == inl ()));
       create "goal"
 entities:
   - name: wall
diff --git a/data/scenarios/Challenges/Mazes/invisible_maze.yaml b/data/scenarios/Challenges/Mazes/invisible_maze.yaml
index bb6b1afd6..43ac73635 100644
--- a/data/scenarios/Challenges/Mazes/invisible_maze.yaml
+++ b/data/scenarios/Challenges/Mazes/invisible_maze.yaml
@@ -10,17 +10,17 @@ objectives:
       try {
         teleport self (27, -17);
         b <- ishere "goal";
-        return (not b)
-      } { return false }
+        pure (not b)
+      } { pure false }
 solution: |
   def tL = turn left end;
   def tR = turn right end;
   def ifC = \test. \then. \else. b <- test; if b then else end;
   def forever = \c. c ; forever c end;
-  def rightBlocked = tR; b <- blocked; tL; return b end;
+  def rightBlocked = tR; b <- blocked; tL; pure b end;
   def mazeStep = ifC rightBlocked {ifC blocked {tL} {move}} {tR; move} end;
   build {
-    forever (mazeStep; ifC (ishere "goal") {grab; return ()} {})
+    forever (mazeStep; ifC (ishere "goal") {grab; pure ()} {})
   }
 robots:
   - name: base
diff --git a/data/scenarios/Challenges/Mazes/loopy_maze.yaml b/data/scenarios/Challenges/Mazes/loopy_maze.yaml
index 0c22cffb0..7b7796d09 100644
--- a/data/scenarios/Challenges/Mazes/loopy_maze.yaml
+++ b/data/scenarios/Challenges/Mazes/loopy_maze.yaml
@@ -10,8 +10,8 @@ objectives:
       try {
         teleport self (27, -17);
         b <- ishere "goal";
-        return (not b)
-      } { return false }
+        pure (not b)
+      } { pure false }
 solution: |
   run "scenarios/Challenges/Mazes/loopy_maze_sol.sw"
 robots:
diff --git a/data/scenarios/Challenges/Mazes/loopy_maze_sol.sw b/data/scenarios/Challenges/Mazes/loopy_maze_sol.sw
index d08060f08..78b675b81 100644
--- a/data/scenarios/Challenges/Mazes/loopy_maze_sol.sw
+++ b/data/scenarios/Challenges/Mazes/loopy_maze_sol.sw
@@ -3,7 +3,7 @@ def tR = turn right end;
 def tB = turn back end;
 def ifM = \p.\t.\e. b <- p; if b t e end;
 def DFS =
-  ifM (ishere "goal") {grab; return ()} {};
+  ifM (ishere "goal") {grab; pure ()} {};
   ifM (ishere "rock") {} {
     place "rock";
     tL; b <- blocked; if b {} {move; DFS};
diff --git a/data/scenarios/Challenges/Ranching/_beekeeping/queenbee.sw b/data/scenarios/Challenges/Ranching/_beekeeping/queenbee.sw
index af8548956..e2794cd3d 100644
--- a/data/scenarios/Challenges/Ranching/_beekeeping/queenbee.sw
+++ b/data/scenarios/Challenges/Ranching/_beekeeping/queenbee.sw
@@ -37,7 +37,7 @@ def moveTuple = \tup.
 
 def randomDir =
     r <- random 4;
-    return $ if (r == 1) {north}
+    pure $ if (r == 1) {north}
         $ elif (r == 2) {west}
         $ elif (r == 3) {south}
         $ else {east};
@@ -136,7 +136,7 @@ def takeStepTowardItem = \item.
         // another bee gets here first
         try {
             harvest;
-            return ();
+            pure ();
         } {};
     } {
         // Include some random motion
@@ -159,7 +159,7 @@ def workerProgram = \structureLoc.
         goToHive structureLoc;
     } {
         takeStepTowardItem "wildflower";
-        return ();
+        pure ();
     };
     workerProgram structureLoc;
     end;
@@ -206,7 +206,7 @@ def createWorkerForStructure = \loc.
         require 1 "wax gland";
         workerProgramInit beename loc;
     };
-    return ();
+    pure ();
     end;
 
 def associateHive = \loc.
@@ -214,7 +214,7 @@ def associateHive = \loc.
    try {
        // Fails if the robot does not exist
        robotnamed beename;
-       return ();
+       pure ();
    } {
        createWorkerForStructure loc;
 
@@ -227,7 +227,7 @@ def associateHive = \loc.
 
 def mapM_ : (a -> Cmd b) -> (rec l. Unit + a * l) -> Cmd Unit = \f. \l.
   case l
-    (\_. return ())
+    (\_. pure ())
     (\cons. f (fst cons); mapM_ f (snd cons))
   end;
 
diff --git a/data/scenarios/Challenges/Ranching/_beekeeping/solution.sw b/data/scenarios/Challenges/Ranching/_beekeeping/solution.sw
index 8db584b7b..5c817e69d 100644
--- a/data/scenarios/Challenges/Ranching/_beekeeping/solution.sw
+++ b/data/scenarios/Challenges/Ranching/_beekeeping/solution.sw
@@ -113,14 +113,14 @@ def collectContiguous = \maxdist. \item. \hadFound. \dist.
             collectContiguous maxdist item true $ dist + 1;
         } {
             if hadFound {
-                return dist;
+                pure dist;
             } {
                 move;
                 collectContiguous maxdist item false $ dist + 1;
             }
         }
     } {
-        return dist;
+        pure dist;
     }
     end;
 
@@ -148,7 +148,7 @@ def collectAllHoneycombs = \targetCount.
 
         collectAllHoneycombs targetCount;
     } {
-        return currentCount;
+        pure currentCount;
     };
     end;
 
@@ -176,7 +176,7 @@ def pickRock =
     isRock <- ishere "rock";
     if isRock {
         grab;
-        return ();
+        pure ();
     } {};
     end;
 
diff --git a/data/scenarios/Challenges/Ranching/_capture/opponent.sw b/data/scenarios/Challenges/Ranching/_capture/opponent.sw
index 43f46e92a..67a0c15de 100644
--- a/data/scenarios/Challenges/Ranching/_capture/opponent.sw
+++ b/data/scenarios/Challenges/Ranching/_capture/opponent.sw
@@ -41,9 +41,9 @@ def countBlocked = \n.
         isBlocked <- blocked;
         turn left;
         lastCount <- countBlocked $ n - 1;
-        return $ lastCount + boolToInt isBlocked;
+        pure $ lastCount + boolToInt isBlocked;
     } {
-        return 0;
+        pure 0;
     }
     end;
 
@@ -54,7 +54,7 @@ def reWatch =
 def locationIsOpen =
     emptyHere <- isempty;
     blockedCount <- countBlocked 4;
-    return $ emptyHere && blockedCount == 0;
+    pure $ emptyHere && blockedCount == 0;
     end;
 
 def faceAwayFrom = \loc.
@@ -84,7 +84,7 @@ def relocateAway =
     findOpenArea;
     try {
         swap marker;
-        return ();
+        pure ();
     } {
         place marker;
     };
diff --git a/data/scenarios/Challenges/Ranching/_fishing/hauler.sw b/data/scenarios/Challenges/Ranching/_fishing/hauler.sw
index 9d1e978d4..53385a5fb 100644
--- a/data/scenarios/Challenges/Ranching/_fishing/hauler.sw
+++ b/data/scenarios/Challenges/Ranching/_fishing/hauler.sw
@@ -20,13 +20,13 @@ def isEnclosureFull : Int * Int -> Cmd Bool = \encl.
     let notFull = c < area in
 
     teleport self prevLoc;
-    return $ not notFull;
+    pure $ not notFull;
     end;
 
 def any : (a -> Cmd Bool) -> (rec l. Unit + a * l) -> Cmd Bool = \p. \l.
   case l
-    (\_. return false)
-    (\c. b <- p (fst c); if b {return true} {any p (snd c)})
+    (\_. pure false)
+    (\c. b <- p (fst c); if b {pure true} {any p (snd c)})
 end;
 
 def isEitherEnclosureFull =
@@ -37,7 +37,7 @@ def isEitherEnclosureFull =
 def tryGrab =
     try {
         grab;
-        return ()
+        pure ()
     } {};
     end;
 
diff --git a/data/scenarios/Challenges/Ranching/_fishing/solution.sw b/data/scenarios/Challenges/Ranching/_fishing/solution.sw
index 2d5d87581..7c8bb528f 100644
--- a/data/scenarios/Challenges/Ranching/_fishing/solution.sw
+++ b/data/scenarios/Challenges/Ranching/_fishing/solution.sw
@@ -17,7 +17,7 @@ def makeRoll =
 def checkIngredients =
     hasTuna <- has "crab";
     hasSeaweed <- has "seaweed";
-    return $ hasTuna && hasSeaweed;
+    pure $ hasTuna && hasSeaweed;
     end;
 
 def catchFish = \rod.
@@ -39,9 +39,9 @@ Precondition:
 At the top-right corner
 */
 def harvestRectangle =
-    intersperse 4 move $ harvest; return ();
+    intersperse 4 move $ harvest; pure ();
     turnAround left;
-    intersperse 4 move $ harvest; return ();
+    intersperse 4 move $ harvest; pure ();
     end;
 
 def harvestIngredients =
@@ -64,8 +64,8 @@ def harvestIngredients =
 
 def find : (a -> Cmd Bool) -> (rec l. Unit + a * l) -> Cmd (Unit + a) = \p. \l.
   case l
-    (\_. return (inl ()))
-    (\cons. h <- p (fst cons); if h {return $ inr (fst cons)} {find p (snd cons)})
+    (\_. pure (inl ()))
+    (\cons. h <- p (fst cons); if h {pure $ inr (fst cons)} {find p (snd cons)})
 end
 
 def tryPlace = \item.
@@ -108,7 +108,7 @@ def placeSerpentine = \placeFunc.
 
     end;
 
-def returnToCorner =
+def pureToCorner =
     turn back;
     move; move;
     turn right;
@@ -143,7 +143,7 @@ def burnTires =
 
         intersperse 2 move $ (
             placeSerpentine $ tryPlace "car tire";
-            returnToCorner;
+            pureToCorner;
             ignite forward;
 
             turn right;
diff --git a/data/scenarios/Challenges/Ranching/_gated-paddock/enclosure-checking.sw b/data/scenarios/Challenges/Ranching/_gated-paddock/enclosure-checking.sw
index b94e46e45..bf8d2cbae 100644
--- a/data/scenarios/Challenges/Ranching/_gated-paddock/enclosure-checking.sw
+++ b/data/scenarios/Challenges/Ranching/_gated-paddock/enclosure-checking.sw
@@ -1,14 +1,14 @@
 def isBlockedOrFenced =
     b <- blocked;
-    return b;
+    pure b;
     end;
 
 def checkIsEnclosed =
     maybePath <- path (inL ()) (inR "water");
-    case maybePath (\_. return True) (\_. return False);
+    case maybePath (\_. pure True) (\_. pure False);
     end;
 
-def boolToInt = \b. if (b) {return 1} {return 0}; end;
+def boolToInt = \b. if (b) {pure 1} {pure 0}; end;
 
 def countAdjacentBlockages =
 
@@ -28,7 +28,7 @@ def countAdjacentBlockages =
     b4 <- isBlockedOrFenced;
     c4 <- boolToInt b4;
 
-    return $ c1 + c2 + c3 + c4;
+    pure $ c1 + c2 + c3 + c4;
     end;
 
 // Step forward, observing left and right.
@@ -44,7 +44,7 @@ def observeLeftAndRight =
 
     turn right;
     move;
-    return $ val1 + val2;
+    pure $ val1 + val2;
     end;
 
 
@@ -65,7 +65,7 @@ def countDiagonalBlockages =
     // Second, step to both sides
     fwdCount <- observeLeftAndRight;
     backCount <- observeLeftAndRight;
-    return $ fwdCount + backCount;
+    pure $ fwdCount + backCount;
     end;
 
 def isStandingOnBridge =
@@ -74,12 +74,12 @@ def isStandingOnBridge =
     if (onFence || onGate) {
         adjCount <- countAdjacentBlockages;
         if (adjCount > 1) {
-            return true;
+            pure true;
         } {
             diagCount <- countDiagonalBlockages;
-            return $ (adjCount + diagCount) > 1;
+            pure $ (adjCount + diagCount) > 1;
         };
-    } {return false};
+    } {pure false};
     end;
 
 def getValForSheepIndex = \predicateCmd. \i.
@@ -91,7 +91,7 @@ def getValForSheepIndex = \predicateCmd. \i.
 
         boolToInt didSucceed;
     } {
-        return 0;
+        pure 0;
     }
     end;
 
@@ -99,7 +99,7 @@ def countSheepWith = \predicateCmd.
     val1 <- getValForSheepIndex predicateCmd 1;
     val2 <- getValForSheepIndex predicateCmd 2;
     val3 <- getValForSheepIndex predicateCmd 3;
-    return $ val1 + val2 + val3;
+    pure $ val1 + val2 + val3;
     end;
 
 justFilledGap <- as base {
@@ -108,7 +108,7 @@ justFilledGap <- as base {
 
 if (justFilledGap) {
     enclosedCount <- countSheepWith checkIsEnclosed;
-    return $ enclosedCount >= 1;
+    pure $ enclosedCount >= 1;
 } {
-    return false;
+    pure false;
 }
diff --git a/data/scenarios/Challenges/Ranching/_gated-paddock/fence-construction.sw b/data/scenarios/Challenges/Ranching/_gated-paddock/fence-construction.sw
index 3f5f387f5..59907a62c 100644
--- a/data/scenarios/Challenges/Ranching/_gated-paddock/fence-construction.sw
+++ b/data/scenarios/Challenges/Ranching/_gated-paddock/fence-construction.sw
@@ -54,10 +54,10 @@ def grabTrees =
 
 def harvestIfClover =
     x <- scan down;
-    case x return (\y.
+    case x pure (\y.
         if (y == "clover") {
             harvest;
-            return ();
+            pure ();
         } {};
     );
     end;
@@ -125,11 +125,11 @@ def harvestCloverField =
     turn right;
     wait 200;
     
-    plantCloverColumn right $ return ();
-    plantCloverColumn left $ return ();
-    plantCloverColumn right $ return ();
-    plantCloverColumn left $ return ();
-    plantCloverColumn right $ return ();
+    plantCloverColumn right $ pure ();
+    plantCloverColumn left $ pure ();
+    plantCloverColumn right $ pure ();
+    plantCloverColumn left $ pure ();
+    plantCloverColumn right $ pure ();
     end;
 
 
@@ -164,10 +164,10 @@ def distributeCloverInPaddock =
 
 def pickupWool =
     x <- scan down;
-    case x return (\y.
+    case x pure (\y.
         if (y == "wool") {
             grab;
-            return ();
+            pure ();
         } {};
     );
     end;
diff --git a/data/scenarios/Challenges/Ranching/_gated-paddock/meandering-sheep.sw b/data/scenarios/Challenges/Ranching/_gated-paddock/meandering-sheep.sw
index a1d64aa94..efb5dfa86 100644
--- a/data/scenarios/Challenges/Ranching/_gated-paddock/meandering-sheep.sw
+++ b/data/scenarios/Challenges/Ranching/_gated-paddock/meandering-sheep.sw
@@ -5,12 +5,12 @@ def elif = \p.\t.\e. {if p t e} end;
 def turnToClover = \direction.
 
   x <- scan direction;
-  case x (\_. return false;) (\y.
+  case x (\_. pure false;) (\y.
     if (y == "clover") {
       turn direction;
-      return true;
+      pure true;
     } {
-      return false;
+      pure false;
     };
   );
   end;
@@ -23,11 +23,11 @@ turn that direction.
 def turnCloverDirection =
 
     foundN <- turnToClover north;
-    if (foundN) {return true} {
+    if (foundN) {pure true} {
       foundE <- turnToClover east;  
-      if (foundE) {return true} {
+      if (foundE) {pure true} {
         foundS <- turnToClover south;
-        if (foundS) {return true} {
+        if (foundS) {pure true} {
           turnToClover west;
         }
       }
@@ -38,7 +38,7 @@ def decideDirection =
 
   let randdir : Cmd Dir =
     d <- random 4;
-    return $ if (d == 0) {
+    pure $ if (d == 0) {
         north
       } $ elif (d == 1) {
         east
@@ -90,7 +90,7 @@ forever (
 
     // Eat clover.
     x <- scan down;
-    case x return (\y.
+    case x pure (\y.
         if (y == "clover") {
             harvest;
             cloverCount <- count "clover";
@@ -113,9 +113,9 @@ forever (
         // Make sure nothing's in the way before we place
         // our wool:
         x <- scan down;
-        case x return (\_.
+        case x pure (\_.
           grab;
-          return ();
+          pure ();
         );
 
         place item;
diff --git a/data/scenarios/Challenges/Ranching/_pied-piper/rat.sw b/data/scenarios/Challenges/Ranching/_pied-piper/rat.sw
index 18145aa6c..e3e3575d4 100644
--- a/data/scenarios/Challenges/Ranching/_pied-piper/rat.sw
+++ b/data/scenarios/Challenges/Ranching/_pied-piper/rat.sw
@@ -20,10 +20,10 @@ def goFoodDir = \f. \r.
     if (d == down) {
         foodHere <- ishere "oats";
         if foodHere {
-            grab; return ()
+            grab; pure ()
         } {};
         f;
-        return ()
+        pure ()
     } {
         turn d;
 
@@ -40,7 +40,7 @@ def goFoodDir = \f. \r.
 def goHomeDir = \f. \r.
     let d = fst r in
     if (d == down) {
-        return ()
+        pure ()
     } {
         turn d;
 
@@ -81,12 +81,12 @@ def pauseAtRandom =
 
 def returnHome = \homeLoc.
     nextDir <- path (inL ()) (inL homeLoc);
-    case nextDir return $ goHomeDir $ returnHome homeLoc;
+    case nextDir pure $ goHomeDir $ returnHome homeLoc;
     end;
 
 def pursueFood = \hadSensedFood. \homeLoc.
     nextDir <- path (inR 5) (inR "oats");
-    case nextDir (\_. if hadSensedFood {returnHome homeLoc} {return ()}) $
+    case nextDir (\_. if hadSensedFood {returnHome homeLoc} {pure ()}) $
         goFoodDir $ pursueFood true homeLoc;
     end;
 
diff --git a/data/scenarios/Challenges/Ranching/_powerset/setup.sw b/data/scenarios/Challenges/Ranching/_powerset/setup.sw
index 6d3395f0e..0e43bccf3 100644
--- a/data/scenarios/Challenges/Ranching/_powerset/setup.sw
+++ b/data/scenarios/Challenges/Ranching/_powerset/setup.sw
@@ -9,7 +9,7 @@ end
 def doN = \n. \f. if (n > 0) {f; doN (n - 1) f} {}; end;
 
 def until = \p. \c. q <- p; if q {} {c; until p c} end;
-def while = \p. until (x <- p; return $ not x) end;
+def while = \p. until (x <- p; pure $ not x) end;
 
 def isDivisibleBy = \dividend. \divisor.
     (dividend / divisor) * divisor == dividend;
@@ -67,7 +67,7 @@ def atLocation = \newLoc. \f.
     teleport self newLoc;
     retval <- f;
     teleport self prevLoc;
-    return retval;
+    pure retval;
     end;
 
 def placeSand =
@@ -86,7 +86,7 @@ def getUnusedRandom = \maxval. \bitmask.
     if (isBitSet bitmask nextRandomVal) {
         getUnusedRandom maxval bitmask;
     } {
-        return nextRandomVal;
+        pure nextRandomVal;
     }
     end;
 
@@ -140,7 +140,7 @@ def naiveRandomStack = \valueFunc. \maxval. \bitmask. \n.
         nextRandomVal <- getUnusedRandom maxval bitmask;
         let newBitmask = bitmask + shiftLeft 1 nextRandomVal in
         naiveRandomStack valueFunc maxval newBitmask $ n - 1;
-        return nextRandomVal;
+        pure nextRandomVal;
     } {
         // We're at the peak of the stack.
         // Now we unwind it.
@@ -148,7 +148,7 @@ def naiveRandomStack = \valueFunc. \maxval. \bitmask. \n.
         // Saves some time in generating the last number by inferring the
         // only remaining possible choice.
         let missingBit = getMissingBit bitmask maxval in
-        return missingBit;
+        pure missingBit;
     };
     valueFunc val;
     end;
@@ -193,7 +193,7 @@ def chooseExclusionValue = \powersetCardinality.
     if (exactlyOneBit false value) {
       chooseExclusionValue powersetCardinality;
     } {
-      return value;
+      pure value;
     }
     end;
 
@@ -208,7 +208,7 @@ def setup = \inputCardinality.
     move;
     exclusionValue <- chooseExclusionValue powersetCardinality;
     naiveRandomStack (columnFunc exclusionValue inputCardinality) powersetCardinality 0 powersetCardinality;
-    return exclusionValue;
+    pure exclusionValue;
     end;
 
 /**
@@ -220,7 +220,7 @@ def getOrdinal : Text -> Cmd Int = \item.
 
 def checkSolutionSum = \runningSum.
     maybeItem <- scan down;
-    case maybeItem (\_. return runningSum) (\item.
+    case maybeItem (\_. pure runningSum) (\item.
         // The bell is the only other item we can place in this
         // scenario besides the fruits.
         if (item != "bell") {
@@ -228,7 +228,7 @@ def checkSolutionSum = \runningSum.
             let binaryValue = shiftLeft 1 $ theOrdinal - 1 in
             move;
             checkSolutionSum $ binaryValue + runningSum;
-        } {return runningSum};
+        } {pure runningSum};
     );
     end;
 
@@ -250,4 +250,4 @@ def go = \distinctCount.
     create sentinelItem;
     end;
 
-go 7;
\ No newline at end of file
+go 7;
diff --git a/data/scenarios/Challenges/Ranching/_powerset/solution.sw b/data/scenarios/Challenges/Ranching/_powerset/solution.sw
index 8198101fe..e534761ef 100644
--- a/data/scenarios/Challenges/Ranching/_powerset/solution.sw
+++ b/data/scenarios/Challenges/Ranching/_powerset/solution.sw
@@ -1,6 +1,6 @@
 def doN = \n. \f. if (n > 0) {f; doN (n - 1) f} {}; end;
 def until = \p. \c. q <- p; if q {} {c; until p c} end;
-def while = \p. until (x <- p; return $ not x) end;
+def while = \p. until (x <- p; pure $ not x) end;
 
 def abs = \n. if (n < 0) {-n} {n} end;
 
@@ -28,7 +28,7 @@ def negateTuple = \t.
 
 def getRelativeLocation = \absCurrentLoc. \absDestLoc.
     let negatedLoc = negateTuple absCurrentLoc in
-    return $ sumTuples negatedLoc absDestLoc;
+    pure $ sumTuples negatedLoc absDestLoc;
     end;
 
 def splitStride = \n.
@@ -73,7 +73,7 @@ def recordFirstEncounter = \stashLoc. \item.
 
 def tryHarvest = \stashLoc.
     maybeItem <- scan down;
-    case maybeItem return (\item.
+    case maybeItem pure (\item.
         hasSome <- has item;
         harvest;
         if hasSome {} {
@@ -105,7 +105,7 @@ def countLine = \tally.
     if emptyhere {
         turn back;
         splitStride tally;
-        return tally;
+        pure tally;
     } {
         move;
         countLine $ tally + 1;
@@ -123,7 +123,7 @@ def placeFinalCopy = \item.
 
 def copyIfNeeded = \targetCount.
     maybeItem <- scan down;
-    case maybeItem return (\item.
+    case maybeItem pure (\item.
         quantity <- count item;
         if (quantity < targetCount) {
             placeFinalCopy item;
diff --git a/data/scenarios/Challenges/Ranching/beekeeping.yaml b/data/scenarios/Challenges/Ranching/beekeeping.yaml
index c0e06add5..489dba826 100644
--- a/data/scenarios/Challenges/Ranching/beekeeping.yaml
+++ b/data/scenarios/Challenges/Ranching/beekeeping.yaml
@@ -33,7 +33,7 @@ objectives:
         `honeycomb`{=entity} production.
     condition: |
       foundStructure <- structures "beehive";
-      return $ case foundStructure (\_. false) (\_. true);
+      pure $ case foundStructure (\_. false) (\_. true);
   - teaser: Collect honeycomb
     goal:
       - |
@@ -67,7 +67,7 @@ objectives:
     condition: |
       as base {
         meadCount <- count "mead";
-        return $ meadCount >= 2;
+        pure $ meadCount >= 2;
       }
   - teaser: Tavern keeper
     optional: true
@@ -76,7 +76,7 @@ objectives:
         Construct a `mead hall`{=structure}.
     condition: |
       foundStructure <- structures "mead hall";
-      return $ case foundStructure (\_. false) (\_. true);
+      pure $ case foundStructure (\_. false) (\_. true);
 robots:
   - name: base
     display:
diff --git a/data/scenarios/Challenges/Ranching/capture.yaml b/data/scenarios/Challenges/Ranching/capture.yaml
index d65a4b5ce..29129fc0e 100644
--- a/data/scenarios/Challenges/Ranching/capture.yaml
+++ b/data/scenarios/Challenges/Ranching/capture.yaml
@@ -25,10 +25,10 @@ objectives:
                   turn left;
                   isSurrounded $ n - 1;
               } {
-                  return false;
+                  pure false;
               }
           } {
-              return true;
+              pure true;
           }
           end;
 
@@ -59,16 +59,16 @@ objectives:
               let localBlockCount = boolToInt isBlocked in
               turn left;
               lastCount <- countBlocked $ n - 1;
-              return $ lastCount + localBlockCount;
+              pure $ lastCount + localBlockCount;
           } {
-              return 0;
+              pure 0;
           }
           end;
 
       opponent <- robotnamed "opponent";
       as opponent {
           blockCount <- countBlocked 4;
-          return $ blockCount > 0 && blockCount < 4;
+          pure $ blockCount > 0 && blockCount < 4;
       };
 robots:
   - name: base
diff --git a/data/scenarios/Challenges/Ranching/fishing.yaml b/data/scenarios/Challenges/Ranching/fishing.yaml
index f120d2f4e..ed1e12f78 100644
--- a/data/scenarios/Challenges/Ranching/fishing.yaml
+++ b/data/scenarios/Challenges/Ranching/fishing.yaml
@@ -29,7 +29,7 @@ objectives:
     condition: |
       as base {
         rollCount <- count "california roll";
-        return $ rollCount >= 16;
+        pure $ rollCount >= 16;
       }
     prerequisite: made_sushi
   - teaser: Sushi chef
@@ -61,12 +61,12 @@ objectives:
     condition: |
       def hasAny : (rec l. Unit + Text * l) -> Cmd Bool = \items.
         case items
-          (\_. return false)
-          (\c. b <- has (fst c); if b {return true} {hasAny (snd c)})
+          (\_. pure false)
+          (\c. b <- has (fst c); if b {pure true} {hasAny (snd c)})
       end;
 
       let prohibited = tagmembers "junk" in
-      h <- hasAny prohibited; return (not h)
+      h <- hasAny prohibited; pure (not h)
   - teaser: No littering
     id: littered
     hidden: true
@@ -78,7 +78,7 @@ objectives:
         Note that certain items can be burned with a `torch`{=entity} to make room.
     condition: |
       def locIsInsideEnclosure = \loc. \dims. \boxPos.
-        return $ snd loc >= snd boxPos
+        pure $ snd loc >= snd boxPos
           && snd loc < snd dims + snd boxPos
           && fst loc >= fst boxPos
           && fst loc < fst dims + fst boxPos;
@@ -86,14 +86,14 @@ objectives:
 
       def junkOutsideEnclosure =
         result <- scan down;
-        case result (\_. return false) (\item.
+        case result (\_. pure false) (\item.
           let isJunk = hastag item "junk" in
           if isJunk {
             enclosures <- structures "rubbish enclosure";
 
-            case enclosures (\_. return false) (\cons0.
+            case enclosures (\_. pure false) (\cons0.
               let enclosure0 = fst cons0 in
-              case (snd cons0) (\_. return false) (\cons1.
+              case (snd cons0) (\_. pure false) (\cons1.
                 let enclosure1 = fst cons1 in
 
                 dims <- floorplan "rubbish enclosure";
@@ -101,11 +101,11 @@ objectives:
 
                 insideFirst <- locIsInsideEnclosure loc dims enclosure0;
                 insideSecond <- locIsInsideEnclosure loc dims enclosure1;
-                return $ not $ insideFirst || insideSecond;
+                pure $ not $ insideFirst || insideSecond;
               )
             )
           } {
-            return false;
+            pure false;
           };
         );
         end;
diff --git a/data/scenarios/Challenges/Ranching/gated-paddock.yaml b/data/scenarios/Challenges/Ranching/gated-paddock.yaml
index 825a9eb91..1bee1a4f5 100644
--- a/data/scenarios/Challenges/Ranching/gated-paddock.yaml
+++ b/data/scenarios/Challenges/Ranching/gated-paddock.yaml
@@ -23,15 +23,15 @@ objectives:
     condition: |-
       def isBlockedOrFenced =
           b <- blocked;
-          return b;
+          pure b;
           end;
 
       def checkIsEnclosed =
           maybePath <- path (inL ()) (inR "water");
-          case maybePath (\_. return True) (\_. return False);
+          case maybePath (\_. pure True) (\_. pure False);
           end;
 
-      def boolToInt = \b. if (b) {return 1} {return 0}; end;
+      def boolToInt = \b. if (b) {pure 1} {pure 0}; end;
 
       def countAdjacentBlockages =
 
@@ -51,7 +51,7 @@ objectives:
           b4 <- isBlockedOrFenced;
           c4 <- boolToInt b4;
 
-          return $ c1 + c2 + c3 + c4;
+          pure $ c1 + c2 + c3 + c4;
           end;
 
       // Step forward, observing left and right.
@@ -67,7 +67,7 @@ objectives:
 
           turn right;
           move;
-          return $ val1 + val2;
+          pure $ val1 + val2;
           end;
 
 
@@ -88,7 +88,7 @@ objectives:
           // Second, step to both sides
           fwdCount <- observeLeftAndRight;
           backCount <- observeLeftAndRight;
-          return $ fwdCount + backCount;
+          pure $ fwdCount + backCount;
           end;
 
       def isStandingOnBridge =
@@ -97,12 +97,12 @@ objectives:
           if (onFence || onGate) {
               adjCount <- countAdjacentBlockages;
               if (adjCount > 1) {
-                  return true;
+                  pure true;
               } {
                   diagCount <- countDiagonalBlockages;
-                  return $ (adjCount + diagCount) > 1;
+                  pure $ (adjCount + diagCount) > 1;
               };
-          } {return false};
+          } {pure false};
           end;
 
       def getValForSheepIndex = \predicateCmd. \i.
@@ -114,7 +114,7 @@ objectives:
 
               boolToInt didSucceed;
           } {
-              return 0;
+              pure 0;
           }
           end;
 
@@ -122,7 +122,7 @@ objectives:
           val1 <- getValForSheepIndex predicateCmd 1;
           val2 <- getValForSheepIndex predicateCmd 2;
           val3 <- getValForSheepIndex predicateCmd 3;
-          return $ val1 + val2 + val3;
+          pure $ val1 + val2 + val3;
           end;
 
       justFilledGap <- as base {
@@ -131,9 +131,9 @@ objectives:
 
       if (justFilledGap) {
           enclosedCount <- countSheepWith checkIsEnclosed;
-          return $ enclosedCount >= 1;
+          pure $ enclosedCount >= 1;
       } {
-          return false;
+          pure false;
       }
   - id: feed_sheep
     teaser: Feed sheep
@@ -155,7 +155,7 @@ objectives:
               r <- robotnumbered i;
               as r {predicateCmd};
           } {
-              return false;
+              pure false;
           };
           end;
 
@@ -164,12 +164,12 @@ objectives:
           if (i > 0) {
               didSucceed <- getTruthForSheepIndex predicateCmd i;
               if didSucceed {
-                  return true;
+                  pure true;
               } {
                   anySheep predicateCmd $ i - 1;
               };
           } {
-              return false;
+              pure false;
           };
           end;
 
diff --git a/data/scenarios/Challenges/Ranching/pied-piper.yaml b/data/scenarios/Challenges/Ranching/pied-piper.yaml
index b65736868..1f478e3e8 100644
--- a/data/scenarios/Challenges/Ranching/pied-piper.yaml
+++ b/data/scenarios/Challenges/Ranching/pied-piper.yaml
@@ -89,7 +89,7 @@ objectives:
     condition: |
       as base {
         loc <- locateme;
-        return $ fst loc == "silo";
+        pure $ fst loc == "silo";
       }
   - teaser: Moldy
     prerequisite: silo
@@ -109,8 +109,8 @@ objectives:
     condition: |
       try {
         r <- robotNamed "rat";
-        return false;
-      } {return true}
+        pure false;
+      } {pure true}
   - teaser: Spelunker
     prerequisite: silo
     goal:
@@ -119,7 +119,7 @@ objectives:
     condition: |
       as base {
         loc <- locateme;
-        return $ fst loc == "tunnel";
+        pure $ fst loc == "tunnel";
       }
 solution: |
   run "scenarios/Challenges/Ranching/_pied-piper/solution.sw"
diff --git a/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/maintainer.sw b/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/maintainer.sw
index 19853d373..5bfe944aa 100644
--- a/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/maintainer.sw	
+++ b/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/maintainer.sw	
@@ -28,12 +28,12 @@ def atLocation = \newLoc. \f.
     teleport self newLoc;
     retval <- f;
     teleport self prevLoc;
-    return retval;
+    pure retval;
     end;
 
 def itemIsHere = \item.
     x <- scan down;
-    case x (\_. return false) (\found. return $ found == item);
+    case x (\_. pure false) (\found. pure $ found == item);
     end;
 
 def getLetterEntityByIndex = \idx.
@@ -52,7 +52,7 @@ def getOrdinal : Text -> Cmd Int = \item.
 
 def getValueHere =
     maybeItem <- scan down;
-    ordNum <- case maybeItem (\_. return 0) getOrdinal;
+    ordNum <- case maybeItem (\_. pure 0) getOrdinal;
     end;
 
 def getIndexesTotal = \boardWidth. \boardHeight. \n.
@@ -61,9 +61,9 @@ def getIndexesTotal = \boardWidth. \boardHeight. \n.
         teleport self (idx/boardHeight, -(mod idx boardWidth));
         valueHere <- getValueHere;
         runningTotal <- getIndexesTotal boardWidth boardHeight $ n - 1;
-        return $ valueHere + runningTotal;
+        pure $ valueHere + runningTotal;
     } {
-        return 0;
+        pure 0;
     }
     end;
 
@@ -77,7 +77,7 @@ def findMissingIndex = \boardWidth. \boardHeight.
     let tileCount = squareCount - 1 in
     let indicesSum = computeTriangularNumber tileCount in
     mySum <- getIndexesTotal boardWidth boardHeight squareCount;
-    return $ indicesSum - mySum;
+    pure $ indicesSum - mySum;
     end;
 
 def replenishInk =
@@ -114,12 +114,12 @@ Checks in the four directions.
 def hasAdjacentBlank = \tileIdx. \n.
     if (n > 0) {
         result <- scan forward;
-        case result (\_. handleLegalMove tileIdx; return true;) (\_.
+        case result (\_. handleLegalMove tileIdx; pure true;) (\_.
             turn left;
             hasAdjacentBlank tileIdx $ n - 1;
         );
     } {
-        return false;
+        pure false;
     }
     end;
 
@@ -141,7 +141,7 @@ Preconditions:
 def handleMarker = \boardWidth. \boardHeight.
     detectReferenceLoc <- whereami;
     result <- detect "sliding-tile" ((0, 0), (boardWidth - 1, boardHeight - 1));
-    case result return (\badLoc.
+    case result pure (\badLoc.
         teleportToDetectResult detectReferenceLoc badLoc;
         missingIdx <- atLocation (0, 0) $ findMissingIndex boardWidth boardHeight;
         markIsLegal <- isLegalMark missingIdx;
@@ -208,4 +208,4 @@ def go = \boardWidth. \boardHeight.
     end;
 
 until (has "flower") $ wait 1;
-go 3 3;
\ No newline at end of file
+go 3 3;
diff --git a/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/setup.sw b/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/setup.sw
index 9d5ae6a01..ec0f5cf6b 100644
--- a/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/setup.sw	
+++ b/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/setup.sw	
@@ -29,7 +29,7 @@ def getOrdinal : Text -> Cmd Int = \item.
 
 def getValueHere =
     maybeItem <- scan down;
-    ordNum <- case maybeItem (\_. return 0) getOrdinal;
+    ordNum <- case maybeItem (\_. pure 0) getOrdinal;
     end;
 
 /**
@@ -81,9 +81,9 @@ def countInnerInversions = \n. \referenceVal. \j.
         let addend = if (referenceVal > valueHere) {1} {0} in
         recursiveSum <- countInnerInversions n referenceVal $ j + 1;
         let foo = recursiveSum in
-        return $ addend + foo;
+        pure $ addend + foo;
     } {
-        return 0;
+        pure 0;
     };
     end
 
@@ -102,9 +102,9 @@ def countInversions = \n. \i.
         turn back;
         subarrayInversions <- countInversions n $ i + 1;
         let foo = subarrayInversions in
-        return $ innerCountFoo + foo;
+        pure $ innerCountFoo + foo;
     } {
-        return 0;
+        pure 0;
     };
     end
 
@@ -115,13 +115,13 @@ Right is a valid tile entity name.
 def scanValid : Dir -> Cmd (Bool + Text) = \d.
     maybeTileForward <- scan d;
     case maybeTileForward
-        (\_. return $ inL false)
+        (\_. pure $ inL false)
         (\x.
             if (x == "sliding-tile") {
-                return $ inL true;
+                pure $ inL true;
             } {
                 y <- getOrdinal x;
-                return $ if (y > 0) {
+                pure $ if (y > 0) {
                     inR x;
                 } {
                     inL false;
@@ -145,14 +145,14 @@ def actOnItemComparison = \maybeNewItem. \original.
             move;
             grab;
             // Abort early from the recursion.
-            return false;
+            pure false;
         } {
             // The new tile is not a sliding tile.
             // We assume it's a blank tile and move there.
             // If it turns out not to be blank, that will
             // be addressed in the outer "observe" loop.
             move;
-            return false;
+            pure false;
         };
     ) (\newItem.
         let isSame = newItem == original in
@@ -160,7 +160,7 @@ def actOnItemComparison = \maybeNewItem. \original.
         if isSame {} {
             say $ "Original was " ++ original ++ "; newItem was " ++ newItem;
         };
-        return isSame;
+        pure isSame;
     );
     end;
 
@@ -179,14 +179,14 @@ def unwind = \keepChecking. \maybeItem.
                 // Our assumption was invalid; we don't have a
                 // valid reference tile to compared the drilled tile to.
                 say "Unexpected drilling; no reference tile.";
-                return false;
+                pure false;
             } {
-                return true;
+                pure true;
             }
         ) (actOnItemComparison maybeItem2);
-        return keepGoing;
+        pure keepGoing;
     } {
-        return false;
+        pure false;
     };
     end;
 
diff --git a/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/solution.sw b/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/solution.sw
index 89755bcd8..0f5691f94 100644
--- a/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/solution.sw	
+++ b/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/solution.sw	
@@ -57,13 +57,13 @@ def subtractTuple = \t1. \t2.
 def getRelativeLocation = \absLoc.
     myloc <- whereami;
     let negatedLoc = negateTuple myloc in
-    return $ sumTuples negatedLoc absLoc;
+    pure $ sumTuples negatedLoc absLoc;
     end;
 
 def getRelativeRectangle : (Int * Int) * (Int * Int) -> Cmd ((Int * Int) * (Int * Int)) = \corners.
     myloc <- whereami;
     let negatedLoc = negateTuple myloc in
-    return $ mapTuple (sumTuples negatedLoc) corners;
+    pure $ mapTuple (sumTuples negatedLoc) corners;
     end;
 
 /**
@@ -191,7 +191,7 @@ def blankCellLocatorCriteria = \rect.
     entCount <- density rect;
     let dims = getDimensions rect in
     let tileCount = getRectArea dims in
-    return $ entCount < tileCount;
+    pure $ entCount < tileCount;
     end;
 
 /**
@@ -209,10 +209,10 @@ def findEmptyCell = \foundCriteria. \rect.
     let tileCount = getRectArea dims in
 
     if (tileCount < 1) {
-        return $ inL ();
+        pure $ inL ();
     } $ elif (tileCount == 1) {
         foundHere <- foundCriteria rect;
-        return $ if foundHere {
+        pure $ if foundHere {
             inR $ fst rect;
         } {
             inL ();
@@ -297,7 +297,7 @@ def placeTile = \boardWidth. \idx. \blankLoc.
     log $ "absolute target loc: " ++ format targetLocAbsolute;
     targetRelativeLoc <- getRelativeLocation targetLocAbsolute;
 
-//    case eitherLetterloc return $ moveSpaceToTile blankLoc targetRelativeLoc;
+//    case eitherLetterloc pure $ moveSpaceToTile blankLoc targetRelativeLoc;
     moveTuple blankLoc;
     end;
 
@@ -412,7 +412,7 @@ def go = \boardWidth.
     corners <- getBoardRectangle;
     eitherBlankLoc <- findEmptyCell blankCellLocatorCriteria corners;
 
-    case eitherBlankLoc return $ placeTile boardWidth 1;
+    case eitherBlankLoc pure $ placeTile boardWidth 1;
 
     moveManually;
     end;
diff --git a/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/validate-board.sw b/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/validate-board.sw
index 239081be0..074afc074 100644
--- a/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/validate-board.sw	
+++ b/data/scenarios/Challenges/Sliding Puzzles/_sliding-puzzle/validate-board.sw	
@@ -4,7 +4,7 @@ def else = id end
 
 def itemIsHere = \item.
     x <- scan down;
-    case x (\_. return false) (\found. return $ found == item);
+    case x (\_. pure false) (\found. pure $ found == item);
     end;
 
 def getOrdinal : Text -> Cmd Int = \item.
@@ -19,10 +19,10 @@ def getOrdinal : Text -> Cmd Int = \item.
 def isMonotonic : Int -> Cmd (Unit + Int) = \expectedVal.
     maybeItem <- scan down;
     case maybeItem
-        (\_. return $ inR expectedVal) // Cell was blank
+        (\_. pure $ inR expectedVal) // Cell was blank
         (\entity.
             intVal <- getOrdinal entity;
-            return $ if (intVal == expectedVal) {
+            pure $ if (intVal == expectedVal) {
                 inR $ expectedVal + 1;
             } {
                 inL ();
@@ -41,11 +41,11 @@ def isMonotonic : Int -> Cmd (Unit + Int) = \expectedVal.
 def loopMonotonicityCheck : Int -> Cmd Bool = \expectedVal.
     isOnBottomBorder <- itemIsHere "border";
     if isOnBottomBorder {
-        return true;
+        pure true;
     } {
         maybeNextVal <- isMonotonic expectedVal;
         case maybeNextVal
-            (\_. return false)
+            (\_. pure false)
             (\nextVal.
                 move;
                 isOnRightBorder <- itemIsHere "border";
@@ -65,7 +65,7 @@ def go =
         teleport self (0, 0);
         loopMonotonicityCheck 1;
     } {
-        return false;
+        pure false;
     };
     end;
 
diff --git a/data/scenarios/Challenges/_blender/apprehension-checker.sw b/data/scenarios/Challenges/_blender/apprehension-checker.sw
index 3dad7aa4e..9ad92e5d9 100644
--- a/data/scenarios/Challenges/_blender/apprehension-checker.sw
+++ b/data/scenarios/Challenges/_blender/apprehension-checker.sw
@@ -2,12 +2,12 @@ def hasMetBase = \r.
     let basename = "base" in
     x <- as r {whoami};
     if (x == basename) {
-        return false;
+        pure false;
     } {
         mr0 <- as r {meet};
         case mr0
-          (\_. return false)
-          (\bot. name <- as bot {whoami}; return $ name == basename);
+          (\_. pure false)
+          (\bot. name <- as bot {whoami}; pure $ name == basename);
     };
     end;
 
@@ -26,11 +26,11 @@ def anyHasMetBase : Int -> Cmd Bool = \idx.
         let foo = intermediate in
         let newIdx = idx + 1 in
         recursiveResult <- anyHasMetBase newIdx;
-        return $ foo || recursiveResult;
+        pure $ foo || recursiveResult;
     } {
       // Terminates the recursion on the
       // lowest index at which a robot does not exist
-      return false;
+      pure false;
     };
     end;
 
diff --git a/data/scenarios/Challenges/_bridge-building/flower-ring-check.sw b/data/scenarios/Challenges/_bridge-building/flower-ring-check.sw
index 57bb581f9..5667d7d42 100644
--- a/data/scenarios/Challenges/_bridge-building/flower-ring-check.sw
+++ b/data/scenarios/Challenges/_bridge-building/flower-ring-check.sw
@@ -3,13 +3,13 @@ def doN = \n. \f. if (n > 0) {f; doN (n - 1) f} {}; end;
 
 def isItemInDirection = \direction. \item. 
     x <- scan direction;
-    return $ case x (\_. false) (\y. y == item);
+    pure $ case x (\_. false) (\y. y == item);
     end;
 
 def isFlankedByItem = \item.
     hasLeft <- isItemInDirection left item;
     hasRight <- isItemInDirection right item;
-    return $ hasLeft && hasRight;
+    pure $ hasLeft && hasRight;
     end;
 
 def flowersInCardinalDirections = \item. \n.
@@ -19,10 +19,10 @@ def flowersInCardinalDirections = \item. \n.
             turn left;
             flowersInCardinalDirections item $ n - 1;
         } {
-            return false;
+            pure false;
         };
     } {
-        return true;
+        pure true;
     }
     end;
 
@@ -38,12 +38,12 @@ def flowersAround = \item.
             isFlanked2 <- isFlankedByItem item;
             turn back;
             move;
-            return isFlanked2;
+            pure isFlanked2;
         } {
-            return false;
+            pure false;
         }
     } {
-        return false;
+        pure false;
     }
     end;
 
diff --git a/data/scenarios/Challenges/_bucket-brigade/hauler.sw b/data/scenarios/Challenges/_bucket-brigade/hauler.sw
index 169e22143..cdc5bbef7 100644
--- a/data/scenarios/Challenges/_bucket-brigade/hauler.sw
+++ b/data/scenarios/Challenges/_bucket-brigade/hauler.sw
@@ -27,8 +27,8 @@ def patrol =
     traverseRoad;
     doNTimesOr 40 (
         briquetteIsHere <- ishere briquette;
-        if briquetteIsHere {grab; return ()} {};
-        return briquetteIsHere;
+        if briquetteIsHere {grab; pure ()} {};
+        pure briquetteIsHere;
     );
     turn back;
     traverseRoad;
@@ -41,4 +41,4 @@ def patrol =
     turn back;
     end;
 
-forever patrol;
\ No newline at end of file
+forever patrol;
diff --git a/data/scenarios/Challenges/_combo-lock/setup.sw b/data/scenarios/Challenges/_combo-lock/setup.sw
index 7a1bd6756..898a65eca 100644
--- a/data/scenarios/Challenges/_combo-lock/setup.sw
+++ b/data/scenarios/Challenges/_combo-lock/setup.sw
@@ -31,7 +31,7 @@ def checkCombo = \noMismatchYet. \stepsTaken. \colorString.
         // Replace the cell watches
         doN stepsTaken (watch down; move);
         turn back;
-        return noMismatchYet;
+        pure noMismatchYet;
     };
     end;
 
@@ -41,7 +41,7 @@ def unlockGate = \n.
     move;
     turn left;
     doN n (grab; move);
-    return ()
+    pure ()
     end;
 
 def doUntilCorrect = \colorString.
@@ -49,7 +49,7 @@ def doUntilCorrect = \colorString.
     if isCorrect {
         let remainingCount = chars colorString in
         unlockGate remainingCount;
-        return true;
+        pure true;
     } {
         wait 1000;
         doUntilCorrect colorString;
@@ -69,7 +69,7 @@ def createCombo = \colorString.
         createCombo $ newColor ++ colorString;
     } {
         turn back;
-        return colorString;
+        pure colorString;
     };
     end;
 
diff --git a/data/scenarios/Challenges/_combo-lock/solution.sw b/data/scenarios/Challenges/_combo-lock/solution.sw
index 3483ff327..3e0bd742e 100644
--- a/data/scenarios/Challenges/_combo-lock/solution.sw
+++ b/data/scenarios/Challenges/_combo-lock/solution.sw
@@ -10,7 +10,7 @@ def cycleCombos = \n.
         if (n > 0) {
             drill down;
             maybeNextEnt <- scan east;
-            case maybeNextEnt return (\_. turn east; move; cycleCombos 3);
+            case maybeNextEnt pure (\_. turn east; move; cycleCombos 3);
             cycleCombos $ n - 1;
         } {
             turn west;
diff --git a/data/scenarios/Challenges/_dimsum/cook.sw b/data/scenarios/Challenges/_dimsum/cook.sw
index e3a79f036..4ad1c2e2e 100644
--- a/data/scenarios/Challenges/_dimsum/cook.sw
+++ b/data/scenarios/Challenges/_dimsum/cook.sw
@@ -1,11 +1,11 @@
 def modifyCart = \cartType. \device.
     result <- scan forward;
-    case result return (\item.
+    case result pure (\item.
         if (item == cartType) {
             use device forward;
-            return ();
+            pure ();
         } {
-            return ();
+            pure ();
         };
     );
     end;
diff --git a/data/scenarios/Challenges/_dimsum/patron.sw b/data/scenarios/Challenges/_dimsum/patron.sw
index 260ea2fba..6f23bcddd 100644
--- a/data/scenarios/Challenges/_dimsum/patron.sw
+++ b/data/scenarios/Challenges/_dimsum/patron.sw
@@ -1,11 +1,11 @@
 def modifyCart = \cartType. \device.
     result <- scan forward;
-    case result return (\item.
+    case result pure (\item.
         if (item == cartType) {
             use device forward;
-            return ();
+            pure ();
         } {
-            return ();
+            pure ();
         };
     );
     end;
diff --git a/data/scenarios/Challenges/_dna/lab.sw b/data/scenarios/Challenges/_dna/lab.sw
index dcfbdc3ac..50199ff2f 100644
--- a/data/scenarios/Challenges/_dna/lab.sw
+++ b/data/scenarios/Challenges/_dna/lab.sw
@@ -55,7 +55,7 @@ def waitUntilSomethingExists =
         watch down;
         wait 1000;
         waitUntilSomethingExists;
-    ) return;
+    ) pure;
     end;
 
 def waitUntilHere = \item.
@@ -81,7 +81,7 @@ def myStandby = \receptacleLoc.
     entToClone <- grab;
     teleport self (36, -11);
     turn back;
-    return $ inR entToClone;
+    pure $ inR entToClone;
     end;
 
 def placeBase = \standbyFunc. \n. 
@@ -102,9 +102,9 @@ def placeBase = \standbyFunc. \n.
         move;
 
         if isGood {
-            return clonedOrganism;
+            pure clonedOrganism;
         } {
-            return $ inL ();
+            pure $ inL ();
         }
     } {
         // Returns the clonedOrganism
@@ -151,7 +151,7 @@ def waitForCloneableOrganism =
         waitUntilOccupied;
 
         thingHere <- scan down;
-        return $ case thingHere (\x. inL x) (\item.
+        pure $ case thingHere (\x. inL x) (\item.
           if (hastag item "organism") {inR item} {inL ()}
         )
     );
diff --git a/data/scenarios/Challenges/_dna/resetter.sw b/data/scenarios/Challenges/_dna/resetter.sw
index a88379175..ffb47ddcc 100644
--- a/data/scenarios/Challenges/_dna/resetter.sw
+++ b/data/scenarios/Challenges/_dna/resetter.sw
@@ -25,7 +25,7 @@ def watchSwitch = \lastState.
     watch down;
     wait 1000;
     found <- scan down;
-    case found return (\item.
+    case found pure (\item.
         if (item != lastState) {
             if (item == "switch (off)") {
                 loc <- whereami;
@@ -41,7 +41,7 @@ def watchSwitch = \lastState.
 def go =
     instant $ (
         found <- scan down;
-        case found return watchSwitch;
+        case found pure watchSwitch;
     );
     end;
 
diff --git a/data/scenarios/Challenges/_dna/solution.sw b/data/scenarios/Challenges/_dna/solution.sw
index e31ea08ea..43e6eb909 100644
--- a/data/scenarios/Challenges/_dna/solution.sw
+++ b/data/scenarios/Challenges/_dna/solution.sw
@@ -105,7 +105,7 @@ def waitForItem : Dir -> Cmd Text = \d.
         watch d;
         wait 1000;
         waitForItem d;
-    ) return;
+    ) pure;
     end;
 
 def waitForSpecificItem = \item. \d.
@@ -179,10 +179,10 @@ def pickFlowerAndWater =
     doN 23 move;
     
     turn right;
-    return dahlia;
+    pure dahlia;
     
-    // return mushroom;
-    // return d;
+    // pure mushroom;
+    // pure d;
     end;
 
 
@@ -245,7 +245,7 @@ def completeDnaTask = \sentinel.
 
 def mapM_ : (a -> Cmd b) -> (rec l. Unit + a * l) -> Cmd Unit = \f. \l.
   case l
-    (\_. return ())
+    (\_. pure ())
     (\c. f (fst c); mapM_ f (snd c))
 end;
 
diff --git a/data/scenarios/Challenges/_flower-count/judge.sw b/data/scenarios/Challenges/_flower-count/judge.sw
index ad6d5e75b..2f2a6b63f 100644
--- a/data/scenarios/Challenges/_flower-count/judge.sw
+++ b/data/scenarios/Challenges/_flower-count/judge.sw
@@ -16,7 +16,7 @@ def judgeCount : Int -> Cmd Unit = \actual.
   wait 1024;
   s <- scan down;
   case s
-    (\_. return ())
+    (\_. pure ())
     (\p.
       try {
         let c = (read p : Int) in
diff --git a/data/scenarios/Challenges/_flower-count/solution.sw b/data/scenarios/Challenges/_flower-count/solution.sw
index 437a37750..df131c03b 100644
--- a/data/scenarios/Challenges/_flower-count/solution.sw
+++ b/data/scenarios/Challenges/_flower-count/solution.sw
@@ -23,14 +23,14 @@ end
 def liftA2 : (a -> b -> c) -> Cmd a -> Cmd b -> Cmd c = \f. \ca. \cb.
   a <- ca;
   b <- cb;
-  return (f a b)
+  pure (f a b)
 end
 
 def add : Cmd Int -> Cmd Int -> Cmd Int = liftA2 (\x. \y. x + y) end
 
 def countCell : Cmd Int =
   s <- scan down;
-  return $ case s
+  pure $ case s
     (\_. 0)
     (\t. if (t == "flower") {1} {0})
 end
@@ -44,13 +44,13 @@ def sum : List Int -> Int = \l.
 end
 
 def for : Int -> (Int -> Cmd a) -> Cmd (List a) = \n. \k.
-  if (n == 0) {return (inl ())} {a <- k n; b <- for (n-1) k; return (inr (a,b))}
+  if (n == 0) {pure (inl ())} {a <- k n; b <- for (n-1) k; pure (inr (a,b))}
 end
 
 def countRow : Int -> Cmd Int = \w.
-  ns <- for (w-1) (\_. n <- countCell; move; return n);
+  ns <- for (w-1) (\_. n <- countCell; move; pure n);
   last <- countCell;
-  return (sum ns + last)
+  pure (sum ns + last)
 end
 
 def isEven : Int -> Bool = \n. (n / 2) * 2 == n end
@@ -66,15 +66,15 @@ def countFlowers : Int * Int -> Int * Int -> Cmd Int = \size. \ll.
   cnts <- for (h-1) (\i.
     cnt <- countRow w;
     if (isEven i) { around right } { around left };
-    return cnt
+    pure cnt
   );
   last <- countRow w;
-  return (sum cnts + last)
+  pure (sum cnts + last)
 end
 
 def acquire : Cmd Text =
-  thing <- atomic (b <- isempty; if b {return ""} {grab});
-  if (thing == "") {acquire} {return thing}
+  thing <- atomic (b <- isempty; if b {pure ""} {grab});
+  if (thing == "") {acquire} {pure thing}
 end
 
 def countAndReport : Int * Int -> Int * Int -> Cmd Unit = \size. \ll.
@@ -92,7 +92,7 @@ def until : Cmd Bool -> Cmd a -> Cmd Unit = \test. \body.
 end
 
 def acquireFlower : Cmd Unit =
-  until (ishere "flower") move; grab; return ()
+  until (ishere "flower") move; grab; pure ()
 end
 
 def go =
@@ -108,7 +108,7 @@ def go =
   turn back;
   goto (20,0);
   res <- meet;
-  case res (\_. return ()) (\truelove. give truelove "flower")
+  case res (\_. pure ()) (\truelove. give truelove "flower")
 end;
 
 go;
diff --git a/data/scenarios/Challenges/_friend/cat.sw b/data/scenarios/Challenges/_friend/cat.sw
index 52cf47968..483fdfcdb 100644
--- a/data/scenarios/Challenges/_friend/cat.sw
+++ b/data/scenarios/Challenges/_friend/cat.sw
@@ -11,7 +11,7 @@ def abs = \n. if (n < 0) {-n} {n} end
 
 def randdir : Cmd Dir =
   d <- random 4;
-  return (
+  pure (
     if (d == 0) {north}
     $ elif (d == 1) {east}
     $ elif (d == 2) {south}
@@ -21,7 +21,7 @@ end
 
 def chooseWait : Cmd Int =
   t <- random (16*2);
-  return (16 + t)
+  pure (16 + t)
 end
 
 def wander =
diff --git a/data/scenarios/Challenges/_gallery/setup.sw b/data/scenarios/Challenges/_gallery/setup.sw
index 9a5ad4b2e..6a3f8e886 100644
--- a/data/scenarios/Challenges/_gallery/setup.sw
+++ b/data/scenarios/Challenges/_gallery/setup.sw
@@ -62,7 +62,7 @@ def getUnusedRandom = \maxval. \bitmask.
     if (isBitSet bitmask nextRandomVal) {
         getUnusedRandom maxval bitmask;
     } {
-        return nextRandomVal;
+        pure nextRandomVal;
     }
     end;
 
@@ -77,7 +77,7 @@ def naiveRandomStack = \valueFunc. \maxval. \bitmask. \n.
         nextRandomVal <- getUnusedRandom maxval bitmask;
         let newBitmask = bitmask + shiftLeft 1 nextRandomVal in
         naiveRandomStack valueFunc maxval newBitmask $ n - 1;
-        return nextRandomVal;
+        pure nextRandomVal;
     } {
         // We're at the peak of the stack.
         // Now we unwind it.
@@ -85,7 +85,7 @@ def naiveRandomStack = \valueFunc. \maxval. \bitmask. \n.
         // Saves some time in generating the last number by inferring the
         // only remaining possible choice.
         let missingBit = getMissingBit bitmask maxval in
-        return missingBit;
+        pure missingBit;
     };
     valueFunc val;
     end;
diff --git a/data/scenarios/Challenges/_gallery/solution.sw b/data/scenarios/Challenges/_gallery/solution.sw
index 193bbebc7..9fe38b966 100644
--- a/data/scenarios/Challenges/_gallery/solution.sw
+++ b/data/scenarios/Challenges/_gallery/solution.sw
@@ -3,7 +3,7 @@ def doN = \n. \f. if (n > 0) {f; doN (n - 1) f} {}; end;
 def countRow = \currentCount.
     emptyHere <- isempty;
     if emptyHere {
-        return currentCount;
+        pure currentCount;
     } {
         move;
         countRow $ 1 + currentCount;
@@ -12,9 +12,9 @@ def countRow = \currentCount.
 
 def getOrdinal = \d.
     maybeEntity <- scan d;
-    case maybeEntity (\nothing. return $ inL nothing) (\item.
+    case maybeEntity (\nothing. pure $ inL nothing) (\item.
         myCount <- count item;
-        return $ inR myCount;
+        pure $ inR myCount;
     );
     end;
 
@@ -72,7 +72,7 @@ def swapAdjacent = \remainingSteps. \previousOrdinal.
     thisOrdinal <- if (remainingSteps > 1) {
         getOrdinal right;
     } {
-        return $ inL ();
+        pure $ inL ();
     };
 
     case thisOrdinal (\_.
@@ -83,9 +83,9 @@ def swapAdjacent = \remainingSteps. \previousOrdinal.
         let shouldSwap = num < previousOrdinal in
         ordinal <- if shouldSwap {
             doSwap;
-            return previousOrdinal;
+            pure previousOrdinal;
         } {
-            return num;
+            pure num;
         };
         move;
         swapAdjacent (remainingSteps - 1) ordinal;
@@ -96,7 +96,7 @@ def doLoop = \unsortedCount.
     if (unsortedCount > 1) {
         prevOrdinal <- getOrdinal right;
         move;
-        case prevOrdinal return $ swapAdjacent unsortedCount;
+        case prevOrdinal pure $ swapAdjacent unsortedCount;
         doLoop $ unsortedCount - 1;
     } {}
     end;
diff --git a/data/scenarios/Challenges/_gopher/gopher.sw b/data/scenarios/Challenges/_gopher/gopher.sw
index 5ebd88092..e8e14b2a2 100644
--- a/data/scenarios/Challenges/_gopher/gopher.sw
+++ b/data/scenarios/Challenges/_gopher/gopher.sw
@@ -7,9 +7,9 @@ def else = \t. t end
 def randSign = \x.
     opposite <- random 2;
     if (opposite == 1) {
-        return (-x);
+        pure (-x);
     } {
-        return x;
+        pure x;
     }
     end;
 
@@ -84,7 +84,7 @@ def waitWhileHere = \e. \remainingTime.
                 // but *before* it executes `grab`, the gopher will crash.
                 // Thus we have wrapped this `grab` in a `try`.
                 grab;
-                return ();
+                pure ();
             } {};
         };
     } {};
@@ -115,7 +115,7 @@ def go = \width. \height. \lastTauntIndex. \startingAmount. \dropping.
             place reward;
         } {
             swap reward;
-            return ();
+            pure ();
         };
 
         baseloc <- as base {whereami};
diff --git a/data/scenarios/Challenges/_gopher/solution.sw b/data/scenarios/Challenges/_gopher/solution.sw
index 0a05354dd..7241dc587 100644
--- a/data/scenarios/Challenges/_gopher/solution.sw
+++ b/data/scenarios/Challenges/_gopher/solution.sw
@@ -34,15 +34,15 @@ def scanDirections = \n.
     let d = getDirection n in
     out <- scan d;
     shouldContinue <- case out
-        (\_. return true)
+        (\_. pure true)
         (\x. if (x == "mound") {
             drill d;
-            return true;
+            pure true;
         } {
             // A "flower" shall serve as
             // a semaphore to terminate the loop,
             // so that the base can `salvage` us.
-            return $ x != "flower";
+            pure $ x != "flower";
         });
 
     if shouldContinue {
@@ -58,7 +58,7 @@ def scanDirections = \n.
 
 def deploySensor =
     _s <- build {scanDirections 0};
-    return ();
+    pure ();
     end;
 
 def isDivisibleBy = \dividend. \divisor.
@@ -100,13 +100,13 @@ def deployGrid = \f. \width. \height.
 def pickupToolkit =
     x <- scan down;
     case x
-        (\_. return false)
+        (\_. pure false)
         (\y. if (y == "toolkit") {
             tk <- grab;
             equip tk;
-            return true
+            pure true
         } {
-            return false
+            pure false
         });
     end;
 
diff --git a/data/scenarios/Challenges/_hackman/ghost.sw b/data/scenarios/Challenges/_hackman/ghost.sw
index 581ae87ea..0c912a7bc 100644
--- a/data/scenarios/Challenges/_hackman/ghost.sw
+++ b/data/scenarios/Challenges/_hackman/ghost.sw
@@ -6,7 +6,7 @@ an intersection. Then randomly choose any available direction
 
 def isItemInDirection = \direction. \item.
     x <- scan direction;
-    return $ case x (\_. false) (\y. y == item);
+    pure $ case x (\_. false) (\y. y == item);
     end;
 
 // A ghost is not blocked by a "gate" when it is leaving the "nursery".
@@ -14,27 +14,27 @@ def canCrossGate =
     isGate <- isItemInDirection forward "gate";
     loc <- whereami;
     let amCentered = loc == (0, 0) in
-    return $ isGate && amCentered;
+    pure $ isGate && amCentered;
     end;
 
 def isBlockedAhead =
     isBlockedByWall <- blocked;
     ghostCanCrossGate <- canCrossGate;
-    return $ isBlockedByWall && not ghostCanCrossGate;
+    pure $ isBlockedByWall && not ghostCanCrossGate;
     end;
 
 def checkLeftBlocked =
     turn left;
     isBlocked <- isBlockedAhead;
     turn right;
-    return isBlocked;
+    pure isBlocked;
     end;
 
 def checkRightBlocked =
     turn right;
     isBlocked <- isBlockedAhead;
     turn left;
-    return isBlocked;
+    pure isBlocked;
     end;
 
 def chooseDirection : Cmd Dir =
@@ -43,7 +43,7 @@ def chooseDirection : Cmd Dir =
     forwardBlocked <- isBlockedAhead;
     if (leftBlocked && rightBlocked && forwardBlocked) {
         say "Dead end; turning back";
-        return back;
+        pure back;
     } {
         // Since we're in the "else" case of all three
         // being blocked, we know that at least one
@@ -51,13 +51,13 @@ def chooseDirection : Cmd Dir =
         // the combinations.
         if (leftBlocked && rightBlocked) {
             // Keep going straight
-            return forward;
+            pure forward;
         } {
             if (leftBlocked && forwardBlocked) {
-                return right;
+                pure right;
             } {
                 if (rightBlocked && forwardBlocked) {
-                    return left;
+                    pure left;
                 } {
                     // Now we have exhaused all of the combinations
                     // of *two* directions being blocked, so we
@@ -67,30 +67,30 @@ def chooseDirection : Cmd Dir =
                         // Decide whether to go right or straight
                         d <- random 2;
                         if (d == 0) {
-                            return right;
+                            pure right;
                         } {
                             // go straight
-                            return forward;
+                            pure forward;
                         }
                     } {
                         if rightBlocked {
                             // Decide whether to go left or straight
                             d <- random 2;
                             if (d == 0) {
-                                return left;
+                                pure left;
                             } {
                                 // go straight
-                                return forward;
+                                pure forward;
                             }
                         } {
                             if forwardBlocked {
                                 // Decide whether to go left or right
                                 d <- random 2;
                                 if (d == 0) {
-                                    return left;
+                                    pure left;
                                 } {
                                     // go right
-                                    return right;
+                                    pure right;
                                 }
                             } {
                                 // No directions are blocked, so we can
@@ -99,13 +99,13 @@ def chooseDirection : Cmd Dir =
 
                                 d <- random 3;
                                 if (d == 0) {
-                                    return left;
+                                    pure left;
                                 } {
                                     if (d == 1) {
-                                        return right;
+                                        pure right;
                                     } {
                                         // go straight
-                                        return forward;
+                                        pure forward;
                                     }
                                 }
                             }
diff --git a/data/scenarios/Challenges/_hackman/solution.sw b/data/scenarios/Challenges/_hackman/solution.sw
index 1242c01ec..7869ded5d 100644
--- a/data/scenarios/Challenges/_hackman/solution.sw
+++ b/data/scenarios/Challenges/_hackman/solution.sw
@@ -3,7 +3,7 @@ def doN = \n. \f. if (n > 0) {f; doN (n - 1) f} {}; end;
 def tryGrab =
     try {
         grab;
-        return ();
+        pure ();
     } {};
     end;
 
@@ -12,7 +12,7 @@ def tryGive = \thing.
     if hasPowerup {
         r <- meet;
         case r
-            (\_. return ())
+            (\_. pure ())
             (\r. give r thing; wait 10);
     } {};
     end;
@@ -140,4 +140,4 @@ def invadeDen =
 eatAllPellets;
 returnToCenter;
 waitToGive;
-invadeDen;
\ No newline at end of file
+invadeDen;
diff --git a/data/scenarios/Challenges/_hanoi/hanoi-count.sw b/data/scenarios/Challenges/_hanoi/hanoi-count.sw
index 1529dee08..8ccdac3c4 100644
--- a/data/scenarios/Challenges/_hanoi/hanoi-count.sw
+++ b/data/scenarios/Challenges/_hanoi/hanoi-count.sw
@@ -9,7 +9,7 @@ end;
 
 def cscan = \d.
   s <- scan d;
-  if (s == inl ()) {return 0} {return 1}
+  if (s == inl ()) {pure 0} {pure 1}
 end;
 
 def count_column =
@@ -24,7 +24,7 @@ def count_column =
   // wait 8;
   // log (format k);
   // wait 8;
-  return (i + j + k)
+  pure (i + j + k)
 end;
 
 
@@ -47,7 +47,7 @@ repeat {
         // log (format y);
         // wait 8;
         // log (format z);
-        return $ i2e (x + y + z)
+        pure $ i2e (x + y + z)
     };
     //let sum = i2e (x + y + z) in
     teleport self (0,-6);
@@ -57,6 +57,6 @@ repeat {
     case counted (\e.
         fail $ "Fatal error: there should always be a count entity at (0,-6)! " ++ format e ++ " " ++ format counted
     ) (\e.
-        if (e == sum) {} {swap sum; return ()}
+        if (e == sum) {} {swap sum; pure ()}
     )
 }
diff --git a/data/scenarios/Challenges/_hanoi/hanoi-increasing.sw b/data/scenarios/Challenges/_hanoi/hanoi-increasing.sw
index 4fcd973bc..139b760fa 100644
--- a/data/scenarios/Challenges/_hanoi/hanoi-increasing.sw
+++ b/data/scenarios/Challenges/_hanoi/hanoi-increasing.sw
@@ -34,15 +34,15 @@ repeat (
         z <- scan north;
         if (z == null) {
           if (y == null) {
-            return true
+            pure true
           } {
-            return $ f x y
+            pure $ f x y
           }
         } {
-          return $ f x y && f y z
+          pure $ f x y && f y z
         }
     };
     try {
-        if o {place "OK"} {grab; return ()}
+        if o {place "OK"} {grab; pure ()}
     } {}
 )
\ No newline at end of file
diff --git a/data/scenarios/Challenges/_hanoi/hanoi-invariant.sw b/data/scenarios/Challenges/_hanoi/hanoi-invariant.sw
index 3304b1529..529a0b9ea 100644
--- a/data/scenarios/Challenges/_hanoi/hanoi-invariant.sw
+++ b/data/scenarios/Challenges/_hanoi/hanoi-invariant.sw
@@ -10,7 +10,7 @@ end;
 
 repeat (
 me <- scan down;
-case me (\_. return ()) (\e.
+case me (\_. pure ()) (\e.
 // if
 //  0. I stand on unlocked X
 //  1. place north of me is NOT empty
@@ -25,13 +25,13 @@ if (isUnlocked e)
       case mn (\_.
         teleport self (0,-6);
         allPlaced <- ishere "three";
-        return (not allPlaced)
+        pure (not allPlaced)
       ) (\_.
-        return true
+        pure true
       );
     };
     if northFullOrAllPlaced {
-      swap ("blocked " ++ e); return ()
+      swap ("blocked " ++ e); pure ()
     } {}
 }
 // if
@@ -56,9 +56,9 @@ if (isUnlocked e)
         o2 <- ishere "OK";
         teleport self (2,-5);
         o3 <- ishere "OK";
-        return (o1 && o2 && o3)
+        pure (o1 && o2 && o3)
       };
-      if (allPlaced && allSorted) {swap (unlock e); return ()} {}
-    ) (\_. return ())
+      if (allPlaced && allSorted) {swap (unlock e); pure ()} {}
+    ) (\_. pure ())
 }
 ))
diff --git a/data/scenarios/Challenges/_hanoi/hanoi-solution.sw b/data/scenarios/Challenges/_hanoi/hanoi-solution.sw
index 20ab288ef..118c52595 100644
--- a/data/scenarios/Challenges/_hanoi/hanoi-solution.sw
+++ b/data/scenarios/Challenges/_hanoi/hanoi-solution.sw
@@ -4,12 +4,12 @@ def rep = \n. \c. if (n == 0) {} {c; rep (n-1) c} end;
 def ifC = \p. \t. \e. res <- p; if res t e end;
 
 def orC = \c1. \c2.
-  b1 <- c1; b2 <- c2; return (b1 || b2)
+  b1 <- c1; b2 <- c2; pure (b1 || b2)
 end;
 
 def somethingHere =
   res <- scan down;
-  return (res != inl ())
+  pure (res != inl ())
 end;
 
 def fwdToThing = until blocked move end;
@@ -26,7 +26,7 @@ def getDisk =
   fwdToThing;
   d <- grab;
   goBack;
-  return d
+  pure d
 end;
 
 def placeDisk = \d.
@@ -49,7 +49,7 @@ def hanoi :
   Int -> // The offset to third column
   Cmd Int
   = \n. \o. \a. \b. \c.
-  if (n == 0) {return o}
+  if (n == 0) {pure o}
   {
     o_new <- hanoi (n-1) o a c b;
     moveToCol o_new a;
diff --git a/data/scenarios/Challenges/_ice-cream/customer.sw b/data/scenarios/Challenges/_ice-cream/customer.sw
index c2ee323f4..40ee509c9 100644
--- a/data/scenarios/Challenges/_ice-cream/customer.sw
+++ b/data/scenarios/Challenges/_ice-cream/customer.sw
@@ -11,13 +11,13 @@ def placeOrder =
     scoopCountExtra <- random 4000;
     let scoopCount = 1000 + scoopCountExtra in
     say $ format scoopCount ++ " scoops of vanilla ice cream, no more, no less! And top it with a cherry, please.";
-    return scoopCount;
+    pure scoopCount;
     end;
 
 def waitForItem = \item.
     hasItem <- has item;
     if hasItem {
-        return ();
+        pure ();
     } {
         wait 1;
         waitForItem item;
diff --git a/data/scenarios/Challenges/_ice-cream/solution.sw b/data/scenarios/Challenges/_ice-cream/solution.sw
index 44926b519..90a920a9a 100644
--- a/data/scenarios/Challenges/_ice-cream/solution.sw
+++ b/data/scenarios/Challenges/_ice-cream/solution.sw
@@ -30,12 +30,12 @@ def getIngredients =
     move;
     move;
     turn right;
-    return (cone, cherry);
+    pure (cone, cherry);
     end;
 
 def meetCustomer =
     maybeCustomer <- meet;
-    case maybeCustomer (\_. meetCustomer) return;
+    case maybeCustomer (\_. meetCustomer) pure;
     end;
 
 def serveScoop = \customer.
diff --git a/data/scenarios/Challenges/_lights-out/assistant.sw b/data/scenarios/Challenges/_lights-out/assistant.sw
index ab504f571..7bbc2e1ec 100644
--- a/data/scenarios/Challenges/_lights-out/assistant.sw
+++ b/data/scenarios/Challenges/_lights-out/assistant.sw
@@ -35,7 +35,7 @@ def mapTuple = \f. \t.
 def replaceWith = \withThis.
     create withThis;
     swap withThis;
-    return ();
+    pure ();
     end;
 
 /** Modifies the cell */
@@ -49,7 +49,7 @@ def invertLight = \e.
 
 def toggleLightHere =
     entHere <- scan down;
-    case entHere return invertLight;
+    case entHere pure invertLight;
     end;
 
 /** Precondition: in the middle of a "cross" */
@@ -79,7 +79,7 @@ def flipSelfAndNeighbors = \newState. \locOffset.
 def togglePending = \state.
     let pendingEntityName = "pending-" ++ state in
     maybePending <- detect pendingEntityName ((1, 1), (6, 6));
-    case maybePending return $ flipSelfAndNeighbors state;
+    case maybePending pure $ flipSelfAndNeighbors state;
     end;
 
 def observe =
@@ -140,9 +140,9 @@ def advanceRowViaTeleport =
 def shouldCorrectTile : (Bool * Bool) -> (Bool * Bool) -> Cmd Bool = \evenOverlaps. \isQuietTiles.
     if (evenOverlaps == isQuietTiles) {
         toggleLightHere;
-        return true;
+        pure true;
     } {
-        return false;
+        pure false;
     }
     end;
 
@@ -161,7 +161,7 @@ def prepareBoardRow = \abortFunc. \rowIdx. \colIdx.
 
         shouldAbort <- abortFunc quietTuple;
         if shouldAbort {
-            return ((0, 0), true);
+            pure ((0, 0), true);
         } {
             let quietCellOn = mapTuple (\x. x && isCurrentlyOn) quietTuple in
             let addend = mapTuple boolToInt quietCellOn in
@@ -169,10 +169,10 @@ def prepareBoardRow = \abortFunc. \rowIdx. \colIdx.
             move;
             retval <- prepareBoardRow abortFunc rowIdx $ colIdx - 1;
             let subTotal = fst retval in
-            return $ (sumTuples addend subTotal, snd retval);
+            pure $ (sumTuples addend subTotal, snd retval);
         }
     } {
-        return ((0, 0), false);
+        pure ((0, 0), false);
     }
     end;
 
@@ -186,21 +186,21 @@ def prepareBoardAllRows = \abortFunc. \boardWidth. \rowIdx.
         let shouldAbort = snd retval in
 
         if shouldAbort {
-            return (0, 0);
+            pure (0, 0);
         } {
             advanceRowViaTeleport;
             totalCommonCount <- prepareBoardAllRows abortFunc boardWidth $ rowIdx - 1;
-            return $ sumTuples rowCommonCount totalCommonCount
+            pure $ sumTuples rowCommonCount totalCommonCount
         }
     } {
-        return (0, 0);
+        pure (0, 0);
     }
     end;
 
 def checkIsSolvable = \boardWidth. \boardHeight.
-    overlapCounts <- prepareBoardAllRows (\_. return false) boardWidth $ boardHeight - 1;
+    overlapCounts <- prepareBoardAllRows (\_. pure false) boardWidth $ boardHeight - 1;
     // say $ "Overlap counts: " ++ format overlapCounts;
-    return $ mapTuple isEven overlapCounts;
+    pure $ mapTuple isEven overlapCounts;
     end;
 
 /** Teleports to a new location to execute a function
@@ -212,7 +212,7 @@ def atLocation = \newLoc. \f.
     teleport self newLoc;
     retval <- f;
     teleport self prevLoc;
-    return retval;
+    pure retval;
     end;
 
 def analyzeSolvability : Int -> Int -> Cmd (Bool * Bool) = \boardWidth. \boardHeight.
@@ -232,7 +232,7 @@ def ensureSolvability = \evenOverlaps. \boardWidth. \boardHeight.
     if isSolvable {} {
         atLocation (0, 0) $
             prepareBoardAllRows (shouldCorrectTile $ mapTuple not evenOverlaps) boardWidth $ boardHeight - 1;
-        return ()
+        pure ()
     }
     end;
 
diff --git a/data/scenarios/Challenges/_lights-out/solution.sw b/data/scenarios/Challenges/_lights-out/solution.sw
index d886c0b6e..2b2b756b8 100644
--- a/data/scenarios/Challenges/_lights-out/solution.sw
+++ b/data/scenarios/Challenges/_lights-out/solution.sw
@@ -12,7 +12,7 @@ def intersperse = \n. \f2. \f1. if (n > 0) {
 /** Precondition: facing "d" direction */
 def toggleToDark = \d.
     onHere <- ishere "on";
-    if onHere {drill d; return ()} {};
+    if onHere {drill d; pure ()} {};
     end;
 
 def visitSingleRow = \rowWidth. \d.
@@ -48,7 +48,7 @@ def goToCorner =
 
 def onInDirection = \d.
     entHere <- scan d;
-    return $ case entHere (\_. false) (\e. e == "on");
+    pure $ case entHere (\_. false) (\e. e == "on");
     end;
 
 /**
diff --git a/data/scenarios/Challenges/_maypole/monitor.sw b/data/scenarios/Challenges/_maypole/monitor.sw
index 2bb6f5148..83c168713 100644
--- a/data/scenarios/Challenges/_maypole/monitor.sw
+++ b/data/scenarios/Challenges/_maypole/monitor.sw
@@ -55,7 +55,7 @@ def getQuadrantIncrement = \oldQuadrant. \newQuadrant.
 
 def getCurrentQuadrant : (Int * Int) -> Cmd Int = \myLoc.
   baseLoc <- as base {whereami};
-  return $ getQuadrant baseLoc myLoc;
+  pure $ getQuadrant baseLoc myLoc;
   end;
 
 def checkNewQuadrant = \myLoc. \prevQuadrant. \quadrantTraversalCount.
@@ -65,9 +65,9 @@ def checkNewQuadrant = \myLoc. \prevQuadrant. \quadrantTraversalCount.
 
   if (changeCount != 0) {
     swap $ "maypole " ++ format currentQuadrant;
-    return ();
+    pure ();
   } {};
-  return (currentQuadrant, newQuadrantCount);
+  pure (currentQuadrant, newQuadrantCount);
   end;
 
 /*
@@ -91,7 +91,7 @@ def monitorAngle : (Int * Int) -> Int -> Int -> Int -> Cmd Unit =
   } {
     create "dizzy";
     swap "bitcoin";
-    return ();
+    pure ();
   }
   end;
 
diff --git a/data/scenarios/Challenges/_telephone/judge.sw b/data/scenarios/Challenges/_telephone/judge.sw
index 62812932e..157d0e2ab 100644
--- a/data/scenarios/Challenges/_telephone/judge.sw
+++ b/data/scenarios/Challenges/_telephone/judge.sw
@@ -6,24 +6,24 @@ end
 
 def andC : Cmd Bool -> Cmd Bool -> Cmd Bool = \c1. \c2.
   b1 <- c1;
-  if b1 {c2} {return false}
+  if b1 {c2} {pure false}
 end
 
 tydef List a = rec l. Unit + a * l end
 
 def for : Int -> (Int -> Cmd a) -> Cmd (List a) = \n. \k.
   if (n == 0)
-    { return $ inl () }
+    { pure $ inl () }
     { x <- k (n-1);
       xs <- for (n-1) k;
-      return (inr (x,xs))
+      pure (inr (x,xs))
     }
 end
 
 def readRow : Cmd (List (Unit + Text)) =
-  r <- for 8 (\_. s <- scan down; move; return s);
+  r <- for 8 (\_. s <- scan down; move; pure s);
   turn back; x 8 move; turn right; move; turn right;
-  return r
+  pure r
 end
 
 tydef Rect = List (List (Unit + Text)) end
@@ -31,24 +31,24 @@ tydef Rect = List (List (Unit + Text)) end
 def readRect : Cmd Rect =
   lst <- for 4 (\_. readRow);
   turn right; x 4 move; turn left;
-  return lst
+  pure lst
 end
 
 def checkCell : Unit + Text -> Cmd Bool = \pat.
   actual <- scan down;
   move;
-  return (actual == pat)
+  pure (actual == pat)
 end
 
 def checkRow : List (Unit + Text) -> Cmd Bool = \row.
   case row
-    (\_. turn back; x 8 move; turn right; move; turn right; return true)
+    (\_. turn back; x 8 move; turn right; move; turn right; pure true)
     (\cons. andC (checkCell (fst cons)) (checkRow (snd cons)))
 end
 
 def checkRect : Rect -> Cmd Bool = \rect.
   case rect
-    (\_. return true)
+    (\_. pure true)
     (\cons. andC (checkRow (fst cons)) (checkRect (snd cons)))
 end
 
diff --git a/data/scenarios/Challenges/_telephone/photocopier.sw b/data/scenarios/Challenges/_telephone/photocopier.sw
index 7683a95ce..ad5ee7134 100644
--- a/data/scenarios/Challenges/_telephone/photocopier.sw
+++ b/data/scenarios/Challenges/_telephone/photocopier.sw
@@ -25,7 +25,7 @@ end
 
 def copy : Cmd Unit =
   watch down; wait 1024;
-  p <- atomic (b <- isempty; if b {return ""} {grab});
+  p <- atomic (b <- isempty; if b {pure ""} {grab});
   if (p == "") {} {followInstructions p}
 end
 
diff --git a/data/scenarios/Challenges/_telephone/shuttle.sw b/data/scenarios/Challenges/_telephone/shuttle.sw
index 4ee877f90..9b96f5d0b 100644
--- a/data/scenarios/Challenges/_telephone/shuttle.sw
+++ b/data/scenarios/Challenges/_telephone/shuttle.sw
@@ -12,11 +12,11 @@ end
 def forever: ∀ a b. {Cmd a} -> Cmd b = \c. force c; forever c end
 
 def notC : Cmd Bool -> Cmd Bool = \c.
-  b <- c; return (not b)
+  b <- c; pure (not b)
 end
 
 def or : Cmd Bool -> Cmd Bool -> Cmd Bool = \c1. \c2.
-  ifC c1 {return true} {c2}
+  ifC c1 {pure true} {c2}
 end
 
 def followTrack : Cmd Unit =
@@ -26,11 +26,11 @@ def followTrack : Cmd Unit =
 end
 
 def pickup : Cmd Text =
-  atomic (b <- isempty; if b {return ""} {grab});
+  atomic (b <- isempty; if b {pure ""} {grab});
 end
 
 def dropoff : Text -> Cmd Bool = \thing.
-  atomic (b <- isempty; if b {place thing} {}; return b)
+  atomic (b <- isempty; if b {place thing} {}; pure b)
 end
 
 def deliver : Text -> Cmd Unit = \thing.
diff --git a/data/scenarios/Challenges/_telephone/solution.sw b/data/scenarios/Challenges/_telephone/solution.sw
index 5d60722f2..2d8816505 100644
--- a/data/scenarios/Challenges/_telephone/solution.sw
+++ b/data/scenarios/Challenges/_telephone/solution.sw
@@ -19,7 +19,7 @@ end
 
 def harvestMay =
   e <- isempty;
-  if e {} {harvest; return ()}
+  if e {} {harvest; pure ()}
 end
 
 def harvestTrees =
@@ -46,18 +46,18 @@ def scanAt : Int -> Int -> Cmd (Unit + Text) = \h. \v.
   x h move; turn right; x v move;
   s <- scan down;
   turn back; x v move; turn left; x h move; turn back;
-  return s
+  pure s
 end
 
 def atTerminal : Cmd a -> Cmd a = \c.
   x 12 move; turn left; x 2 move;
   a <- c;
   turn back; x 2 move; turn right; x 12 move; turn back;
-  return a
+  pure a
 end
 
 def waitToPlace : Text -> Cmd Unit = \t.
-  success <- atomic (b <- isempty; if b {place t} {}; return b);
+  success <- atomic (b <- isempty; if b {place t} {}; pure b);
   if success {} { watch down; wait 1024; waitToPlace t }
 end
 
@@ -68,7 +68,7 @@ def go =
     for 4 (\v.
       res <- scanAt (h-1) (v-1);
       case res
-        (\_. return ())
+        (\_. pure ())
         (\t. atTerminal (p <- print "paper" (format ((h-1,v-1),t)); waitToPlace p))
     )
   )
diff --git a/data/scenarios/Challenges/_wolf-goat-cabbage/multi-item-possession.sw b/data/scenarios/Challenges/_wolf-goat-cabbage/multi-item-possession.sw
index 1e7825896..e56d77367 100644
--- a/data/scenarios/Challenges/_wolf-goat-cabbage/multi-item-possession.sw
+++ b/data/scenarios/Challenges/_wolf-goat-cabbage/multi-item-possession.sw
@@ -1,4 +1,4 @@
 has_wolf <- has "wolf";
 has_goat <- has "goat";
 has_cabbage <- has "cabbage";
-return $ (has_wolf && has_goat) || (has_goat && has_cabbage) || (has_wolf && has_cabbage);
\ No newline at end of file
+pure $ (has_wolf && has_goat) || (has_goat && has_cabbage) || (has_wolf && has_cabbage);
\ No newline at end of file
diff --git a/data/scenarios/Challenges/_wolf-goat-cabbage/together-on-east-bank.sw b/data/scenarios/Challenges/_wolf-goat-cabbage/together-on-east-bank.sw
index 7f9d80648..327444c0c 100644
--- a/data/scenarios/Challenges/_wolf-goat-cabbage/together-on-east-bank.sw
+++ b/data/scenarios/Challenges/_wolf-goat-cabbage/together-on-east-bank.sw
@@ -1,7 +1,7 @@
 def get_x_coord = \r.
     as r {
         pos <- whereami;
-        return $ fst pos;
+        pure $ fst pos;
     };
     end;
 
@@ -10,16 +10,16 @@ def all_on_bank = \baseX. \robotName.
     thisX <- get_x_coord r;
 
     as r {
-        try {grab; return ()} {};
+        try {grab; pure ()} {};
         move;
-        try {grab; return ()} {};
+        try {grab; pure ()} {};
         move;
-        try {grab; return ()} {};
+        try {grab; pure ()} {};
 
         has_wolf <- has "wolf";
         has_goat <- has "goat";
         has_cabbage <- has "cabbage";
-        return $ baseX == thisX && has_wolf && has_goat && has_cabbage;
+        pure $ baseX == thisX && has_wolf && has_goat && has_cabbage;
     };
     end;
 
diff --git a/data/scenarios/Challenges/_wolf-goat-cabbage/unattended-together.sw b/data/scenarios/Challenges/_wolf-goat-cabbage/unattended-together.sw
index f5db838bf..eeedb91f0 100644
--- a/data/scenarios/Challenges/_wolf-goat-cabbage/unattended-together.sw
+++ b/data/scenarios/Challenges/_wolf-goat-cabbage/unattended-together.sw
@@ -1,7 +1,7 @@
 def get_x_coord = \r.
     as r {
         pos <- whereami;
-        return $ fst pos;
+        pure $ fst pos;
     };
     end;
 
@@ -10,19 +10,19 @@ def is_unattended_together = \baseX. \robotName.
     thisX <- get_x_coord r;
 
     if (baseX == thisX) {
-        return false;
+        pure false;
     } {
         as r {
-            try {grab; return ()} {};
+            try {grab; pure ()} {};
             move;
-            try {grab; return ()} {};
+            try {grab; pure ()} {};
             move;
-            try {grab; return ()} {};
+            try {grab; pure ()} {};
 
             has_wolf <- has "wolf";
             has_goat <- has "goat";
             has_cabbage <- has "cabbage";
-            return $ (has_wolf && has_goat) || (has_goat && has_cabbage);
+            pure $ (has_wolf && has_goat) || (has_goat && has_cabbage);
         };        
     }
     end;
@@ -32,4 +32,4 @@ baseX <- get_x_coord base;
 west_bad <- is_unattended_together baseX "west_detector";
 east_bad <- is_unattended_together baseX "east_detector";
 
-return $ west_bad || east_bad;
\ No newline at end of file
+pure $ west_bad || east_bad;
\ No newline at end of file
diff --git a/data/scenarios/Challenges/_word-search/create-puzzle.sw b/data/scenarios/Challenges/_word-search/create-puzzle.sw
index 8b748a48f..5563eb04a 100644
--- a/data/scenarios/Challenges/_word-search/create-puzzle.sw
+++ b/data/scenarios/Challenges/_word-search/create-puzzle.sw
@@ -11,15 +11,15 @@ def intersperse = \n. \f2. \f1. if (n > 0) {
 
 def whichOrdinal = \str.
     if (str == "capital C") {
-        return 0;
+        pure 0;
     } {
         if (str == "capital O") {
-            return 1;
+            pure 1;
         } {
             if (str == "capital W") {
-                return 2;
+                pure 2;
             } {
-                return (-1);
+                pure (-1);
             }
         }
     }
@@ -30,7 +30,7 @@ Returns -1 if not a recognized letter.
 */
 def getAdjacentOrdinal = \d.
     maybeEntity <- scan d;
-    str <- case maybeEntity (\_. return "") (\s. return s);
+    str <- case maybeEntity (\_. pure "") (\s. pure s);
     whichOrdinal str;
     end;
 
@@ -48,12 +48,12 @@ def iterN = \n. \f.
 
 def chooseLetter = \i.
     if (i == 0) {
-        return "capital C";
+        pure "capital C";
     } {
         if (i == 1) {
-            return "capital O";
+            pure "capital O";
         } {
-            return "capital W";
+            pure "capital W";
         }
     };
     end;
@@ -73,16 +73,16 @@ def getExcludedVerticalLetter =
         teleport self currentLoc;
 
         if (doubleNorthOrdinal == 2) {
-            return 0;
+            pure 0;
         } {
             if (doubleNorthOrdinal == 0) {
-                return 2;
+                pure 2;
             } {
-                return (-1);
+                pure (-1);
             }
         }
     } {
-        return (-1);
+        pure (-1);
     }
     end;
 
@@ -106,18 +106,18 @@ def reRoll = \excludedVertical. \expectedFwdOrdinal. \expectedBkwdOrdinal.
 
     if excludeZero {
         if (excludedVertical == 2) {
-            return 1;
+            pure 1;
         } {
             // Zero is the only excluded value,
             // so just offset a choice between 0 and 1 upward by 1, 
             // to make it a choice between 1 and 2.
             val <- random 2;
-            return $ val + 1;
+            pure $ val + 1;
         };
     } {
         if excludeTwo {
             if (excludedVertical == 0) {
-                return 1;
+                pure 1;
             } {
                 // Two is the only excluded value,
                 // so make it a choice between 0 and 1.
@@ -129,7 +129,7 @@ def reRoll = \excludedVertical. \expectedFwdOrdinal. \expectedBkwdOrdinal.
                 // so just offset a choice between 0 and 1 upward by 1, 
                 // to make it a choice between 1 and 2.
                 val <- random 2;
-                return $ val + 1;
+                pure $ val + 1;
             } {
                 if (excludedVertical == 2) {
                     // Two is the only excluded value,
@@ -150,7 +150,7 @@ def singleTile = \expectedFwdOrdinal. \expectedBkwdOrdinal.
     letterIndex <- reRoll excludedVertical expectedFwdOrdinal expectedBkwdOrdinal;
     chosenLetter <- chooseLetter letterIndex;
     place chosenLetter;
-    return letterIndex;
+    pure letterIndex;
     end;
 
 def crossBack = \_n.
@@ -169,15 +169,15 @@ def layTilesRow = \expectedFwdOrdinal. \expectedBkwdOrdinal. \n.
         move;
 
         newFwdOrdinal <- if (placedIndex == expectedFwdOrdinal || placedIndex == 0) {
-            return $ placedIndex + 1;
+            pure $ placedIndex + 1;
         } {
-            return 0;
+            pure 0;
         };
 
         newBkwdOrdinal <- if (placedIndex == expectedBkwdOrdinal || placedIndex == 2) {
-            return $ placedIndex - 1;
+            pure $ placedIndex - 1;
         } {
-            return 2;
+            pure 2;
         };
 
         layTilesRow newFwdOrdinal newBkwdOrdinal $ n - 1;
diff --git a/data/scenarios/Challenges/_word-search/solution.sw b/data/scenarios/Challenges/_word-search/solution.sw
index 6cb986159..6c636bd5b 100644
--- a/data/scenarios/Challenges/_word-search/solution.sw
+++ b/data/scenarios/Challenges/_word-search/solution.sw
@@ -20,17 +20,17 @@ def waitUntilUnblocked =
 def whichOrdinal =
     isC <- ishere "capital C";
     if (isC) {
-        return 0;
+        pure 0;
     } {
         isO <- ishere "capital O";
         if (isO) {
-            return 1;
+            pure 1;
         } {
             isW <- ishere "capital W";
             if (isW) {
-                return 2;
+                pure 2;
             } {
-                return (-1);
+                pure (-1);
             }
         }
     }
@@ -57,23 +57,23 @@ def traverseRow = \expectedOrdinal. \colCount.
     // considered a "match".
     let shouldAdvance = theFoundOrdinal == expectedOrdinal || theFoundOrdinal == 0 in
     newExpectedOrdinal <- if shouldAdvance {
-        return $ theFoundOrdinal + 1;
+        pure $ theFoundOrdinal + 1;
     } {
         // Reset the progress
-        return 0;
+        pure 0;
     };
 
     if (newExpectedOrdinal == 3) {
         turn back;
 
         intersperse 3 move highlightLetter;
-        return true;
+        pure true;
     } {
         if (colCount > 1) {
             move;
             traverseRow newExpectedOrdinal (colCount - 1);
         } {
-          return false;
+          pure false;
         };
     };
     end;
@@ -92,18 +92,18 @@ in either direction.
 def traverseCols = \width. \height. 
     didWin <- traverseRow 0 width;
     if didWin {
-        return true;
+        pure true;
     } {
         turn back;
         didWinBackward <- traverseRow 0 width;
         if didWinBackward {
-            return true;
+            pure true;
         } {
             if (height > 1) {
                 advanceRow;
                 traverseCols width $ height - 1;
             } {
-                return false;
+                pure false;
             };
         }
     } 
@@ -115,7 +115,7 @@ def solve = \boardWidth. \boardHeight.
 
     wonHorizontally <- traverseCols boardWidth boardHeight;
     if wonHorizontally {
-        return true;
+        pure true;
     } {
         // If we did not find a horizontal solution,
         // look for vertical solutions.
diff --git a/data/scenarios/Challenges/arbitrage.yaml b/data/scenarios/Challenges/arbitrage.yaml
index 1243e9429..0af5ae2fe 100644
--- a/data/scenarios/Challenges/arbitrage.yaml
+++ b/data/scenarios/Challenges/arbitrage.yaml
@@ -20,7 +20,7 @@ objectives:
     condition: |
       as base {
         pcount <- count "paperclip";
-        return $ pcount >= 100;
+        pure $ pcount >= 100;
       };
   - teaser: Beginner capital
     goal:
@@ -32,7 +32,7 @@ objectives:
     condition: |
       as base {
         pcount <- count "paperclip";
-        return $ pcount >= 20;
+        pure $ pcount >= 20;
       };
 attrs:
   - name: shopA
diff --git a/data/scenarios/Challenges/blender.yaml b/data/scenarios/Challenges/blender.yaml
index fb1cbabc0..c8e7f4254 100644
--- a/data/scenarios/Challenges/blender.yaml
+++ b/data/scenarios/Challenges/blender.yaml
@@ -28,12 +28,12 @@ objectives:
           let basename = "base" in
           x <- as r {whoami};
           if (x == basename) {
-              return false;
+              pure false;
           } {
               mr0 <- as r {meet};
               case mr0
-                (\_. return false)
-                (\bot. name <- as bot {whoami}; return $ name == basename);
+                (\_. pure false)
+                (\bot. name <- as bot {whoami}; pure $ name == basename);
           };
           end;
 
@@ -52,11 +52,11 @@ objectives:
               let foo = intermediate in
               let newIdx = idx + 1 in
               recursiveResult <- anyHasMetBase newIdx;
-              return $ foo || recursiveResult;
+              pure $ foo || recursiveResult;
           } {
             // Terminates the recursion on the
             // lowest index at which a robot does not exist
-            return false;
+            pure false;
           };
           end;
 
diff --git a/data/scenarios/Challenges/bridge-building.yaml b/data/scenarios/Challenges/bridge-building.yaml
index 8308e89df..f9c586192 100644
--- a/data/scenarios/Challenges/bridge-building.yaml
+++ b/data/scenarios/Challenges/bridge-building.yaml
@@ -72,7 +72,7 @@ objectives:
       as base {
         c <- whereami;
         let x = fst c in
-        return $ x >= 1 && x <= 2 && snd c == -30;
+        pure $ x >= 1 && x <= 2 && snd c == -30;
       };
   - id: get_pebble
     teaser: Off-piste
diff --git a/data/scenarios/Challenges/chess_horse.yaml b/data/scenarios/Challenges/chess_horse.yaml
index e77ae2d68..d2e4e794b 100644
--- a/data/scenarios/Challenges/chess_horse.yaml
+++ b/data/scenarios/Challenges/chess_horse.yaml
@@ -12,8 +12,8 @@ objectives:
         bloc <- as base {whereami};
         king <- robotNamed "king";
         kloc <- as king {whereami};
-        return (bloc == kloc)
-      } { return false }
+        pure (bloc == kloc)
+      } { pure false }
 solution: |
   move; move; move; turn right; move; turn left; move;
 robots:
diff --git a/data/scenarios/Challenges/dimsum.yaml b/data/scenarios/Challenges/dimsum.yaml
index 00d1cdae4..d2d3edfb0 100644
--- a/data/scenarios/Challenges/dimsum.yaml
+++ b/data/scenarios/Challenges/dimsum.yaml
@@ -12,7 +12,7 @@ objectives:
       p <- robotnamed "patron";
       as p {
         foodCount <- count "food";
-        return $ foodCount >= 2;
+        pure $ foodCount >= 2;
       }
 solution: |
   run "scenarios/Challenges/_dimsum/solution.sw"
diff --git a/data/scenarios/Challenges/dna.yaml b/data/scenarios/Challenges/dna.yaml
index d09956a06..ff57ffabd 100644
--- a/data/scenarios/Challenges/dna.yaml
+++ b/data/scenarios/Challenges/dna.yaml
@@ -87,8 +87,8 @@ objectives:
     condition: |
       def hasAll : (rec l. Unit + Text * l) -> Cmd Bool = \l.
         case l
-          (\_. return true)
-          (\c. b1 <- has (fst c); if b1 {hasAll (snd c)} {return false})
+          (\_. pure true)
+          (\c. b1 <- has (fst c); if b1 {hasAll (snd c)} {pure false})
       end;
 
       k <- robotnamed "keeper";
@@ -107,15 +107,15 @@ objectives:
     condition: |
       def hasCount : (rec l. Unit + Text * l) -> Cmd Int = \l.
         case l
-          (\_. return 0)
-          (\c. b <- has (fst c); n <- hasCount (snd c); return (if b {1} {0} + n))
+          (\_. pure 0)
+          (\c. b <- has (fst c); n <- hasCount (snd c); pure (if b {1} {0} + n))
       end;
 
       k <- robotnamed "keeper";
       as k {
         let organisms = tagmembers "organism" in
         c <- hasCount organisms;
-        return $ c >= 4;
+        pure $ c >= 4;
       }
 robots:
   - name: base
diff --git a/data/scenarios/Challenges/flower-count.yaml b/data/scenarios/Challenges/flower-count.yaml
index 2dbda868f..0e6dfd503 100644
--- a/data/scenarios/Challenges/flower-count.yaml
+++ b/data/scenarios/Challenges/flower-count.yaml
@@ -40,7 +40,7 @@ objectives:
       f <- as base { has "flower" };
       judge <- robotNamed "judge";
       g <- as judge { has "gold" };
-      return (not g && f)
+      pure (not g && f)
   - id: out_of_time
     hidden: true
     optional: true
@@ -53,7 +53,7 @@ objectives:
         Perhaps you would like to go back in time and try again.
     condition: |
       truelove <- robotNamed "truelove";
-      as truelove { b <- ishere "fuse"; return (not b) }
+      as truelove { b <- ishere "fuse"; pure (not b) }
   - id: joinpoint
     hidden: true
     teaser: Follow instructions!
@@ -64,7 +64,7 @@ objectives:
           - not: pick_flower
           - not: out_of_time
     condition: |
-      return true
+      pure true
   - id: win
     teaser: Give a flower
     prerequisite: joinpoint
diff --git a/data/scenarios/Challenges/friend.yaml b/data/scenarios/Challenges/friend.yaml
index feb3a8d51..63a45acff 100644
--- a/data/scenarios/Challenges/friend.yaml
+++ b/data/scenarios/Challenges/friend.yaml
@@ -63,7 +63,7 @@ objectives:
       c <- robotNamed "cat";
       catLoc <- as c {whereami};
       baseLoc <- as base {whereami};
-      return (catLoc == baseLoc)
+      pure (catLoc == baseLoc)
 solution: |
   run "scenarios/Challenges/_friend/friend-solution.sw"
 entities:
diff --git a/data/scenarios/Challenges/gallery.yaml b/data/scenarios/Challenges/gallery.yaml
index bba64700c..a8558c34b 100644
--- a/data/scenarios/Challenges/gallery.yaml
+++ b/data/scenarios/Challenges/gallery.yaml
@@ -51,13 +51,13 @@ objectives:
     condition: |
       def ensureMonotonicDecreasing = \itemCount. \prevOrdinal.
         maybeEntity <- scan down;
-        case maybeEntity (\_. return $ itemCount == 0) (\item.
+        case maybeEntity (\_. pure $ itemCount == 0) (\item.
             myCount <- count item;
             if (myCount <= prevOrdinal) {
               move;
               ensureMonotonicDecreasing (itemCount - 1) myCount;
             } {
-              return false;
+              pure false;
             };
         );
         end;
@@ -75,7 +75,7 @@ objectives:
           let bustCount = length busts in
           ensureMonotonicDecreasing bustCount $ bustCount + 1;
         } {
-          return false;
+          pure false;
         }
       };
 robots:
diff --git a/data/scenarios/Challenges/gopher.yaml b/data/scenarios/Challenges/gopher.yaml
index 055036003..eae5a67e6 100644
--- a/data/scenarios/Challenges/gopher.yaml
+++ b/data/scenarios/Challenges/gopher.yaml
@@ -20,9 +20,9 @@ objectives:
     condition: |
       try {
         robotnamed "gopher";
-        return false;
+        pure false;
       } {
-        return true;
+        pure true;
       }
   - teaser: Recover equipment
     prerequisite: defeat_gopher
@@ -34,7 +34,7 @@ objectives:
     condition: |
       as base {
         sCount <- count "scanner";
-        return $ sCount >= 120;
+        pure $ sCount >= 120;
       }
 robots:
   - name: base
diff --git a/data/scenarios/Challenges/hackman.yaml b/data/scenarios/Challenges/hackman.yaml
index 064548825..1f9929839 100644
--- a/data/scenarios/Challenges/hackman.yaml
+++ b/data/scenarios/Challenges/hackman.yaml
@@ -29,7 +29,7 @@ objectives:
       as base {
         pcount <- count "pellet";
         donut_count <- count "donut";
-        return $ pcount >= 181;
+        pure $ pcount >= 181;
       };
   - id: donut_given
     teaser: Give to ghost
@@ -47,12 +47,12 @@ objectives:
           };
 
           if hasPowerup {
-              return true;
+              pure true;
           } {
               if (n > 1) {
                   anyHasPowerup $ n - 1;
               } {
-                  return false;
+                  pure false;
               }
           }
           end;
@@ -68,7 +68,7 @@ objectives:
       loc <- as base {
         whereami;
       };
-      return $ loc == (0, 0);
+      pure $ loc == (0, 0);
   - teaser: World wrap
     hidden: true
     optional: true
@@ -109,10 +109,10 @@ objectives:
               if (n > 1) {
                   allHasPowerup $ n - 1;
               } {
-                  return true;
+                  pure true;
               }
           } {
-              return false;
+              pure false;
           }
           end;
 
diff --git a/data/scenarios/Challenges/hanoi.yaml b/data/scenarios/Challenges/hanoi.yaml
index 300302f19..cb68b2945 100644
--- a/data/scenarios/Challenges/hanoi.yaml
+++ b/data/scenarios/Challenges/hanoi.yaml
@@ -13,7 +13,7 @@ objectives:
       y <- ishere "blocked two";
       teleport self (2,-3);
       z <- ishere "blocked three";
-      return (x && y && z)
+      pure (x && y && z)
 solution: |
   run "scenarios/Challenges/_hanoi/hanoi-solution.sw"
 robots:
diff --git a/data/scenarios/Challenges/lights-out.yaml b/data/scenarios/Challenges/lights-out.yaml
index b4148f91c..0857b7d6c 100644
--- a/data/scenarios/Challenges/lights-out.yaml
+++ b/data/scenarios/Challenges/lights-out.yaml
@@ -27,13 +27,13 @@ objectives:
           if (n > 0) {
               onHere <- ishere "on";
               if onHere {
-                  return false;
+                  pure false;
               } {
                   move;
                   isRowDark $ n - 1;
               }
           } {
-              return true;
+              pure true;
           }
           end;
 
@@ -44,10 +44,10 @@ objectives:
                   advanceRow;
                   areAllOff rowWidth $ n - 1;
               } {
-                  return false;
+                  pure false;
               }
           } {
-              return true;
+              pure true;
           }
           end;
 
@@ -57,7 +57,7 @@ objectives:
               teleport self (0, 0);
               turn east;
               areAllOff 5 5;
-          } {return false};
+          } {pure false};
           end;
 
       j <- robotnamed "judge";
diff --git a/data/scenarios/Challenges/pack-tetrominoes.yaml b/data/scenarios/Challenges/pack-tetrominoes.yaml
index f159aaab7..973205f0b 100644
--- a/data/scenarios/Challenges/pack-tetrominoes.yaml
+++ b/data/scenarios/Challenges/pack-tetrominoes.yaml
@@ -22,7 +22,7 @@ objectives:
     condition: |
       def found = \s.
         fs <- structures s;
-        return $ case fs (\_. false) (\_. true);
+        pure $ case fs (\_. false) (\_. true);
         end;
 
       foundT <- found "tee";
@@ -30,7 +30,7 @@ objectives:
       foundI <- found "line";
       foundZ <- found "zee";
       foundQ <- found "square";
-      return $ foundT && foundJ && foundI && foundZ && foundQ;
+      pure $ foundT && foundJ && foundI && foundZ && foundQ;
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Challenges/telephone.yaml b/data/scenarios/Challenges/telephone.yaml
index 5b3a60412..9b59537c7 100644
--- a/data/scenarios/Challenges/telephone.yaml
+++ b/data/scenarios/Challenges/telephone.yaml
@@ -28,7 +28,7 @@ objectives:
     condition: |
       as base {
         pcount <- count "paper";
-        return $ pcount >= 8;
+        pure $ pcount >= 8;
       };
   - teaser: Duplicate the design
     prerequisite: 'paper'
diff --git a/data/scenarios/Challenges/teleport.yaml b/data/scenarios/Challenges/teleport.yaml
index 3e0b049e4..f42e4eaf5 100644
--- a/data/scenarios/Challenges/teleport.yaml
+++ b/data/scenarios/Challenges/teleport.yaml
@@ -11,7 +11,7 @@ objectives:
     condition: |
       try {
         as base {has "lambda"}
-      } { return false }
+      } { pure false }
 solution: |
   def w2 = turn back; turn back end;
   def w10 = w2; w2; w2; w2; w2 end;
diff --git a/data/scenarios/Challenges/wave.yaml b/data/scenarios/Challenges/wave.yaml
index 70693a0b9..16fc7224b 100644
--- a/data/scenarios/Challenges/wave.yaml
+++ b/data/scenarios/Challenges/wave.yaml
@@ -21,7 +21,7 @@ objectives:
       - |
         The robots caught you!
     condition: |
-      as base {x <- meet; return $ case x (\_. false) (\_. true)};
+      as base {x <- meet; pure $ case x (\_. false) (\_. true)};
 robots:
   - name: base
     dir: north
diff --git a/data/scenarios/Challenges/word-search.yaml b/data/scenarios/Challenges/word-search.yaml
index aee3ea6ed..46c3af498 100644
--- a/data/scenarios/Challenges/word-search.yaml
+++ b/data/scenarios/Challenges/word-search.yaml
@@ -15,7 +15,7 @@ objectives:
         Diagonal appearances are not valid.
     condition: |
       foundStructures <- structures "cow";
-      return $ case foundStructures (\_. false) (\_. true);
+      pure $ case foundStructures (\_. false) (\_. true);
 robots:
   - name: base
     display:
diff --git a/data/scenarios/Fun/GoL.yaml b/data/scenarios/Fun/GoL.yaml
index a71047118..a353eb586 100644
--- a/data/scenarios/Fun/GoL.yaml
+++ b/data/scenarios/Fun/GoL.yaml
@@ -33,7 +33,7 @@ robots:
         h <- scan down;
         f <- scan forward;
         b <- scan back;
-        return (cnt h + cnt f + cnt b)
+        pure (cnt h + cnt f + cnt b)
       end;
       def mod : Int -> Int -> Int = \a. \b. a - (a/b)*b end;
       def waitUntil = \p.
@@ -42,20 +42,20 @@ robots:
       end;
       forever (
         h <- scan down;
-        alive <- return (h != inl ());
+        alive <- pure (h != inl ());
         n1 <- count3;
         turn left; move; turn right; n2 <- count3;
         turn right; move; move; turn left; n3 <- count3;
         turn left; move; turn right;
-        total <- return (n1 + n2 + n3 - if alive {1} {0});
+        total <- pure (n1 + n2 + n3 - if alive {1} {0});
         if (alive && (total < 2 || total > 3))
-          { grab; return () }
+          { grab; pure () }
           { if (not alive && total == 3)
             { place "rock" }
             {}
           };
         // synchronize
-        waitUntil (t <- time; return (mod t 0x20 == 0))
+        waitUntil (t <- time; pure (mod t 0x20 == 0))
       )
 world:
   dsl: |
diff --git a/data/scenarios/Fun/_logo-burst/drone.sw b/data/scenarios/Fun/_logo-burst/drone.sw
index 1bdf2c261..399deacf6 100644
--- a/data/scenarios/Fun/_logo-burst/drone.sw
+++ b/data/scenarios/Fun/_logo-burst/drone.sw
@@ -8,7 +8,7 @@ def else = \t. t end
 
 def randdir : Cmd Dir =
   d <- random 4;
-  return (
+  pure (
     if (d == 0) {north}
     $ elif (d == 1) {east}
     $ elif (d == 2) {south}
diff --git a/data/scenarios/Fun/_snake/snake.sw b/data/scenarios/Fun/_snake/snake.sw
index 839b4d741..5f86111ad 100644
--- a/data/scenarios/Fun/_snake/snake.sw
+++ b/data/scenarios/Fun/_snake/snake.sw
@@ -86,7 +86,7 @@ def doAtLoc = \currLoc. \targetLoc. \func.
   teleport self targetLoc;
   x <- func;
   teleport self currLoc;
-  return x;
+  pure x;
   end;
 
 def moveTail = \tailList.
@@ -95,7 +95,7 @@ def moveTail = \tailList.
     let maybeShifted = pop tailList in
     case maybeShifted (\_.
       // Nothing to pick up or replace
-      return tailList;
+      pure tailList;
 
     ) (\newPair.
 
@@ -105,10 +105,10 @@ def moveTail = \tailList.
         grabbedItem <- doAtLoc newLoc farthestTail grab;
         place grabbedItem;
 
-        return $ snoc newLoc newInit;
+        pure $ snoc newLoc newInit;
     );
   } {
-    return tailList;
+    pure tailList;
   }
   end;
 
@@ -119,11 +119,11 @@ def moveOneStep = \tailList.
     targetLoc <- as r {whereami};
 
     maybeD <- getDir targetLoc;
-    case maybeD (\_. say "Dead!"; return "") (\d.
+    case maybeD (\_. say "Dead!"; pure "") (\d.
       turn $ fst d;
       newList <- moveTail tailList;
       move;
-      return newList
+      pure newList
     );
     end
 
@@ -139,10 +139,10 @@ def moveToApple = \tailList.
     modifiedTailList <- try {
       make "tail";
       swap "tail";
-      return $ snoc myLoc tailList;
+      pure $ snoc myLoc tailList;
     } {
       grab;
-      return tailList;
+      pure tailList;
     };
     // Need to move here so that we get out of the way
     // if the tail gets elongated
diff --git a/data/scenarios/Fun/horton.yaml b/data/scenarios/Fun/horton.yaml
index 027ae5a65..d49a557da 100644
--- a/data/scenarios/Fun/horton.yaml
+++ b/data/scenarios/Fun/horton.yaml
@@ -12,7 +12,7 @@ objectives:
     condition: |
       as base {
         flowerCount <- count "flower";
-        return $ flowerCount >= 399;
+        pure $ flowerCount >= 399;
       }
 robots:
   - name: Horton
@@ -41,7 +41,7 @@ solution: |
 
   def followRoute =
       nextDir <- path (inL ()) (inR "flower");
-      case nextDir return $ goDir followRoute;
+      case nextDir pure $ goDir followRoute;
       end;
 
   followRoute;
diff --git a/data/scenarios/Fun/snake.yaml b/data/scenarios/Fun/snake.yaml
index bf06e40be..546955b1c 100644
--- a/data/scenarios/Fun/snake.yaml
+++ b/data/scenarios/Fun/snake.yaml
@@ -13,7 +13,7 @@ objectives:
       r <- robotnamed "snake";
       as r {
         appleCoreCount <- count "apple core";
-        return $ appleCoreCount >= 40;
+        pure $ appleCoreCount >= 40;
       }
 robots:
   - name: base
diff --git a/data/scenarios/Speedruns/forester.yaml b/data/scenarios/Speedruns/forester.yaml
index b1d9f6b43..e88351ca2 100644
--- a/data/scenarios/Speedruns/forester.yaml
+++ b/data/scenarios/Speedruns/forester.yaml
@@ -5,7 +5,7 @@ description: Race to harvest 1024 `tree`{=entity}s as quickly as possible. See t
 objectives:
   - goal:
       - Harvest 1024 `tree`{=entity}s as quickly as possible!
-    condition: as base {n <- count "tree"; return (n >= 1024)}
+    condition: as base {n <- count "tree"; pure (n >= 1024)}
 robots:
   - name: base
     loc: [0, 0]
diff --git a/data/scenarios/Testing/1024-sand.yaml b/data/scenarios/Testing/1024-sand.yaml
index fa0e5dd9c..271770d08 100644
--- a/data/scenarios/Testing/1024-sand.yaml
+++ b/data/scenarios/Testing/1024-sand.yaml
@@ -6,7 +6,7 @@ description: |
   https://github.com/swarm-game/swarm/pull/1024
 objectives:
   - condition: |
-      as base {n <- count "sand"; return (n >= 32)}
+      as base {n <- count "sand"; pure (n >= 32)}
     goal:
       - Obtain 32 sand.
 solution: |
diff --git a/data/scenarios/Testing/1034-custom-attributes.yaml b/data/scenarios/Testing/1034-custom-attributes.yaml
index 7962d10af..e39da70bd 100644
--- a/data/scenarios/Testing/1034-custom-attributes.yaml
+++ b/data/scenarios/Testing/1034-custom-attributes.yaml
@@ -61,7 +61,7 @@ objectives:
       - Do nothing
     condition: |
       pos <- as base {whereami};
-      return $ fst pos > 7;
+      pure $ fst pos > 7;
 entities:
   - name: color1
     display:
diff --git a/data/scenarios/Testing/1140-detect-command.yaml b/data/scenarios/Testing/1140-detect-command.yaml
index 6dade590c..543d74fbc 100644
--- a/data/scenarios/Testing/1140-detect-command.yaml
+++ b/data/scenarios/Testing/1140-detect-command.yaml
@@ -10,7 +10,7 @@ objectives:
 solution: |
   def doN = \n. \f. if (n > 0) {f; doN (n - 1) f} {}; end;
   loc <- detect "tree" ((0, 0), (6, 4));
-  case loc return (\delta.
+  case loc pure (\delta.
     let xDelta = fst delta in
     let yDelta = snd delta in
     if (yDelta < 0) {turn south;} {};
@@ -18,7 +18,7 @@ solution: |
     if (xDelta < 0) {turn west;} {turn east};
     doN xDelta move; // x-movement
     grab;
-    return ();
+    pure ();
   );
 robots:
   - name: base
diff --git a/data/scenarios/Testing/1157-drill-return-value.yaml b/data/scenarios/Testing/1157-drill-return-value.yaml
index 773812e23..f58db5936 100644
--- a/data/scenarios/Testing/1157-drill-return-value.yaml
+++ b/data/scenarios/Testing/1157-drill-return-value.yaml
@@ -8,7 +8,7 @@ objectives:
       as base {ishere "gumball"};
 solution: |
   out <- drill forward;
-  case out return place;
+  case out pure place;
 robots:
   - name: base
     dir: north
diff --git a/data/scenarios/Testing/1171-resonate-command.yaml b/data/scenarios/Testing/1171-resonate-command.yaml
index 692613b8c..322670c62 100644
--- a/data/scenarios/Testing/1171-resonate-command.yaml
+++ b/data/scenarios/Testing/1171-resonate-command.yaml
@@ -9,7 +9,7 @@ objectives:
       j <- robotnamed "judge";
       as j {
         c <- resonate "tree" ((0, 0), (8, -2));
-        return $ c >= 7;
+        pure $ c >= 7;
       };
 solution: |
   def doN = \n. \f. if (n > 0) {f; doN (n - 1) f} {}; end;
diff --git a/data/scenarios/Testing/1171-sniff-command.yaml b/data/scenarios/Testing/1171-sniff-command.yaml
index 3807ccf84..66213d5a9 100644
--- a/data/scenarios/Testing/1171-sniff-command.yaml
+++ b/data/scenarios/Testing/1171-sniff-command.yaml
@@ -16,9 +16,9 @@ solution: |
         turn back;
         move;
         turn left;
-        return oldDistance;
+        pure oldDistance;
       } {
-        return newDistance;
+        pure newDistance;
       };
       homeIn item d;
     } {
diff --git a/data/scenarios/Testing/1218-stride-command.yaml b/data/scenarios/Testing/1218-stride-command.yaml
index f741fe5b3..1cd9b7bd7 100644
--- a/data/scenarios/Testing/1218-stride-command.yaml
+++ b/data/scenarios/Testing/1218-stride-command.yaml
@@ -26,7 +26,7 @@ solution: |
   try {
     stride 5;
     grab;
-    return ();
+    pure ();
   } {
     log "Could not grab northern flower";
   };
@@ -42,7 +42,7 @@ solution: |
   try {
     stride 5;
     grab;
-    return ();
+    pure ();
   } {
     log "Could not grab southern flower";
   };
diff --git a/data/scenarios/Testing/1256-halt-command.yaml b/data/scenarios/Testing/1256-halt-command.yaml
index d6b6c6a4e..f9120916e 100644
--- a/data/scenarios/Testing/1256-halt-command.yaml
+++ b/data/scenarios/Testing/1256-halt-command.yaml
@@ -9,7 +9,7 @@ objectives:
       as base {has "flower"}
 solution: |
   mr <- meet;
-  case mr (\_. return ()) (\r. halt r; turn west; move; salvage )
+  case mr (\_. pure ()) (\r. halt r; turn west; move; salvage )
 robots:
   - name: base
     dir: south
diff --git a/data/scenarios/Testing/1271-wall-boundaries.yaml b/data/scenarios/Testing/1271-wall-boundaries.yaml
index 67f657500..dc355c52b 100644
--- a/data/scenarios/Testing/1271-wall-boundaries.yaml
+++ b/data/scenarios/Testing/1271-wall-boundaries.yaml
@@ -17,7 +17,7 @@ objectives:
     condition: |
       as base {
         hasFence <- has "fence";
-        return $ not hasFence;
+        pure $ not hasFence;
       }
 solution: |
   def doN = \n. \f. if (n > 0) {f; doN (n - 1) f} {}; end;
diff --git a/data/scenarios/Testing/1295-density-command.yaml b/data/scenarios/Testing/1295-density-command.yaml
index aaba569e6..703cfa91f 100644
--- a/data/scenarios/Testing/1295-density-command.yaml
+++ b/data/scenarios/Testing/1295-density-command.yaml
@@ -9,7 +9,7 @@ objectives:
       j <- robotnamed "judge";
       as j {
         c <- density ((0, 0), (3, 3));
-        return $ c <= 13;
+        pure $ c <= 13;
       };
 solution: |
   def doN = \n. \f. if (n > 0) {f; doN (n - 1) f} {}; end;
diff --git a/data/scenarios/Testing/1320-world-DSL/constant.yaml b/data/scenarios/Testing/1320-world-DSL/constant.yaml
index bfb7d2179..757372bf3 100644
--- a/data/scenarios/Testing/1320-world-DSL/constant.yaml
+++ b/data/scenarios/Testing/1320-world-DSL/constant.yaml
@@ -5,7 +5,7 @@ description: |
   single cell value.
 objectives:
   - condition: |
-      as base { n <- count "tree"; return (n >= 4) }
+      as base { n <- count "tree"; pure (n >= 4) }
     goal:
       - Get 4 trees
 solution: |
diff --git a/data/scenarios/Testing/1320-world-DSL/erase.yaml b/data/scenarios/Testing/1320-world-DSL/erase.yaml
index 014b36246..368646915 100644
--- a/data/scenarios/Testing/1320-world-DSL/erase.yaml
+++ b/data/scenarios/Testing/1320-world-DSL/erase.yaml
@@ -4,7 +4,7 @@ description: |
   Test that we can erase entities when overlaying
 objectives:
   - condition: |
-      as base { n <- count "tree"; return (n == 0) }
+      as base { n <- count "tree"; pure (n == 0) }
     goal:
       - Get rid of your trees.
 solution: |
diff --git a/data/scenarios/Testing/1320-world-DSL/override.yaml b/data/scenarios/Testing/1320-world-DSL/override.yaml
index 6f87e7f30..ee9cf2e08 100644
--- a/data/scenarios/Testing/1320-world-DSL/override.yaml
+++ b/data/scenarios/Testing/1320-world-DSL/override.yaml
@@ -4,7 +4,7 @@ description: |
   Test that later entities override earlier ones when overlaying
 objectives:
   - condition: |
-      as base { n <- count "tree"; return (n == 1) }
+      as base { n <- count "tree"; pure (n == 1) }
     goal:
       - Get a tree.
 solution: |
diff --git a/data/scenarios/Testing/1320-world-DSL/reflect.yaml b/data/scenarios/Testing/1320-world-DSL/reflect.yaml
index 397e61259..f2d7ea48e 100644
--- a/data/scenarios/Testing/1320-world-DSL/reflect.yaml
+++ b/data/scenarios/Testing/1320-world-DSL/reflect.yaml
@@ -8,7 +8,7 @@ objectives:
   - goal:
       - Pick up four trees
     condition: |
-      as base {n <- count "tree"; return (n >= 4)}
+      as base {n <- count "tree"; pure (n >= 4)}
 robots:
   - name: base
     loc: [0, 0]
@@ -27,7 +27,7 @@ solution: |
   def x = \n. \c. if (n==0) {} {c; x (n-1) c} end
   def ifC = \p. \t. \e. b <- p; if b t e end
   def findTree = ifC (ishere "tree") {whereami} {move; findTree} end
-  def ell = \d. turn right; x (2*d) move; grab; return () end
+  def ell = \d. turn right; x (2*d) move; grab; pure () end
   def grabTrees = \loc. let x = fst loc in let y = snd loc in grab; ell y; ell x; ell y end
   n <- random 10;
   x (n+1) move; turn right; move;
diff --git a/data/scenarios/Testing/1322-wait-with-instant.yaml b/data/scenarios/Testing/1322-wait-with-instant.yaml
index 17309e73c..22077360e 100644
--- a/data/scenarios/Testing/1322-wait-with-instant.yaml
+++ b/data/scenarios/Testing/1322-wait-with-instant.yaml
@@ -19,7 +19,7 @@ objectives:
 
       let xDiff = fst hareloc - fst tortoiseloc in
 
-      return $ fst hareloc == 0 && xDiff == 3;
+      pure $ fst hareloc == 0 && xDiff == 3;
 solution: |
   noop;
 robots:
diff --git a/data/scenarios/Testing/1341-command-count.yaml b/data/scenarios/Testing/1341-command-count.yaml
index 51a2b886a..7f5960502 100644
--- a/data/scenarios/Testing/1341-command-count.yaml
+++ b/data/scenarios/Testing/1341-command-count.yaml
@@ -22,7 +22,7 @@ solution: |
     move;
     grab;
   } {
-    return "";
+    pure "";
   };
 robots:
   - name: base
diff --git a/data/scenarios/Testing/1356-portals/_automatic-waypoint-patrol/program.sw b/data/scenarios/Testing/1356-portals/_automatic-waypoint-patrol/program.sw
index a28daa23e..16a3ace43 100644
--- a/data/scenarios/Testing/1356-portals/_automatic-waypoint-patrol/program.sw
+++ b/data/scenarios/Testing/1356-portals/_automatic-waypoint-patrol/program.sw
@@ -15,7 +15,7 @@ def negateTuple = \t.
 
 def getRelativeLocation = \absCurrentLoc. \absDestLoc.
     let negatedLoc = negateTuple absCurrentLoc in
-    return $ sumTuples negatedLoc absDestLoc;
+    pure $ sumTuples negatedLoc absDestLoc;
     end;
 
 def moveTuple = \tup.
diff --git a/data/scenarios/Testing/1356-portals/_portals-flip-and-rotate/solution.sw b/data/scenarios/Testing/1356-portals/_portals-flip-and-rotate/solution.sw
index b7566d118..691fa1930 100644
--- a/data/scenarios/Testing/1356-portals/_portals-flip-and-rotate/solution.sw
+++ b/data/scenarios/Testing/1356-portals/_portals-flip-and-rotate/solution.sw
@@ -16,7 +16,7 @@ def negateTuple = \t.
 
 def getRelativeLocation = \absCurrentLoc. \absDestLoc.
     let negatedLoc = negateTuple absCurrentLoc in
-    return $ sumTuples negatedLoc absDestLoc;
+    pure $ sumTuples negatedLoc absDestLoc;
     end;
 
 def moveTuple = \tup.
diff --git a/data/scenarios/Testing/1430-built-robot-ownership.yaml b/data/scenarios/Testing/1430-built-robot-ownership.yaml
index 337bfb952..c4d804c7e 100644
--- a/data/scenarios/Testing/1430-built-robot-ownership.yaml
+++ b/data/scenarios/Testing/1430-built-robot-ownership.yaml
@@ -48,7 +48,7 @@ objectives:
     condition: |
       r2 <- robotNumbered 2;
       r3 <- robotNumbered 3;
-      return true
+      pure true
 known: []
 world:
   palette:
diff --git a/data/scenarios/Testing/144-subworlds/_subworld-located-robots/solution.sw b/data/scenarios/Testing/144-subworlds/_subworld-located-robots/solution.sw
index 8b012b9fb..d0029c905 100644
--- a/data/scenarios/Testing/144-subworlds/_subworld-located-robots/solution.sw
+++ b/data/scenarios/Testing/144-subworlds/_subworld-located-robots/solution.sw
@@ -6,5 +6,5 @@ f <- grab;
 
 doN 5 move;
 r <- meet;
-case r return $ \j. give j f;
+case r pure $ \j. give j f;
 
diff --git a/data/scenarios/Testing/144-subworlds/_subworld-mapped-robots/judges.sw b/data/scenarios/Testing/144-subworlds/_subworld-mapped-robots/judges.sw
index 8ca85ea5f..47de8c346 100644
--- a/data/scenarios/Testing/144-subworlds/_subworld-mapped-robots/judges.sw
+++ b/data/scenarios/Testing/144-subworlds/_subworld-mapped-robots/judges.sw
@@ -2,7 +2,7 @@
 def getRobotNumber = \n.
     r <- robotnumbered n;
     if (r == self) {
-        return n;
+        pure n;
     } {getRobotNumber $ n + 1};
     end;
 
@@ -10,7 +10,7 @@ def amLowestRecursive = \targetName. \idx.
     r <- robotnumbered idx;
     thisName <- as r {whoami};
     if (thisName == targetName) {
-        return $ r == self;
+        pure $ r == self;
     } {amLowestRecursive targetName $ idx + 1};
     end;
 
@@ -49,4 +49,4 @@ def go =
     if amFirst {waitToReceive} {waitToGive};
     end;
 
-go;
\ No newline at end of file
+go;
diff --git a/data/scenarios/Testing/144-subworlds/_subworld-mapped-robots/solution.sw b/data/scenarios/Testing/144-subworlds/_subworld-mapped-robots/solution.sw
index b43d0fd76..aaf9e306b 100644
--- a/data/scenarios/Testing/144-subworlds/_subworld-mapped-robots/solution.sw
+++ b/data/scenarios/Testing/144-subworlds/_subworld-mapped-robots/solution.sw
@@ -4,5 +4,5 @@ def doN = \n. \f. if (n > 0) {f; doN (n - 1) f} {}; end;
 doN 16 move;
 
 r <- meet;
-case r return $ \j. give j "bitcoin";
+case r pure $ \j. give j "bitcoin";
 
diff --git a/data/scenarios/Testing/144-subworlds/teleport-and-query.yaml b/data/scenarios/Testing/144-subworlds/teleport-and-query.yaml
index be02c8e8a..a9e3d25f8 100644
--- a/data/scenarios/Testing/144-subworlds/teleport-and-query.yaml
+++ b/data/scenarios/Testing/144-subworlds/teleport-and-query.yaml
@@ -10,7 +10,7 @@ objectives:
     condition: |
       as base {
         myloc <- locateme;
-        return $ fst myloc == "underground";
+        pure $ fst myloc == "underground";
       }
 solution: |
   noop
diff --git a/data/scenarios/Testing/1533-sow-command.yaml b/data/scenarios/Testing/1533-sow-command.yaml
index ce68b85ef..07ba038ee 100644
--- a/data/scenarios/Testing/1533-sow-command.yaml
+++ b/data/scenarios/Testing/1533-sow-command.yaml
@@ -34,7 +34,7 @@ objectives:
       r <- robotnamed "kudzubot";
       as r {
         kCount <- resonate "kudzu" ((0, 0), (10, 4));
-        return $ kCount >= 45;
+        pure $ kCount >= 45;
       }
 solution: |
   def doN = \n. \f. if (n > 0) {f; doN (n - 1) f} {}; end;
diff --git a/data/scenarios/Testing/1533-sow-seed-maturation.yaml b/data/scenarios/Testing/1533-sow-seed-maturation.yaml
index c5796f361..37078d2c1 100644
--- a/data/scenarios/Testing/1533-sow-seed-maturation.yaml
+++ b/data/scenarios/Testing/1533-sow-seed-maturation.yaml
@@ -22,7 +22,7 @@ objectives:
     condition: |
       as base {
         x <- has "acorn";
-        return $ not x;
+        pure $ not x;
       }
   - goal:
       - |
diff --git a/data/scenarios/Testing/1535-ping/1535-out-of-range.yaml b/data/scenarios/Testing/1535-ping/1535-out-of-range.yaml
index 96352c712..f9094c2f1 100644
--- a/data/scenarios/Testing/1535-ping/1535-out-of-range.yaml
+++ b/data/scenarios/Testing/1535-ping/1535-out-of-range.yaml
@@ -11,7 +11,7 @@ objectives:
       r <- robotnamed "buddy";
       as r {
         response <- ping base;
-        return $ case response (\_. true) (\_. false);
+        pure $ case response (\_. true) (\_. false);
       }
 solution: |
   run "scenarios/Testing/1535-ping/_1535-out-of-range/solution.sw"
diff --git a/data/scenarios/Testing/1535-ping/_1535-in-range/solution.sw b/data/scenarios/Testing/1535-ping/_1535-in-range/solution.sw
index bda152a68..c24d21f2f 100644
--- a/data/scenarios/Testing/1535-ping/_1535-in-range/solution.sw
+++ b/data/scenarios/Testing/1535-ping/_1535-in-range/solution.sw
@@ -7,9 +7,9 @@ def goToBuddy = \loc.
     let longitudinalDist = snd loc in
     absFwd <- if (longitudinalDist < 0) {
         turn back;
-        return $ -longitudinalDist;
+        pure $ -longitudinalDist;
     } {
-        return longitudinalDist;
+        pure longitudinalDist;
     };
     doN absFwd move;
     if (longitudinalDist < 0) {
@@ -19,10 +19,10 @@ def goToBuddy = \loc.
     let lateralDist = fst loc in
     absSide <- if (lateralDist < 0) {
         turn left;
-        return $ -lateralDist;
+        pure $ -lateralDist;
     } {
         turn right;
-        return lateralDist;
+        pure lateralDist;
     };
     doN absSide move;
     end;
@@ -30,7 +30,7 @@ def goToBuddy = \loc.
 def checkNeedToMove = \f. \loc.
     wait 3;
     if (loc == (0, 0)) {
-        return ()
+        pure ()
     } {
         goToBuddy loc;
         f;
@@ -39,7 +39,7 @@ def checkNeedToMove = \f. \loc.
 
 def pingLoop = \buddy.
     maybeLoc <- ping buddy;
-    case maybeLoc return $ checkNeedToMove $ pingLoop buddy;
+    case maybeLoc pure $ checkNeedToMove $ pingLoop buddy;
     end;
 
 def giveToBuddy = \buddy.
@@ -50,7 +50,7 @@ def giveToBuddy = \buddy.
 def go =
     move;
     maybeBuddy <- meet;
-    case maybeBuddy return giveToBuddy;
+    case maybeBuddy pure giveToBuddy;
     grab;
     end;
 
diff --git a/data/scenarios/Testing/1569-pathfinding-cache/1569-harvest-batch.yaml b/data/scenarios/Testing/1569-pathfinding-cache/1569-harvest-batch.yaml
index 86dced56d..b2107703d 100644
--- a/data/scenarios/Testing/1569-pathfinding-cache/1569-harvest-batch.yaml
+++ b/data/scenarios/Testing/1569-pathfinding-cache/1569-harvest-batch.yaml
@@ -12,7 +12,7 @@ objectives:
     condition: |
       as base {
         fCount <- count "flower";
-        return $ fCount >= 4;
+        pure $ fCount >= 4;
       };
 solution: |
   def goDir = \f. \result.
@@ -22,7 +22,7 @@ solution: |
 
   def followRoute =
       nextDir <- path (inL ()) (inR "flower");
-      case nextDir return $ goDir followRoute;
+      case nextDir pure $ goDir followRoute;
       end;
 
   followRoute;
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1575-bounding-box-overlap.yaml b/data/scenarios/Testing/1575-structure-recognizer/1575-bounding-box-overlap.yaml
index 179ab3194..476f785be 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1575-bounding-box-overlap.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1575-bounding-box-overlap.yaml
@@ -18,7 +18,7 @@ objectives:
         case l (\_. 0) (\cons. 1 + length (snd cons))
       end;
       foundStructures <- structures "chevron";
-      return $ length foundStructures >= 3
+      pure $ length foundStructures >= 3
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1575-browse-structures.yaml b/data/scenarios/Testing/1575-structure-recognizer/1575-browse-structures.yaml
index 471dff73d..96cd853df 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1575-browse-structures.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1575-browse-structures.yaml
@@ -15,7 +15,7 @@ objectives:
         Build a `precious`{=structure} structure
     condition: |
       foundStructure <- structures "precious";
-      return $ case foundStructure (\_. false) (\_. true);
+      pure $ case foundStructure (\_. false) (\_. true);
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1575-construction-count.yaml b/data/scenarios/Testing/1575-structure-recognizer/1575-construction-count.yaml
index d7445f08c..f0f142814 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1575-construction-count.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1575-construction-count.yaml
@@ -13,7 +13,7 @@ objectives:
         case l (\_. 0) (\cons. 1 + length (snd cons))
       end;
       foundGreen <- structures "green_jewel";
-      return $ length foundGreen >= 12;
+      pure $ length foundGreen >= 12;
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1575-ensure-disjoint.yaml b/data/scenarios/Testing/1575-structure-recognizer/1575-ensure-disjoint.yaml
index 1b923dea2..e654fdccc 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1575-ensure-disjoint.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1575-ensure-disjoint.yaml
@@ -20,7 +20,7 @@ objectives:
         case l (\_. 0) (\cons. 1 + length (snd cons))
       end;
       foundStructures <- structures "chessboard";
-      return $ length foundStructures >= 2
+      pure $ length foundStructures >= 2
   - id: premature_win
     teaser: Don't count win early
     optional: true
@@ -35,7 +35,7 @@ objectives:
       robotHasSilver <- as base {has "silver"};
 
       foundStructures <- structures "chessboard";
-      return $ length foundStructures >= 2 && robotHasSilver;
+      pure $ length foundStructures >= 2 && robotHasSilver;
 robots:
   - name: base
     dir: south
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1575-ensure-single-recognition.yaml b/data/scenarios/Testing/1575-structure-recognizer/1575-ensure-single-recognition.yaml
index e4d9cdcae..a6f67eb8c 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1575-ensure-single-recognition.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1575-ensure-single-recognition.yaml
@@ -17,7 +17,7 @@ objectives:
         case l (\_. 0) (\cons. 1 + length (snd cons))
       end;
       foundStructures <- structures "chessboard";
-      return $ length foundStructures >= 2;
+      pure $ length foundStructures >= 2;
   - id: premature_win
     teaser: Don't count win early
     optional: true
@@ -32,7 +32,7 @@ objectives:
       robotHasGold <- as base {has "gold"};
 
       foundStructures <- structures "chessboard";
-      return $ length foundStructures >= 2 && robotHasGold;
+      pure $ length foundStructures >= 2 && robotHasGold;
 robots:
   - name: base
     dir: south
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1575-floorplan-command.yaml b/data/scenarios/Testing/1575-structure-recognizer/1575-floorplan-command.yaml
index 0a31fcef5..36388ed75 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1575-floorplan-command.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1575-floorplan-command.yaml
@@ -12,7 +12,7 @@ objectives:
     condition: |
       def isRight = \x. case x (\_. false) (\_. true); end;
       foundBox <- structures "wooden box";
-      return $ isRight foundBox;
+      pure $ isRight foundBox;
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1575-handle-overlapping.yaml b/data/scenarios/Testing/1575-structure-recognizer/1575-handle-overlapping.yaml
index 74510709a..8269e30d9 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1575-handle-overlapping.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1575-handle-overlapping.yaml
@@ -11,7 +11,7 @@ objectives:
         Build a `precious`{=structure} structure
     condition: |
       foundStructure <- structures "precious";
-      return $ case foundStructure (\_. false) (\_. true);
+      pure $ case foundStructure (\_. false) (\_. true);
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1575-interior-entity-placement.yaml b/data/scenarios/Testing/1575-structure-recognizer/1575-interior-entity-placement.yaml
index 964b5f62c..4aa6d0985 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1575-interior-entity-placement.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1575-interior-entity-placement.yaml
@@ -18,7 +18,7 @@ objectives:
         Place the `rock`{=entity} entity back inside the `pigpen`{=structure}.
     condition: |
       foundBoxes <- structures "pigpen";
-      case foundBoxes (\_. return false) (\cons.
+      case foundBoxes (\_. pure false) (\cons.
         let structPos = fst cons in
         j <- robotnamed "judge";
         as j {
@@ -28,7 +28,7 @@ objectives:
           teleport self structPos;
 
           rockCount <- resonate "rock" ((0, 0), structBounds);
-          return $ rockCount > 0;
+          pure $ rockCount > 0;
         }
       );
   - teaser: Grab rock
@@ -50,7 +50,7 @@ objectives:
     condition: |
       def isRight = \x. case x (\_. false) (\_. true); end;
       foundBoxes <- structures "pigpen";
-      return $ isRight foundBoxes;
+      pure $ isRight foundBoxes;
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1575-nested-structure-definition.yaml b/data/scenarios/Testing/1575-structure-recognizer/1575-nested-structure-definition.yaml
index 323e33e3b..7784ddd8c 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1575-nested-structure-definition.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1575-nested-structure-definition.yaml
@@ -14,7 +14,7 @@ objectives:
         The `double ring`{=structure} structure should be recognized again.
     condition: |
       foundStructure <- structures "double ring";
-      return $ case foundStructure (\_. false) (\_. true);
+      pure $ case foundStructure (\_. false) (\_. true);
   - teaser: Pre-recognized
     id: pre_recognized
     prerequisite:
@@ -24,7 +24,7 @@ objectives:
         Pre-placed structure must be recognized
     condition: |
       foundStructure <- structures "double ring";
-      return $ case foundStructure (\_. false) (\_. true);
+      pure $ case foundStructure (\_. false) (\_. true);
   - teaser: Grab tree
     id: grab_tree
     goal:
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1575-overlapping-tiebreaker-by-largest.yaml b/data/scenarios/Testing/1575-structure-recognizer/1575-overlapping-tiebreaker-by-largest.yaml
index 792cbabff..f8f286e32 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1575-overlapping-tiebreaker-by-largest.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1575-overlapping-tiebreaker-by-largest.yaml
@@ -12,7 +12,7 @@ objectives:
         Build a 3x3 structure
     condition: |
       foundStructure <- structures "large";
-      return $ case foundStructure (\_. false) (\_. true);
+      pure $ case foundStructure (\_. false) (\_. true);
   - id: wrong_structure
     teaser: Don't recognize small structure
     optional: true
@@ -21,7 +21,7 @@ objectives:
         The small structure shouldn't be recognized.
     condition: |
       foundStructure <- structures "small";
-      return $ case foundStructure (\_. false) (\_. true);
+      pure $ case foundStructure (\_. false) (\_. true);
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1575-overlapping-tiebreaker-by-location.yaml b/data/scenarios/Testing/1575-structure-recognizer/1575-overlapping-tiebreaker-by-location.yaml
index 2b15adee1..64526276a 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1575-overlapping-tiebreaker-by-location.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1575-overlapping-tiebreaker-by-location.yaml
@@ -13,7 +13,7 @@ objectives:
         Build a `topleft`{=structure} structure
     condition: |
       foundStructure <- structures "topleft";
-      return $ case foundStructure (\_. false) (\_. true);
+      pure $ case foundStructure (\_. false) (\_. true);
   - id: wrong_structure
     teaser: Don't recognize small structure
     optional: true
@@ -22,7 +22,7 @@ objectives:
         The `bottomright`{=structure} structure shouldn't be recognized.
     condition: |
       foundStructure <- structures "bottomright";
-      return $ case foundStructure (\_. false) (\_. true);
+      pure $ case foundStructure (\_. false) (\_. true);
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1575-placement-occlusion.yaml b/data/scenarios/Testing/1575-structure-recognizer/1575-placement-occlusion.yaml
index abd3df85a..994271aab 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1575-placement-occlusion.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1575-placement-occlusion.yaml
@@ -16,7 +16,7 @@ objectives:
       def isRight = \x. case x (\_. false) (\_. true); end;
 
       foundGreen <- structures "green_jewel";
-      return $ isRight foundGreen;
+      pure $ isRight foundGreen;
   - id: complete_red_structure
     optional: true
     teaser: Complete red structure
@@ -27,7 +27,7 @@ objectives:
       def isRight = \x. case x (\_. false) (\_. true); end;
 
       foundRed <- structures "red_jewel";
-      return $ isRight foundRed;
+      pure $ isRight foundRed;
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1575-remove-structure.yaml b/data/scenarios/Testing/1575-structure-recognizer/1575-remove-structure.yaml
index e43a9ae67..2d47f1de2 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1575-remove-structure.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1575-remove-structure.yaml
@@ -12,7 +12,7 @@ objectives:
         Remove a piece of the structure to destroy it
     condition: |
       foundStructures <- structures "chessboard";
-      return $ case foundStructures (\_. true) (\_. false);
+      pure $ case foundStructures (\_. true) (\_. false);
   - id: complete_structure
     teaser: Complete structure
     goal:
@@ -20,7 +20,7 @@ objectives:
         Build a `chessboard`{=structure} structure
     condition: |
       foundStructures <- structures "chessboard";
-      return $ case foundStructures (\_. false) (\_. true);
+      pure $ case foundStructures (\_. false) (\_. true);
 robots:
   - name: base
     dir: south
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1575-swap-structure.yaml b/data/scenarios/Testing/1575-structure-recognizer/1575-swap-structure.yaml
index 81e10bb4d..a43069860 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1575-swap-structure.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1575-swap-structure.yaml
@@ -16,7 +16,7 @@ objectives:
       foundBlue <- structures "blue_jewel";
       foundGreen <- structures "green_jewel";
       foundRed <- structures "red_jewel";
-      return $ isRight foundBlue && not (isRight foundRed) && not (isRight foundRed);
+      pure $ isRight foundBlue && not (isRight foundRed) && not (isRight foundRed);
   - id: complete_green_structure
     teaser: Complete green structure
     prerequisite: complete_red_structure
@@ -28,7 +28,7 @@ objectives:
 
       foundGreen <- structures "green_jewel";
       foundRed <- structures "red_jewel";
-      return $ isRight foundGreen && not (isRight foundRed);
+      pure $ isRight foundGreen && not (isRight foundRed);
   - id: complete_red_structure
     teaser: Complete red structure
     goal:
@@ -38,7 +38,7 @@ objectives:
       def isRight = \x. case x (\_. false) (\_. true); end;
 
       foundRed <- structures "red_jewel";
-      return $ isRight foundRed;
+      pure $ isRight foundRed;
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1644-rotated-preplacement-recognition.yaml b/data/scenarios/Testing/1575-structure-recognizer/1644-rotated-preplacement-recognition.yaml
index 13e63a1de..189a61478 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1644-rotated-preplacement-recognition.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1644-rotated-preplacement-recognition.yaml
@@ -10,7 +10,7 @@ objectives:
         Have a `tee`{=structure} structure
     condition: |
       foundStructure <- structures "tee";
-      return $ case foundStructure (\_. false) (\_. true);
+      pure $ case foundStructure (\_. false) (\_. true);
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/1644-rotated-recognition.yaml b/data/scenarios/Testing/1575-structure-recognizer/1644-rotated-recognition.yaml
index a37a1233e..b88ead935 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/1644-rotated-recognition.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/1644-rotated-recognition.yaml
@@ -10,7 +10,7 @@ objectives:
         Build a `tee`{=structure} structure
     condition: |
       foundStructure <- structures "tee";
-      return $ case foundStructure (\_. false) (\_. true);
+      pure $ case foundStructure (\_. false) (\_. true);
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/2115-encroaching-upon-exterior-transparent-cells.yaml b/data/scenarios/Testing/1575-structure-recognizer/2115-encroaching-upon-exterior-transparent-cells.yaml
index b0b9399a6..0ca89c5c9 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/2115-encroaching-upon-exterior-transparent-cells.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/2115-encroaching-upon-exterior-transparent-cells.yaml
@@ -20,7 +20,7 @@ objectives:
       def isRight = \x. case x (\_. false) (\_. true); end;
 
       foundBoxes <- structures "chevron";
-      return $ isRight foundBoxes;
+      pure $ isRight foundBoxes;
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/2115-encroaching-upon-interior-transparent-cells.yaml b/data/scenarios/Testing/1575-structure-recognizer/2115-encroaching-upon-interior-transparent-cells.yaml
index db8c365c0..b8dcb6751 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/2115-encroaching-upon-interior-transparent-cells.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/2115-encroaching-upon-interior-transparent-cells.yaml
@@ -19,7 +19,7 @@ objectives:
     condition: |
       def isRight = \x. case x (\_. false) (\_. true); end;
       foundBoxes <- structures "pigpen";
-      return $ isRight foundBoxes;
+      pure $ isRight foundBoxes;
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/2201-initial-recognition-overlap.yaml b/data/scenarios/Testing/1575-structure-recognizer/2201-initial-recognition-overlap.yaml
index 5fd96bdd7..82f6fbb41 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/2201-initial-recognition-overlap.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/2201-initial-recognition-overlap.yaml
@@ -22,7 +22,7 @@ objectives:
         case l (\_. 0) (\c. 1 + length (snd c))
       end;
       foundStructures <- structures "square";
-      return $ length foundStructures == 1;
+      pure $ length foundStructures == 1;
 robots:
   - name: base
     dir: north
diff --git a/data/scenarios/Testing/1575-structure-recognizer/2201-piecewise-lines.yaml b/data/scenarios/Testing/1575-structure-recognizer/2201-piecewise-lines.yaml
index 29ed33a38..b0f9dd485 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/2201-piecewise-lines.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/2201-piecewise-lines.yaml
@@ -16,7 +16,7 @@ objectives:
     condition: |
       def isRight = \x. case x (\_. false) (\_. true); end;
       foundStructures <- structures "spaceship";
-      return $ isRight foundStructures;
+      pure $ isRight foundStructures;
 robots:
   - name: base
     dir: east
diff --git a/data/scenarios/Testing/1575-structure-recognizer/2201-preclude-overlapping-recognition.yaml b/data/scenarios/Testing/1575-structure-recognizer/2201-preclude-overlapping-recognition.yaml
index bc3e5a7e8..951e0efe2 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/2201-preclude-overlapping-recognition.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/2201-preclude-overlapping-recognition.yaml
@@ -11,7 +11,7 @@ objectives:
     condition: |
       def isRight = \x. case x (\_. false) (\_. true); end;
       foundStructures <- structures "line";
-      return $ isRight foundStructures;
+      pure $ isRight foundStructures;
   - teaser: Recognize second structure
     id: found_elbow
     optional: true
@@ -21,7 +21,7 @@ objectives:
     condition: |
       def isRight = \x. case x (\_. false) (\_. true); end;
       foundStructures <- structures "elbow";
-      return $ isRight foundStructures;
+      pure $ isRight foundStructures;
   - teaser: Grab tree
     prerequisite:
       not: found_elbow
diff --git a/data/scenarios/Testing/1575-structure-recognizer/2229-position-uniqueness-multiple-orientations.yaml b/data/scenarios/Testing/1575-structure-recognizer/2229-position-uniqueness-multiple-orientations.yaml
index ae472ceb9..93c2a6fa1 100644
--- a/data/scenarios/Testing/1575-structure-recognizer/2229-position-uniqueness-multiple-orientations.yaml
+++ b/data/scenarios/Testing/1575-structure-recognizer/2229-position-uniqueness-multiple-orientations.yaml
@@ -20,7 +20,7 @@ objectives:
         case l (\_. 0) (\c. 1 + length (snd c))
       end;
       foundStructures <- structures "thingy";
-      return $ length foundStructures == 2;
+      pure $ length foundStructures == 2;
 robots:
   - name: base
     dir: north
diff --git a/data/scenarios/Testing/1631-tags.yaml b/data/scenarios/Testing/1631-tags.yaml
index eb7d05038..d80d1fb4b 100644
--- a/data/scenarios/Testing/1631-tags.yaml
+++ b/data/scenarios/Testing/1631-tags.yaml
@@ -16,8 +16,8 @@ objectives:
     condition: |
       def hasAny : (rec l. Unit + Text * l) -> Cmd Bool = \items.
         case items
-          (\_. return false)
-          (\c. b <- has (fst c); if b {return true} {hasAny (snd c)})
+          (\_. pure false)
+          (\c. b <- has (fst c); if b {pure true} {hasAny (snd c)})
       end;
 
       let prohibited = tagmembers "fruit" in
@@ -28,15 +28,15 @@ objectives:
 solution: |
   def findTarget =
     result <- scan down;
-    isTarget <- case result (\_. return false) (\item.
+    isTarget <- case result (\_. pure false) (\item.
       let isEdible = hastag item "edible" in
       let isFruit = hastag item "fruit" in
-      return $ isEdible && not isFruit;
+      pure $ isEdible && not isFruit;
     );
 
     if isTarget {
       grab;
-      return ();
+      pure ();
     } {
       move;
       findTarget;
diff --git a/data/scenarios/Testing/1642-biomes.yaml b/data/scenarios/Testing/1642-biomes.yaml
index 662029c1c..b7a3fa1da 100644
--- a/data/scenarios/Testing/1642-biomes.yaml
+++ b/data/scenarios/Testing/1642-biomes.yaml
@@ -15,7 +15,7 @@ objectives:
     condition: |
       as base {
         c <- count "wheat";
-        return $ c == 2;
+        pure $ c == 2;
       }
   - id: western_wheat_regrew
     optional: true
@@ -40,7 +40,7 @@ objectives:
       baseLoc <- as base {whereami};
       as goalbot {
         goalLoc <- whereami;
-        return $ baseLoc == goalLoc;
+        pure $ baseLoc == goalLoc;
       }
     prerequisite:
       logic:
diff --git a/data/scenarios/Testing/1721-custom-walkable-entities.yaml b/data/scenarios/Testing/1721-custom-walkable-entities.yaml
index a8d590a6f..2b61bea2f 100644
--- a/data/scenarios/Testing/1721-custom-walkable-entities.yaml
+++ b/data/scenarios/Testing/1721-custom-walkable-entities.yaml
@@ -16,7 +16,7 @@ objectives:
       m <- robotnamed "monkey";
       as m {
         p <- path (inR 10) (inR "banana");
-        return $ case p (\_. false) (\_. true);
+        pure $ case p (\_. false) (\_. true);
       };
   - id: placed_tree
     teaser: Tree placed
@@ -26,7 +26,7 @@ objectives:
       - Tree must be placed
     condition: |
       x <- as base {has "tree"};
-      return $ not x;
+      pure $ not x;
 solution: |
   move;
   move;
diff --git a/data/scenarios/Testing/1721-walkability-whitelist-path-cache.yaml b/data/scenarios/Testing/1721-walkability-whitelist-path-cache.yaml
index 1522a5cf9..7b9b0b693 100644
--- a/data/scenarios/Testing/1721-walkability-whitelist-path-cache.yaml
+++ b/data/scenarios/Testing/1721-walkability-whitelist-path-cache.yaml
@@ -10,12 +10,12 @@ objectives:
 solution: |
   def goDir = \f. \result.
     let d = fst result in
-    if (d == down) {return ()} {turn d; move; f;}
+    if (d == down) {pure ()} {turn d; move; f;}
     end;
 
   def followRoute =
       nextDir <- path (inL ()) (inR "platform");
-      case nextDir return $ goDir followRoute;
+      case nextDir pure $ goDir followRoute;
       end;
 
   followRoute;
diff --git a/data/scenarios/Testing/1747-volume-command.yaml b/data/scenarios/Testing/1747-volume-command.yaml
index 22e5c5d4a..d77109f26 100644
--- a/data/scenarios/Testing/1747-volume-command.yaml
+++ b/data/scenarios/Testing/1747-volume-command.yaml
@@ -11,7 +11,7 @@ objectives:
       as base {
         let targetVolume = 14 in
         vol <- volume targetVolume;
-        return $case vol (\_. false) (\x. x == targetVolume);
+        pure $case vol (\_. false) (\x. x == targetVolume);
       }
 solution: |
   move;
diff --git a/data/scenarios/Testing/1777-capability-cost.yaml b/data/scenarios/Testing/1777-capability-cost.yaml
index 04006210d..4fb09faa8 100644
--- a/data/scenarios/Testing/1777-capability-cost.yaml
+++ b/data/scenarios/Testing/1777-capability-cost.yaml
@@ -17,10 +17,10 @@ objectives:
         judge <- robotnamed "judge";
         as judge {
           maybePath <- path (inL ()) (inR "packing peanut");
-          return $ case maybePath (\_. true) (\d. false);
+          pure $ case maybePath (\_. true) (\d. false);
         }
       } {
-        return false;
+        pure false;
       };
 solution: |
   move;
diff --git a/data/scenarios/Testing/1780-structure-merge-expansion/coordinate-offset-propagation.yaml b/data/scenarios/Testing/1780-structure-merge-expansion/coordinate-offset-propagation.yaml
index 70dc93707..cf6ff0286 100644
--- a/data/scenarios/Testing/1780-structure-merge-expansion/coordinate-offset-propagation.yaml
+++ b/data/scenarios/Testing/1780-structure-merge-expansion/coordinate-offset-propagation.yaml
@@ -15,7 +15,7 @@ objectives:
   - goal:
       - Enjoy the view.
     condition: |
-      return true
+      pure true
 solution: |
   noop
 known: [boulder, log, pixel (R), pixel (G), pixel (B), gold]
diff --git a/data/scenarios/Testing/1780-structure-merge-expansion/sequential-placement.yaml b/data/scenarios/Testing/1780-structure-merge-expansion/sequential-placement.yaml
index 206f9fffa..326bc01b9 100644
--- a/data/scenarios/Testing/1780-structure-merge-expansion/sequential-placement.yaml
+++ b/data/scenarios/Testing/1780-structure-merge-expansion/sequential-placement.yaml
@@ -21,7 +21,7 @@ objectives:
         g <- countColor "pixel (G)";
         b <- countColor "pixel (B)";
         y <- countColor "gold";
-        return $ r == 3 && g == 3 && b == 3 && y == 3;
+        pure $ r == 3 && g == 3 && b == 3 && y == 3;
       }
 solution: |
   noop
diff --git a/data/scenarios/Testing/1780-structure-merge-expansion/simultaneous-north-and-west-offset.yaml b/data/scenarios/Testing/1780-structure-merge-expansion/simultaneous-north-and-west-offset.yaml
index 7b52bdf55..4bc830f4b 100644
--- a/data/scenarios/Testing/1780-structure-merge-expansion/simultaneous-north-and-west-offset.yaml
+++ b/data/scenarios/Testing/1780-structure-merge-expansion/simultaneous-north-and-west-offset.yaml
@@ -12,7 +12,7 @@ objectives:
   - goal:
       - Enjoy the view.
     condition: |
-      return true
+      pure true
 solution: |
   noop
 known: [pixel (R), gold]
diff --git a/data/scenarios/Testing/201-require/1664-require-system-robot-children.yaml b/data/scenarios/Testing/201-require/1664-require-system-robot-children.yaml
index b8a63f71b..2619c4caa 100644
--- a/data/scenarios/Testing/201-require/1664-require-system-robot-children.yaml
+++ b/data/scenarios/Testing/201-require/1664-require-system-robot-children.yaml
@@ -11,9 +11,9 @@ objectives:
   - condition: |
       try {
         robotnamed "childbot";
-        return true;
+        pure true;
       } {
-        return false;
+        pure false;
       }
 solution: |
   noop;
diff --git a/data/scenarios/Testing/201-require/201-require-device-creative.yaml b/data/scenarios/Testing/201-require/201-require-device-creative.yaml
index 6f01061de..a46c16f39 100644
--- a/data/scenarios/Testing/201-require/201-require-device-creative.yaml
+++ b/data/scenarios/Testing/201-require/201-require-device-creative.yaml
@@ -11,8 +11,8 @@ objectives:
         boatEquipped <- as r {equipped "boat"};
         b1 <- as r {count "boat"};
         b0 <- as base {count "boat"};
-        return (p == (2,0) && b0 == 0 && boatEquipped && b1 == 0);
-      } { return false }
+        pure (p == (2,0) && b0 == 0 && boatEquipped && b1 == 0);
+      } { pure false }
 creative: true
 solution: |
   build {require "boat"; move; move}
diff --git a/data/scenarios/Testing/201-require/201-require-device-creative1.yaml b/data/scenarios/Testing/201-require/201-require-device-creative1.yaml
index 9fd363eda..8c34d7f12 100644
--- a/data/scenarios/Testing/201-require/201-require-device-creative1.yaml
+++ b/data/scenarios/Testing/201-require/201-require-device-creative1.yaml
@@ -11,8 +11,8 @@ objectives:
         boatEquipped <- as r {equipped "boat"};
         b1 <- as r {count "boat"};
         b0 <- as base {count "boat"};
-        return (p == (2,0) && b0 == 1 && boatEquipped && b1 == 0);
-      } { return false }
+        pure (p == (2,0) && b0 == 1 && boatEquipped && b1 == 0);
+      } { pure false }
 creative: true
 solution: |
   build {require "boat"; move; move}
diff --git a/data/scenarios/Testing/201-require/201-require-device.yaml b/data/scenarios/Testing/201-require/201-require-device.yaml
index 4c259cf4a..d9d39d604 100644
--- a/data/scenarios/Testing/201-require/201-require-device.yaml
+++ b/data/scenarios/Testing/201-require/201-require-device.yaml
@@ -11,8 +11,8 @@ objectives:
         boatEquipped <- as r {equipped "boat"};
         b1 <- as r {count "boat"};
         b0 <- as base {count "boat"};
-        return (p == (2,0) && b0 == 0 && boatEquipped && b1 == 0);
-      } { return false }
+        pure (p == (2,0) && b0 == 0 && boatEquipped && b1 == 0);
+      } { pure false }
 solution: |
   build {require "boat"; move; require "boat"; move}
 robots:
diff --git a/data/scenarios/Testing/201-require/201-require-entities-def.yaml b/data/scenarios/Testing/201-require/201-require-entities-def.yaml
index 3b38167cd..79ace4cc6 100644
--- a/data/scenarios/Testing/201-require/201-require-entities-def.yaml
+++ b/data/scenarios/Testing/201-require/201-require-entities-def.yaml
@@ -10,8 +10,8 @@ objectives:
         p <- as r {whereami};
         r1 <- as r {count "rock"};
         r0 <- as base {count "rock"};
-        return (p == (4,0) && r0 == 5 && r1 == 1);
-      } { return false }
+        pure (p == (4,0) && r0 == 5 && r1 == 1);
+      } { pure false }
 solution: |
   def mp = move; place "rock" end;
   build { log "hi"; require 5 "rock"; mp; mp; mp; mp }
diff --git a/data/scenarios/Testing/201-require/201-require-entities.yaml b/data/scenarios/Testing/201-require/201-require-entities.yaml
index 70341fe81..576168a21 100644
--- a/data/scenarios/Testing/201-require/201-require-entities.yaml
+++ b/data/scenarios/Testing/201-require/201-require-entities.yaml
@@ -9,8 +9,8 @@ objectives:
         r <- robotNumbered 1;
         r1 <- as r {count "rock"};
         r0 <- as base {count "rock"};
-        return (r0 == 5 && r1 == 3);
-      } { return false }
+        pure (r0 == 5 && r1 == 3);
+      } { pure false }
 solution: |
   build { require 5 "rock"; move; place "rock"; move; place "rock" }
 robots:
diff --git a/data/scenarios/Testing/201-require/533-reprogram-simple.yaml b/data/scenarios/Testing/201-require/533-reprogram-simple.yaml
index 8092326e1..b2d792fbd 100644
--- a/data/scenarios/Testing/201-require/533-reprogram-simple.yaml
+++ b/data/scenarios/Testing/201-require/533-reprogram-simple.yaml
@@ -19,12 +19,12 @@ objectives:
         solarEquipped <- as fred {equipped "solar panel"};
         treadsEquipped <- as fred {equipped "treads"};
 
-        return (p == (2,0)
+        pure (p == (2,0)
                 && boatEquipped && drillEquipped && solarEquipped && treadsEquipped
                 && base_boats == 1 && base_solar == 1
                 && base_treads == 1 && base_drills == 1
                );
-      } { return false }
+      } { pure false }
 solution: |
   fred <- build {require "boat"; setname "fred"};
   wait 5;
diff --git a/data/scenarios/Testing/201-require/533-reprogram.yaml b/data/scenarios/Testing/201-require/533-reprogram.yaml
index 5553c8579..71b4a9944 100644
--- a/data/scenarios/Testing/201-require/533-reprogram.yaml
+++ b/data/scenarios/Testing/201-require/533-reprogram.yaml
@@ -19,12 +19,12 @@ objectives:
         drillEquipped <- as fred {equipped "metal drill"};
         fred_rocks <- as fred {count "rock"};
 
-        return (p == (2,0) && boatEquipped && drillEquipped
+        pure (p == (2,0) && boatEquipped && drillEquipped
                 && base_boats == 1 && base_solar == 1
                 && base_treads == 1 && base_drills == 1
                 && base_rocks == 42 && fred_rocks == 8
                );
-      } { return false }
+      } { pure false }
 solution: |
   fred <- build {require "boat"; require 5 "rock"; setname "fred"};
   wait 5;
diff --git a/data/scenarios/Testing/2239-custom-entity.yaml b/data/scenarios/Testing/2239-custom-entity.yaml
index 9057775d5..e1432576a 100644
--- a/data/scenarios/Testing/2239-custom-entity.yaml
+++ b/data/scenarios/Testing/2239-custom-entity.yaml
@@ -9,7 +9,7 @@ objectives:
     condition: |
       as base {
         loc <- whereami;
-        return $ loc == (1,0)
+        pure $ loc == (1,0)
       };
 entities:
   - name: rock
diff --git a/data/scenarios/Testing/373-drill.yaml b/data/scenarios/Testing/373-drill.yaml
index 6c76dfdde..e47cb3a7c 100644
--- a/data/scenarios/Testing/373-drill.yaml
+++ b/data/scenarios/Testing/373-drill.yaml
@@ -9,8 +9,8 @@ objectives:
         i <- as base {has "iron ore"};
         c <- as base {has "copper ore"};
         s <- as base {has "rock"};
-        return (i && c && s)
-      } { return false }
+        pure (i && c && s)
+      } { pure false }
 solution: |
   build {
     move; drill forward; turn back; move; turn left;
diff --git a/data/scenarios/Testing/378-objectives.yaml b/data/scenarios/Testing/378-objectives.yaml
index 3f4ffd61c..c907617b4 100644
--- a/data/scenarios/Testing/378-objectives.yaml
+++ b/data/scenarios/Testing/378-objectives.yaml
@@ -10,12 +10,12 @@ objectives:
     condition: |
       try {
         n <- as base {count "tree"};
-        return (n >= 3)
-      } { return false }
+        pure (n >= 3)
+      } { pure false }
   - goal:
       - Nice job.  Now, build a harvester.
     condition: |
-      try { as base {has "harvester"} } {return false}
+      try { as base {has "harvester"} } {pure false}
     prerequisite: get_trees
 solution: |
   build {turn right; move; move; grab; move; grab; move; grab; turn back; move; move; move; move};
diff --git a/data/scenarios/Testing/394-build-drill.yaml b/data/scenarios/Testing/394-build-drill.yaml
index 8d60dd0b3..a95b87de4 100644
--- a/data/scenarios/Testing/394-build-drill.yaml
+++ b/data/scenarios/Testing/394-build-drill.yaml
@@ -7,12 +7,12 @@ creative: True
 objectives:
   - condition: |
       try {
-        as base {l <- has "detonator"; return (not l)}
-      } { return false }
+        as base {l <- has "detonator"; pure (not l)}
+      } { pure false }
 # When testing, add `s <- build {...}; n <- as s {whoami}; log n`
 solution: |
   def forever = \c. c ; forever c end;
-  def unblock = try {drill forward; return ()} {} end;
+  def unblock = try {drill forward; pure ()} {} end;
   def doPush = unblock; move end;
   log "Hi, I am base";
   r <- build {
diff --git a/data/scenarios/Testing/397-wrong-missing.yaml b/data/scenarios/Testing/397-wrong-missing.yaml
index 3e10289ec..e4f4b80c4 100644
--- a/data/scenarios/Testing/397-wrong-missing.yaml
+++ b/data/scenarios/Testing/397-wrong-missing.yaml
@@ -8,7 +8,7 @@ description: |
   https://github.com/swarm-game/swarm/issues/397
 objectives:
   - condition: |
-      t <- time; return (t == 2)
+      t <- time; pure (t == 2)
     goal:
       - |
         This is a dummy condition that just ensures the base has had
diff --git a/data/scenarios/Testing/428-drowning-destroy.yaml b/data/scenarios/Testing/428-drowning-destroy.yaml
index ca01b7158..e9e14d6ab 100644
--- a/data/scenarios/Testing/428-drowning-destroy.yaml
+++ b/data/scenarios/Testing/428-drowning-destroy.yaml
@@ -8,13 +8,13 @@ objectives:
       def isAlive = \n. try {
         _ <- robotNamed n;
         log ("Is alive: " ++ n);
-        return true
-      } { return false } end;
+        pure true
+      } { pure false } end;
       try {
         baseAlive <- isAlive "base";
         botAlive <- isAlive "bot";
-        return (baseAlive && not botAlive);
-      } { return false; }
+        pure (baseAlive && not botAlive);
+      } { pure false; }
 solution: |
   log "start";
   wait 5;
diff --git a/data/scenarios/Testing/475-wait-one.yaml b/data/scenarios/Testing/475-wait-one.yaml
index cdc7f4b2e..97b7d2737 100644
--- a/data/scenarios/Testing/475-wait-one.yaml
+++ b/data/scenarios/Testing/475-wait-one.yaml
@@ -11,7 +11,7 @@ objectives:
       try {
         teleport self (0,0);
         ishere "lambda"
-      } { return false }
+      } { pure false }
 solution: |
   log "I will win next tick!"; wait 1; place "lambda";
 robots:
diff --git a/data/scenarios/Testing/479-atomic-race.yaml b/data/scenarios/Testing/479-atomic-race.yaml
index 491bf02d9..d89835e5f 100644
--- a/data/scenarios/Testing/479-atomic-race.yaml
+++ b/data/scenarios/Testing/479-atomic-race.yaml
@@ -28,7 +28,7 @@ objectives:
       try {
         p <- robotNamed "planter";
         as p {has "Win"}
-      } { return false }
+      } { pure false }
 solution: |
   def forever = \c. force c ; forever c end
   def tryharvest =
diff --git a/data/scenarios/Testing/479-atomic.yaml b/data/scenarios/Testing/479-atomic.yaml
index 2d40084e2..5fc877e4d 100644
--- a/data/scenarios/Testing/479-atomic.yaml
+++ b/data/scenarios/Testing/479-atomic.yaml
@@ -11,14 +11,14 @@ objectives:
   - condition: |
       try {
         p <- robotNamed "planter";
-        as p {n <- count "weed"; return (n == 0)}
-      } { return false }
+        as p {n <- count "weed"; pure (n == 0)}
+      } { pure false }
 solution: |
   def forever = \c. force c ; forever c end
   def tryharvest =
     atomic (
       b <- ishere "weed";
-      if b {harvest; return ()} {}
+      if b {harvest; pure ()} {}
     );
     n <- random 4; wait n
   end;
diff --git a/data/scenarios/Testing/490-harvest.yaml b/data/scenarios/Testing/490-harvest.yaml
index ad1096342..8528eb0da 100644
--- a/data/scenarios/Testing/490-harvest.yaml
+++ b/data/scenarios/Testing/490-harvest.yaml
@@ -11,9 +11,9 @@ objectives:
         t1 <- ishere "tree";
         move;
         t2 <- ishere "tree";
-        t3 <- as base {n <- count "water"; return (n == 4)};
-        return (t1 && not t2 && t3)
-      } { return false }
+        t3 <- as base {n <- count "water"; pure (n == 4)};
+        pure (t1 && not t2 && t3)
+      } { pure false }
 solution: |
   move; harvest; move; grab; turn right; move; grab; grab; harvest; grab
 robots:
diff --git a/data/scenarios/Testing/504-teleport-self.yaml b/data/scenarios/Testing/504-teleport-self.yaml
index d375d32e5..dc4b884bb 100644
--- a/data/scenarios/Testing/504-teleport-self.yaml
+++ b/data/scenarios/Testing/504-teleport-self.yaml
@@ -9,7 +9,7 @@ objectives:
         create "tree"; place "tree";
         teleport self (5,0);
         ishere "tree"
-      } { return false }
+      } { pure false }
 solution: |
   place "tree"
 robots:
diff --git a/data/scenarios/Testing/508-capability-subset.yaml b/data/scenarios/Testing/508-capability-subset.yaml
index 3e6b86010..cac1de285 100644
--- a/data/scenarios/Testing/508-capability-subset.yaml
+++ b/data/scenarios/Testing/508-capability-subset.yaml
@@ -10,8 +10,8 @@ objectives:
         p <- as r {whereami};
         g <- as base {count "GPS"};
         t <- as base {count "Tardis"};
-        return (p == (5,0) && g == 1 && t == 0);
-      } { return false }
+        pure (p == (5,0) && g == 1 && t == 0);
+      } { pure false }
 solution: |
   log "Hi, I can build teleporting robots!";
   try {
diff --git a/data/scenarios/Testing/555-teleport-location.yaml b/data/scenarios/Testing/555-teleport-location.yaml
index d04c7d786..db8c96996 100644
--- a/data/scenarios/Testing/555-teleport-location.yaml
+++ b/data/scenarios/Testing/555-teleport-location.yaml
@@ -8,7 +8,7 @@ objectives:
   - condition: |
       try {
         as base {has "rock"}
-      } { return false }
+      } { pure false }
 solution: |
   fred <- robotNamed "fred";
   myLoc <- whereami;
diff --git a/data/scenarios/Testing/562-lodestone.yaml b/data/scenarios/Testing/562-lodestone.yaml
index bbdb6bf5e..15fb683bf 100644
--- a/data/scenarios/Testing/562-lodestone.yaml
+++ b/data/scenarios/Testing/562-lodestone.yaml
@@ -7,7 +7,7 @@ objectives:
     condition: |
       try {
         as base {has "drill bit"};
-      } { return false }
+      } { pure false }
 solution: |
   run "scenarios/Testing/562-lodestone.sw"
 robots:
diff --git a/data/scenarios/Testing/684-swap.yaml b/data/scenarios/Testing/684-swap.yaml
index b1d387a89..f9dfe5547 100644
--- a/data/scenarios/Testing/684-swap.yaml
+++ b/data/scenarios/Testing/684-swap.yaml
@@ -9,10 +9,10 @@ objectives:
         as base {
           l <- has "lambda";
           b <- has "bitcoin";
-          return $ l && b
+          pure $ l && b
         }
       } {
-        return false
+        pure false
       }
 solution: |
   swap "bitcoin"; swap "gold coin";
@@ -41,7 +41,7 @@ robots:
         case d (\_.
           say "Fatal error: swap does not work atomically!"
         ) (\_.
-          return ()
+          pure ()
         )
       )
 world:
diff --git a/data/scenarios/Testing/699-movement-fail/699-move-blocked.yaml b/data/scenarios/Testing/699-movement-fail/699-move-blocked.yaml
index c709c789b..f2b103de1 100644
--- a/data/scenarios/Testing/699-movement-fail/699-move-blocked.yaml
+++ b/data/scenarios/Testing/699-movement-fail/699-move-blocked.yaml
@@ -5,12 +5,12 @@ description: |
   https://github.com/swarm-game/swarm/issues/699
 objectives:
   - condition: |
-      def isAliveOn = \name.\loc. try {r <- robotNamed name; l <- as r {whereami}; return $ l == loc} {return false} end;
-      def isDead = \name. try {robotNamed name; return false} {return true} end;
+      def isAliveOn = \name.\loc. try {r <- robotNamed name; l <- as r {whereami}; pure $ l == loc} {pure false} end;
+      def isDead = \name. try {robotNamed name; pure false} {pure true} end;
       a1 <- isAliveOn "one" (0,-1);
       a2 <- isAliveOn "two" (1,-2);
       b <- isAliveOn "base" (0,0);
-      return (a1 && a2 && b)
+      pure (a1 && a2 && b)
 solution: |
   move; say "Fatal error: base was able to move into a boulder and not fail!"
 robots:
diff --git a/data/scenarios/Testing/699-movement-fail/699-move-liquid.yaml b/data/scenarios/Testing/699-movement-fail/699-move-liquid.yaml
index e410a74cf..f3e916171 100644
--- a/data/scenarios/Testing/699-movement-fail/699-move-liquid.yaml
+++ b/data/scenarios/Testing/699-movement-fail/699-move-liquid.yaml
@@ -5,13 +5,13 @@ description: |
   https://github.com/swarm-game/swarm/issues/699
 objectives:
   - condition: |
-      def isAliveOn = \name.\loc. try {r <- robotNamed name; l <- as r {whereami}; return $ l == loc} {return false} end;
-      def isDead = \name. try {robotNamed name; return false} {return true} end;
+      def isAliveOn = \name.\loc. try {r <- robotNamed name; l <- as r {whereami}; pure $ l == loc} {pure false} end;
+      def isDead = \name. try {robotNamed name; pure false} {pure true} end;
       d1 <- isDead "one";
       a2 <- isAliveOn "two" (1,-2);
       a3 <- isAliveOn "three" (1,-3);
       b <- isAliveOn "base" (0,0);
-      return (d1 && a2 && a3 && b)
+      pure (d1 && a2 && a3 && b)
 solution: |
   move; say "Fatal error: base was able to move into water and not fail!"
 robots:
diff --git a/data/scenarios/Testing/699-movement-fail/699-teleport-blocked.yaml b/data/scenarios/Testing/699-movement-fail/699-teleport-blocked.yaml
index b67f0a53e..913409424 100644
--- a/data/scenarios/Testing/699-movement-fail/699-teleport-blocked.yaml
+++ b/data/scenarios/Testing/699-movement-fail/699-teleport-blocked.yaml
@@ -6,14 +6,14 @@ description: |
 creative: true
 objectives:
   - condition: |
-      def isAliveOn = \name.\loc. try {r <- robotNamed name; l <- as r {whereami}; return $ l == loc} {return false} end;
-      def isDead = \name. try {robotNamed name; return false} {return true} end;
+      def isAliveOn = \name.\loc. try {r <- robotNamed name; l <- as r {whereami}; pure $ l == loc} {pure false} end;
+      def isDead = \name. try {robotNamed name; pure false} {pure true} end;
       d1 <- isDead "one";
       d2 <- isDead "two";
       a3 <- isAliveOn "three" (3,0);
       a4 <- isAliveOn "four" (3,0);
       b <- isAliveOn "base" (5,0);
-      return (d1 && d2 && a3 && a4 && b)
+      pure (d1 && d2 && a3 && a4 && b)
 solution: |
   o <- robotNamed "one"; reprogram o {t <- robotNamed "two"; teleport t (3,0); teleport self (3,0)};
   try {teleport self (3,0)} {teleport self (5,0)}
diff --git a/data/scenarios/Testing/710-multi-robot.yaml b/data/scenarios/Testing/710-multi-robot.yaml
index d4aa9fb9c..ce08316ff 100644
--- a/data/scenarios/Testing/710-multi-robot.yaml
+++ b/data/scenarios/Testing/710-multi-robot.yaml
@@ -67,8 +67,8 @@ robots:
       until (
         try {
           loc <- as base {whereami};
-          return (loc == l)
-        } { return false }
+          pure (loc == l)
+        } { pure false }
       );
       create "Win"
   - name: check2
@@ -83,8 +83,8 @@ robots:
       until (
         try {
           loc <- as base {whereami};
-          return (loc == l)
-        } { return false }
+          pure (loc == l)
+        } { pure false }
       );
       create "Win"
   - name: check3
@@ -99,8 +99,8 @@ robots:
       until (
         try {
           loc <- as base {whereami};
-          return (loc == l)
-        } { return false }
+          pure (loc == l)
+        } { pure false }
       );
       create "Win"
   - name: horizon
@@ -111,7 +111,7 @@ robots:
     inventory: [[1, horizontal wall]]
     program: |
       def until = \c. b <- c; if b {} {until c} end;
-      n <- (s <- scan down; case s (\_. fail "Fatal error: missing room number!") return);
+      n <- (s <- scan down; case s (\_. fail "Fatal error: missing room number!") pure);
       c1 <- robotNamed ("check" ++ n);
       until (as c1 {has "Win"});
       swap "horizontal wall"
@@ -123,7 +123,7 @@ robots:
     inventory: [[1, vertical wall]]
     program: |
       def until = \c. b <- c; if b {} {until c} end;
-      n <- (s <- scan down; case s (\_. fail "Fatal error: missing room number!") return);
+      n <- (s <- scan down; case s (\_. fail "Fatal error: missing room number!") pure);
       c1 <- robotNamed ("check" ++ n);
       until (as c1 {has "Win"});
       swap "vertical wall"
@@ -135,7 +135,7 @@ robots:
     inventory: [[1, upper right corner]]
     program: |
       def until = \c. b <- c; if b {} {until c} end;
-      n <- (s <- scan down; case s (\_. fail "Fatal error: missing room number!") return);
+      n <- (s <- scan down; case s (\_. fail "Fatal error: missing room number!") pure);
       c1 <- robotNamed ("check" ++ n);
       until (as c1 {has "Win"});
       swap "upper right corner"
@@ -147,7 +147,7 @@ robots:
     inventory: [[1, lower right corner]]
     program: |
       def until = \c. b <- c; if b {} {until c} end;
-      n <- (s <- scan down; case s (\_. fail "Fatal error: missing room number!") return);
+      n <- (s <- scan down; case s (\_. fail "Fatal error: missing room number!") pure);
       c1 <- robotNamed ("check" ++ n);
       until (as c1 {has "Win"});
       swap "lower right corner"
@@ -159,7 +159,7 @@ robots:
     inventory: [[1, upper left corner]]
     program: |
       def until = \c. b <- c; if b {} {until c} end;
-      n <- (s <- scan down; case s (\_. fail "Fatal error: missing room number!") return);
+      n <- (s <- scan down; case s (\_. fail "Fatal error: missing room number!") pure);
       c1 <- robotNamed ("check" ++ n);
       until (as c1 {has "Win"});
       swap "upper left corner"
diff --git a/data/scenarios/Testing/795-prerequisite/795-prerequisite-and.yaml b/data/scenarios/Testing/795-prerequisite/795-prerequisite-and.yaml
index 3a5c94c68..35078257f 100644
--- a/data/scenarios/Testing/795-prerequisite/795-prerequisite-and.yaml
+++ b/data/scenarios/Testing/795-prerequisite/795-prerequisite-and.yaml
@@ -7,7 +7,7 @@ objectives:
   - goal:
       - Achieve both of two other objectives
     condition: |
-      return true;
+      pure true;
     prerequisite:
       previewable: true
       logic:
diff --git a/data/scenarios/Testing/795-prerequisite/795-prerequisite-mutually-exclusive.yaml b/data/scenarios/Testing/795-prerequisite/795-prerequisite-mutually-exclusive.yaml
index c7e97790c..0b30b98d4 100644
--- a/data/scenarios/Testing/795-prerequisite/795-prerequisite-mutually-exclusive.yaml
+++ b/data/scenarios/Testing/795-prerequisite/795-prerequisite-mutually-exclusive.yaml
@@ -7,7 +7,7 @@ objectives:
   - goal:
       - Either obtain a "furnace" WITHOUT a "flower", or obtain both a "wooden gear" and "flower".
     condition: |
-      return true;
+      pure true;
     prerequisite:
       previewable: true
       logic:
diff --git a/data/scenarios/Testing/795-prerequisite/795-prerequisite-or.yaml b/data/scenarios/Testing/795-prerequisite/795-prerequisite-or.yaml
index d19876862..3b9131f7d 100644
--- a/data/scenarios/Testing/795-prerequisite/795-prerequisite-or.yaml
+++ b/data/scenarios/Testing/795-prerequisite/795-prerequisite-or.yaml
@@ -7,7 +7,7 @@ objectives:
   - goal:
       - Achieve one of two other objectives
     condition: |
-      return true;
+      pure true;
     prerequisite:
       previewable: true
       logic:
diff --git a/data/scenarios/Testing/836-pathfinding/836-automatic-waypoint-navigation.yaml b/data/scenarios/Testing/836-pathfinding/836-automatic-waypoint-navigation.yaml
index 04b826ffd..b1273da96 100644
--- a/data/scenarios/Testing/836-pathfinding/836-automatic-waypoint-navigation.yaml
+++ b/data/scenarios/Testing/836-pathfinding/836-automatic-waypoint-navigation.yaml
@@ -11,7 +11,7 @@ objectives:
     condition: |
       as base {
         eggCount <- count "egg";
-        return $ eggCount >= 64;
+        pure $ eggCount >= 64;
       };
 attrs:
   - name: easter_egg
diff --git a/data/scenarios/Testing/836-pathfinding/836-no-path-exists1.yaml b/data/scenarios/Testing/836-pathfinding/836-no-path-exists1.yaml
index 8a2ce51af..5e2637076 100644
--- a/data/scenarios/Testing/836-pathfinding/836-no-path-exists1.yaml
+++ b/data/scenarios/Testing/836-pathfinding/836-no-path-exists1.yaml
@@ -10,7 +10,7 @@ objectives:
     condition: |
       as base {
         nextDir <- path (inL ()) (inR "flower");
-        return $ case nextDir (\_. true) (\_. false);
+        pure $ case nextDir (\_. true) (\_. false);
       };
 solution: |
   noop;
diff --git a/data/scenarios/Testing/836-pathfinding/836-no-path-exists2.yaml b/data/scenarios/Testing/836-pathfinding/836-no-path-exists2.yaml
index 42d66372c..af9dfcef8 100644
--- a/data/scenarios/Testing/836-pathfinding/836-no-path-exists2.yaml
+++ b/data/scenarios/Testing/836-pathfinding/836-no-path-exists2.yaml
@@ -16,7 +16,7 @@ objectives:
     condition: |
       as base {
         nextDir <- path (inL ()) (inR "flower");
-        return $ case nextDir (\_. true) (\_. false);
+        pure $ case nextDir (\_. true) (\_. false);
       };
 solution: |
   noop;
diff --git a/data/scenarios/Testing/836-pathfinding/836-path-exists-distance-limit-reachable.yaml b/data/scenarios/Testing/836-pathfinding/836-path-exists-distance-limit-reachable.yaml
index 4e7582c38..04aaea8eb 100644
--- a/data/scenarios/Testing/836-pathfinding/836-path-exists-distance-limit-reachable.yaml
+++ b/data/scenarios/Testing/836-pathfinding/836-path-exists-distance-limit-reachable.yaml
@@ -9,7 +9,7 @@ objectives:
     condition: |
       as base {
         nextDir <- path (inR 3) (inR "flower");
-        return $ case nextDir (\_. false) (\_. true);
+        pure $ case nextDir (\_. false) (\_. true);
       };
 solution: |
   move;
diff --git a/data/scenarios/Testing/836-pathfinding/836-path-exists-distance-limit-unreachable.yaml b/data/scenarios/Testing/836-pathfinding/836-path-exists-distance-limit-unreachable.yaml
index 55c905670..42a052ff6 100644
--- a/data/scenarios/Testing/836-pathfinding/836-path-exists-distance-limit-unreachable.yaml
+++ b/data/scenarios/Testing/836-pathfinding/836-path-exists-distance-limit-unreachable.yaml
@@ -9,7 +9,7 @@ objectives:
     condition: |
       as base {
         nextDir <- path (inR 3) (inR "flower");
-        return $ case nextDir (\_. true) (\_. false);
+        pure $ case nextDir (\_. true) (\_. false);
       };
 solution: |
   turn back;
diff --git a/data/scenarios/Testing/836-pathfinding/836-path-exists-find-entity-unwalkable.yaml b/data/scenarios/Testing/836-pathfinding/836-path-exists-find-entity-unwalkable.yaml
index 3c25270c8..9583b20fd 100644
--- a/data/scenarios/Testing/836-pathfinding/836-path-exists-find-entity-unwalkable.yaml
+++ b/data/scenarios/Testing/836-pathfinding/836-path-exists-find-entity-unwalkable.yaml
@@ -9,7 +9,7 @@ objectives:
     condition: |
       as base {
         itemAhead <- scan forward;
-        return $ case itemAhead (\_. false) (\item. item == "water");
+        pure $ case itemAhead (\_. false) (\item. item == "water");
       };
 solution: |
   run "scenarios/Testing/836-pathfinding/_836-path-exists/find-entity-unwalkable-solution.sw";
diff --git a/data/scenarios/Testing/836-pathfinding/_836-automatic-waypoint-navigation/gardener.sw b/data/scenarios/Testing/836-pathfinding/_836-automatic-waypoint-navigation/gardener.sw
index 303e585e0..e152e9458 100644
--- a/data/scenarios/Testing/836-pathfinding/_836-automatic-waypoint-navigation/gardener.sw
+++ b/data/scenarios/Testing/836-pathfinding/_836-automatic-waypoint-navigation/gardener.sw
@@ -6,7 +6,7 @@ def harvestPlant =
   } {
     wait 50;
     harvest;
-    return ();
+    pure ();
   };
   end;
 
diff --git a/data/scenarios/Testing/836-pathfinding/_836-automatic-waypoint-navigation/patrol.sw b/data/scenarios/Testing/836-pathfinding/_836-automatic-waypoint-navigation/patrol.sw
index 678147e48..a876dbbe8 100644
--- a/data/scenarios/Testing/836-pathfinding/_836-automatic-waypoint-navigation/patrol.sw
+++ b/data/scenarios/Testing/836-pathfinding/_836-automatic-waypoint-navigation/patrol.sw
@@ -2,8 +2,8 @@ def goDir = \f. \r.
   let d = fst r in
   if (d == down) {
     eggHere <- ishere "egg";
-    if eggHere {grab; return ()} {};
-    return ()
+    if eggHere {grab; pure ()} {};
+    pure ()
   } {
     turn d;
 
@@ -19,7 +19,7 @@ def goDir = \f. \r.
 
 def followRoute = \loc.
     nextDir <- path (inL ()) (inL loc);
-    case nextDir return $ goDir $ followRoute loc;
+    case nextDir pure $ goDir $ followRoute loc;
     end;
 
 def visitNextWaypoint = \nextWpIdx.
diff --git a/data/scenarios/Testing/836-pathfinding/_836-path-exists/find-entity-solution.sw b/data/scenarios/Testing/836-pathfinding/_836-path-exists/find-entity-solution.sw
index 39c4e3d7c..a144cd849 100644
--- a/data/scenarios/Testing/836-pathfinding/_836-path-exists/find-entity-solution.sw
+++ b/data/scenarios/Testing/836-pathfinding/_836-path-exists/find-entity-solution.sw
@@ -1,7 +1,7 @@
 def goDir = \f. \r.
   let d = fst r in
   if (d == down) {
-    grab; return ()
+    grab; pure ()
   } {
     turn d; move; f;
   }
@@ -9,7 +9,7 @@ def goDir = \f. \r.
 
 def followRoute =
     nextDir <- path (inL ()) (inR "flower");
-    case nextDir return $ goDir followRoute;
+    case nextDir pure $ goDir followRoute;
     end;
 
 followRoute;
diff --git a/data/scenarios/Testing/836-pathfinding/_836-path-exists/find-entity-unwalkable-solution.sw b/data/scenarios/Testing/836-pathfinding/_836-path-exists/find-entity-unwalkable-solution.sw
index 7235a8d80..a877931c1 100644
--- a/data/scenarios/Testing/836-pathfinding/_836-path-exists/find-entity-unwalkable-solution.sw
+++ b/data/scenarios/Testing/836-pathfinding/_836-path-exists/find-entity-unwalkable-solution.sw
@@ -1,13 +1,13 @@
 def goDir = \goalItem. \f. \r.
   let d = fst r in
   if (d == down) {
-    grab; return ()
+    grab; pure ()
   } {
     turn d;
     itemAhead <- scan forward;
     let isGoalAhead = case itemAhead (\_. false) (\item. item == goalItem) in
     if isGoalAhead {
-      return ();
+      pure ();
     } {
       move; f;
     };
@@ -17,7 +17,7 @@ def goDir = \goalItem. \f. \r.
 def followRoute =
     let goalItem = "water" in
     nextDir <- path (inL ()) (inR goalItem);
-    case nextDir return $ goDir goalItem followRoute;
+    case nextDir pure $ goDir goalItem followRoute;
     end;
 
 followRoute;
diff --git a/data/scenarios/Testing/836-pathfinding/_836-path-exists/find-location-solution.sw b/data/scenarios/Testing/836-pathfinding/_836-path-exists/find-location-solution.sw
index 70b2a83f7..98897b973 100644
--- a/data/scenarios/Testing/836-pathfinding/_836-path-exists/find-location-solution.sw
+++ b/data/scenarios/Testing/836-pathfinding/_836-path-exists/find-location-solution.sw
@@ -1,7 +1,7 @@
 def goDir = \f. \r.
   let d = fst r in
   if (d == down) {
-    grab; return ()
+    grab; pure ()
   } {
     turn d; move; f;
   }
@@ -9,7 +9,7 @@ def goDir = \f. \r.
 
 def followRoute =
     nextDir <- path (inL ()) (inL (4, 0));
-    case nextDir return $ goDir followRoute;
+    case nextDir pure $ goDir followRoute;
     end;
 
 followRoute;
diff --git a/data/scenarios/Testing/858-inventory/858-counting-objective.yaml b/data/scenarios/Testing/858-inventory/858-counting-objective.yaml
index d476abdb7..cc0c54c65 100644
--- a/data/scenarios/Testing/858-inventory/858-counting-objective.yaml
+++ b/data/scenarios/Testing/858-inventory/858-counting-objective.yaml
@@ -12,7 +12,7 @@ objectives:
     condition: |-
       as base {
         itemCount <- count "tree";
-        return $ itemCount > 0;
+        pure $ itemCount > 0;
       };
 robots:
   - name: base
diff --git a/data/scenarios/Testing/858-inventory/858-nonpossession-objective.yaml b/data/scenarios/Testing/858-inventory/858-nonpossession-objective.yaml
index f875063e3..01509a7bc 100644
--- a/data/scenarios/Testing/858-inventory/858-nonpossession-objective.yaml
+++ b/data/scenarios/Testing/858-inventory/858-nonpossession-objective.yaml
@@ -10,7 +10,7 @@ objectives:
     condition: |-
       as base {
         x <- has "tree";
-        return $ not x;
+        pure $ not x;
       };
 robots:
   - name: base
diff --git a/data/scenarios/Testing/920-meet.yaml b/data/scenarios/Testing/920-meet.yaml
index 2a9e64d42..0bb51b26d 100644
--- a/data/scenarios/Testing/920-meet.yaml
+++ b/data/scenarios/Testing/920-meet.yaml
@@ -10,19 +10,19 @@ objectives:
       teleport self (0,0);
       def all : (rec l. Unit + a * l) -> (a -> Cmd Bool) -> Cmd Bool =
         \xs. \f. case xs
-          (\_. return true)
-          (\cons. b <- f (fst cons); if b {all (snd cons) f} {return false})
+          (\_. pure true)
+          (\cons. b <- f (fst cons); if b {all (snd cons) f} {pure false})
       end;
       rs <- meetAll;
       b1 <- all rs (\r. as r {has "boat"});
       n2 <- as r0 { count "boat" };
-      return (b0 && b1 && (n2 == 2))
+      pure (b0 && b1 && (n2 == 2))
 solution: |
   mr0 <- meet;
-  case mr0 (\_. return ()) (\r0. give r0 "boat");
+  case mr0 (\_. pure ()) (\r0. give r0 "boat");
   def forM_ : (rec l. Unit + a * l) -> (a -> Cmd b) -> Cmd Unit =
     \xs. \f. case xs
-      (\_. return ())
+      (\_. pure ())
       (\cons. f (fst cons); forM_ (snd cons) f)
   end;
   rs <- meetAll;
diff --git a/data/scenarios/Testing/955-heading.yaml b/data/scenarios/Testing/955-heading.yaml
index 1470473ac..52930b4d5 100644
--- a/data/scenarios/Testing/955-heading.yaml
+++ b/data/scenarios/Testing/955-heading.yaml
@@ -7,7 +7,7 @@ objectives:
   - condition: |
       loc <- as base {whereami};
       h <- as base {heading};
-      return $ loc == (1,-1) && h == north
+      pure $ loc == (1,-1) && h == north
 solution: |
   h <- heading; turn east; move; move; turn right; move; move; turn h
 robots:
diff --git a/data/scenarios/Testing/956-GPS.yaml b/data/scenarios/Testing/956-GPS.yaml
index 7829c76bc..0eb2b6d92 100644
--- a/data/scenarios/Testing/956-GPS.yaml
+++ b/data/scenarios/Testing/956-GPS.yaml
@@ -6,7 +6,7 @@ description: |
 objectives:
   - condition: |
       loc <- as base {whereami};
-      return $ loc == (0,0)
+      pure $ loc == (0,0)
     goal:
       - |
         The goal is to move back to the origin after being
diff --git a/data/scenarios/Testing/961-custom-capabilities.yaml b/data/scenarios/Testing/961-custom-capabilities.yaml
index 3307048b5..84e8b881b 100644
--- a/data/scenarios/Testing/961-custom-capabilities.yaml
+++ b/data/scenarios/Testing/961-custom-capabilities.yaml
@@ -7,7 +7,7 @@ description: |
 objectives:
   - condition: |
       mover <- robotNumbered 1;
-      as mover { loc <- whereami; return $ loc == (0,1) }
+      as mover { loc <- whereami; pure $ loc == (0,1) }
     goal:
       - |
         Get a robot to (0,1)!
diff --git a/data/scenarios/Testing/Achievements/RobotIntoWater.yaml b/data/scenarios/Testing/Achievements/RobotIntoWater.yaml
index 3a302f260..ae43a66dd 100644
--- a/data/scenarios/Testing/Achievements/RobotIntoWater.yaml
+++ b/data/scenarios/Testing/Achievements/RobotIntoWater.yaml
@@ -6,12 +6,12 @@ objectives:
     goal:
       - Build a robot
     condition: |
-      try {robotNumbered 1; return True} {return False}
+      try {robotNumbered 1; pure True} {pure False}
   - goal:
       - Drown it
     prerequisite: build
     condition: |
-      try {robotNumbered 1; return False} {return True}
+      try {robotNumbered 1; pure False} {pure True}
 solution: |
   build { turn right; move; move; move }
 robots:
diff --git a/data/scenarios/Testing/_Validation/795-prerequisite-nonexistent-reference.yaml b/data/scenarios/Testing/_Validation/795-prerequisite-nonexistent-reference.yaml
index 10dfe2839..aee7affef 100644
--- a/data/scenarios/Testing/_Validation/795-prerequisite-nonexistent-reference.yaml
+++ b/data/scenarios/Testing/_Validation/795-prerequisite-nonexistent-reference.yaml
@@ -7,7 +7,7 @@ objectives:
   - goal:
       - Achieve one of two other objectives
     condition: |
-      return true;
+      pure true;
     prerequisite: shave_furnace
   - id: have_furnace
     goal:
diff --git a/data/scenarios/Tutorials/backstory.yaml b/data/scenarios/Tutorials/backstory.yaml
index 06e934d91..b79f86a2f 100644
--- a/data/scenarios/Tutorials/backstory.yaml
+++ b/data/scenarios/Tutorials/backstory.yaml
@@ -18,7 +18,7 @@ objectives:
       try {
         l <- robotNamed "listener";
         as l {has "READY"}
-      } { return false }
+      } { pure false }
 solution: |
   say "Ready!"
 entities:
diff --git a/data/scenarios/Tutorials/bind2.yaml b/data/scenarios/Tutorials/bind2.yaml
index ec8e8f9ae..b905efced 100644
--- a/data/scenarios/Tutorials/bind2.yaml
+++ b/data/scenarios/Tutorials/bind2.yaml
@@ -19,8 +19,8 @@ objectives:
       try {
         p <- robotnamed "floorspot";
         w <- as p {ishere "Hastur"};
-        return (not w);
-      } { return false }
+        pure (not w);
+      } { pure false }
   - goal:
       - |
         Your robot obtained the misplaced artifact! Next you need to put it back
@@ -58,8 +58,8 @@ objectives:
       try {
         p <- robotnamed "pedestal";
         w <- as p {ishere "Hastur"};
-        return w;
-      } { return false }
+        pure w;
+      } { pure false }
     prerequisite: grab_artifact
 solution: |
   build {
diff --git a/data/scenarios/Tutorials/build.yaml b/data/scenarios/Tutorials/build.yaml
index d47158431..344fce649 100644
--- a/data/scenarios/Tutorials/build.yaml
+++ b/data/scenarios/Tutorials/build.yaml
@@ -25,8 +25,8 @@ objectives:
         b <- ishere "flower";
         move;
         c <- ishere "flower";
-        return (a || b || c);
-      } { return false }
+        pure (a || b || c);
+      } { pure false }
 solution: |
   build {turn right; move; move; harvest; turn right; move; place "flower"}
 robots:
diff --git a/data/scenarios/Tutorials/conditionals.yaml b/data/scenarios/Tutorials/conditionals.yaml
index b569212fe..2a927951d 100644
--- a/data/scenarios/Tutorials/conditionals.yaml
+++ b/data/scenarios/Tutorials/conditionals.yaml
@@ -51,19 +51,19 @@ objectives:
         **TIP:** the two branches of an `if` must have the same type. In particular,
         `if ... {grab} {}`{=snippet} is not
         allowed, because `{grab}` has type `{Cmd Text}`{=type} whereas `{}`{=snippet} has type `{Cmd Unit}`{=type}.
-        In this case `{grab; return ()}` has the right type.
+        In this case `{grab; pure ()}` has the right type.
     condition: |
       try {
         n <- as base {count "very small rock"};
-        return (n == 4)
-      } { return false}
+        pure (n == 4)
+      } { pure false}
 solution: |
   def tL = turn left end;
   def tB = turn back end;
   def x4 = \c. c;c;c;c end;
   def VSR = "very small rock" end;
   def ifC = \c.\t.\e. b <- c; if b t e end;
-  def pick = move; ifC (ishere VSR) {grab; return ()} {} end;
+  def pick = move; ifC (ishere VSR) {grab; pure ()} {} end;
   def pickrow = x4 pick; turn back; x4 move end;
   build {
     turn south; x4 (move; tL; pickrow; tL); tB; x4 move; x4 (give base VSR)
diff --git a/data/scenarios/Tutorials/craft.yaml b/data/scenarios/Tutorials/craft.yaml
index 7808b6b15..61f5c81e4 100644
--- a/data/scenarios/Tutorials/craft.yaml
+++ b/data/scenarios/Tutorials/craft.yaml
@@ -19,7 +19,7 @@ objectives:
     condition: |
       try {
         as base {has "branch predictor"}
-      } { return false }
+      } { pure false }
 solution: |
   make "branch"; make "branch predictor"
 robots:
diff --git a/data/scenarios/Tutorials/crash-secret.sw b/data/scenarios/Tutorials/crash-secret.sw
index 578b532c2..de1003a98 100644
--- a/data/scenarios/Tutorials/crash-secret.sw
+++ b/data/scenarios/Tutorials/crash-secret.sw
@@ -1,7 +1,7 @@
 // A for cycle from start to end (excluded) that carries a state.
 def foreachF = \s.\e.\com.\state.
   if (s >= e) {
-    return state
+    pure state
   } {
     n <- com state s;
     foreachF (s+1) e com n
@@ -23,7 +23,7 @@ myLoc <- whereami;
 
 def foldM : (rec l. Unit + a * l) -> b -> (b -> a -> Cmd b) -> Cmd b =
   \xs. \b. \f. case xs
-    (\_. return b)
+    (\_. pure b)
     (\cons. b' <- f b (fst cons); foldM (snd cons) b' f)
 end
 
@@ -34,22 +34,22 @@ def tryGive: Text -> (Actor -> Bool) -> Cmd (Actor -> Bool) = \msg. \ok.
   foldM rs ok $ \f.\rob.
     if (not $ f rob) {
       log $ "skipping the robot " ++ format rob ++ "because it already has a Win";
-      return f
+      pure f
     } {
       robLoc <- as rob {whereami};
       if (robLoc != myLoc) {
         log $ "the robot" ++ format rob ++ "is not in my cell";
-        return f;
+        pure f;
       } {
         try {
           reprogram rob { log msg; };
           log $ "successfully reprogrammed robot " ++ format rob;
           give rob "Win";
           log $ "successfully gave Win to robot " ++ format rob;
-          return (\r. (rob != r && f rob));
+          pure (\r. (rob != r && f rob));
         } {
           log $ "the robot " ++ format rob ++ "is missing a logger!";
-          return f;
+          pure f;
         };
       }
     }
diff --git a/data/scenarios/Tutorials/crash.yaml b/data/scenarios/Tutorials/crash.yaml
index 35862c65f..dbdc15dc4 100644
--- a/data/scenarios/Tutorials/crash.yaml
+++ b/data/scenarios/Tutorials/crash.yaml
@@ -26,7 +26,7 @@ objectives:
     condition: |
       try {
         as base {has "Win"}
-      } { return false }
+      } { pure false }
 entities:
   - name: Win
     display:
diff --git a/data/scenarios/Tutorials/def.yaml b/data/scenarios/Tutorials/def.yaml
index 0a93b606c..460252b77 100644
--- a/data/scenarios/Tutorials/def.yaml
+++ b/data/scenarios/Tutorials/def.yaml
@@ -32,7 +32,7 @@ objectives:
     condition: |
       try {
         as base {has "flower"}
-      } { return false }
+      } { pure false }
 solution: |
   def m2 = move; move end;
   def m4 = m2; m2 end;
diff --git a/data/scenarios/Tutorials/equip.yaml b/data/scenarios/Tutorials/equip.yaml
index ff41e6c52..99678515e 100644
--- a/data/scenarios/Tutorials/equip.yaml
+++ b/data/scenarios/Tutorials/equip.yaml
@@ -18,8 +18,8 @@ objectives:
     condition: |
       try {
         _ <- robotNumbered 1;
-        return true;
-      } { return false }
+        pure true;
+      } { pure false }
 solution: |
   turn south; move; grab; equip "3D printer"; build {};
 robots:
diff --git a/data/scenarios/Tutorials/farming.sw b/data/scenarios/Tutorials/farming.sw
index 5481d8cda..89cef5154 100644
--- a/data/scenarios/Tutorials/farming.sw
+++ b/data/scenarios/Tutorials/farming.sw
@@ -27,7 +27,7 @@ def plant_field : Text -> Cmd Unit = \thing.
 end;
 def harvest_field : Text -> Cmd Unit = \thing.
   x4 (
-    x12 (move; ifC (ishere thing) {harvest; return ()} {});
+    x12 (move; ifC (ishere thing) {harvest; pure ()} {});
     next_row
   );
   tL; m4; tR
diff --git a/data/scenarios/Tutorials/farming.yaml b/data/scenarios/Tutorials/farming.yaml
index 1a14ba759..3c662c0e6 100644
--- a/data/scenarios/Tutorials/farming.yaml
+++ b/data/scenarios/Tutorials/farming.yaml
@@ -34,9 +34,9 @@ objectives:
       try {
         as base {
           n <- count "lambda";
-          return (n >= 256)
+          pure (n >= 256)
         }
-      } { return false }
+      } { pure false }
   - goal:
       - Congratulations!  You have completed the most difficult simulated exercise and are ready to begin exploring the new planet in earnest.  Of course there is much more remaining to explore in the world, and many additional programming language features to unlock.
       - |
@@ -45,7 +45,7 @@ objectives:
       - Afterwards, you will return to the menu where you can select "Classic game" for the complete game experience.  Or, play a "Creative game" if you just want to play around with programming robots, without any constraints or need to collect resources.  You could also choose to redo some tutorial scenarios, or explore the other challenge scenarios listed in the menu.
       - Now go forth and build your swarm!
     condition: |
-      try {as base {has "curry"}} {return false}
+      try {as base {has "curry"}} {pure false}
     prerequisite: get_many_lambdas
 solution: |
   run "scenarios/Tutorials/farming.sw";
diff --git a/data/scenarios/Tutorials/give.yaml b/data/scenarios/Tutorials/give.yaml
index c924008ae..368abaefe 100644
--- a/data/scenarios/Tutorials/give.yaml
+++ b/data/scenarios/Tutorials/give.yaml
@@ -16,7 +16,7 @@ objectives:
           b0 <- has "bit (0)";
           lx <- has "LaTeX";
           co <- has "copper ore";
-          return $ bd && b0 && lx && co;
+          pure $ bd && b0 && lx && co;
       };
 solution: |
   build {
diff --git a/data/scenarios/Tutorials/grab.yaml b/data/scenarios/Tutorials/grab.yaml
index 97f5cccc6..d913997e3 100644
--- a/data/scenarios/Tutorials/grab.yaml
+++ b/data/scenarios/Tutorials/grab.yaml
@@ -10,8 +10,8 @@ objectives:
     condition: |
       try {
         t <- as base {count "tree"};
-        return (t >= 6);
-      } { return false }
+        pure (t >= 6);
+      } { pure false }
 solution: |
   move;
   move; grab;
diff --git a/data/scenarios/Tutorials/lambda.yaml b/data/scenarios/Tutorials/lambda.yaml
index 6cc8cbdd6..54f5fa32a 100644
--- a/data/scenarios/Tutorials/lambda.yaml
+++ b/data/scenarios/Tutorials/lambda.yaml
@@ -23,8 +23,8 @@ objectives:
       try {
         teleport self (32,-16);
         b <- ishere "flower";
-        return (not b)
-      } { return false }
+        pure (not b)
+      } { pure false }
 solution: |
   def x4 = \c. c; c; c; c end;
   def m2 = move; move end;
diff --git a/data/scenarios/Tutorials/move.yaml b/data/scenarios/Tutorials/move.yaml
index dcaa2769d..5c63bf365 100644
--- a/data/scenarios/Tutorials/move.yaml
+++ b/data/scenarios/Tutorials/move.yaml
@@ -16,7 +16,7 @@ objectives:
 
         You can open this popup window at any time to remind yourself of the goal using **Ctrl+G**.
     condition: |
-      as base {l <- whereami; return (l == (2,0))}
+      as base {l <- whereami; pure (l == (2,0))}
   - id: move_along_corridor
     teaser: Down the corridor
     goal:
@@ -35,7 +35,7 @@ objectives:
 
         You can open this popup window at any time to remind yourself of the goal using **Ctrl+G**.
     condition: |
-      as base {l <- whereami; return (l == (8,0))}
+      as base {l <- whereami; pure (l == (8,0))}
     prerequisite: move_to_first_flower
   - id: move_northeast_corner
     teaser: To northeast corner
@@ -59,7 +59,7 @@ objectives:
 
         You can open this popup window at any time to remind yourself of the goal using **Ctrl+G**.
     condition: |
-      as base {l <- whereami; return (l == (8,4))}
+      as base {l <- whereami; pure (l == (8,4))}
     prerequisite: move_along_corridor
   - goal:
       - Good job! You are now ready to move and turn on your own.
@@ -67,7 +67,7 @@ objectives:
       - Remember you can press the upward arrow on your keyboard to select previous commands.
       - You can open this popup window at any time to remind yourself of the goal using **Ctrl+G**.
     condition: |
-      as base {l <- whereami; return (l == (8,8))}
+      as base {l <- whereami; pure (l == (8,8))}
     prerequisite: move_northeast_corner
 solution: |
   // 0
@@ -145,7 +145,7 @@ robots:
   - name: 1P wall
     system: true
     program: |
-      def main = \a. return noop end
+      def main = \a. pure noop end
       instant (
         run "scenarios/Tutorials/move_surveil.sw";
         main [entity="wall", room=1]
diff --git a/data/scenarios/Tutorials/move_check.sw b/data/scenarios/Tutorials/move_check.sw
index a00ec1c27..6a8f06220 100644
--- a/data/scenarios/Tutorials/move_check.sw
+++ b/data/scenarios/Tutorials/move_check.sw
@@ -13,7 +13,7 @@ end
 def waitForBaseAt = \l. \get_timeout.
     loc <- as base {whereami};
     if (loc == l) {
-        return true
+        pure true
     } {
         wait (get_timeout l loc);
         waitForBaseAt l get_timeout
diff --git a/data/scenarios/Tutorials/place.yaml b/data/scenarios/Tutorials/place.yaml
index 67a6fe94b..ff5cd977c 100644
--- a/data/scenarios/Tutorials/place.yaml
+++ b/data/scenarios/Tutorials/place.yaml
@@ -30,8 +30,8 @@ objectives:
     condition: |
       try {
         t <- as base {count "spruce"};
-        return (t >= 6);
-      } { return false }
+        pure (t >= 6);
+      } { pure false }
 entities:
   # faster tree (the normal one grows 500-600 ticks)
   - name: spruce
diff --git a/data/scenarios/Tutorials/require.yaml b/data/scenarios/Tutorials/require.yaml
index b33d5000a..56d6f5415 100644
--- a/data/scenarios/Tutorials/require.yaml
+++ b/data/scenarios/Tutorials/require.yaml
@@ -11,7 +11,7 @@ objectives:
     condition: |
       try {
         as base {has "periwinkle"}
-      } { return false }
+      } { pure false }
 entities:
   - name: periwinkle
     display:
diff --git a/data/scenarios/Tutorials/requireinv.yaml b/data/scenarios/Tutorials/requireinv.yaml
index 28b96d70a..faeba4c6d 100644
--- a/data/scenarios/Tutorials/requireinv.yaml
+++ b/data/scenarios/Tutorials/requireinv.yaml
@@ -25,8 +25,8 @@ objectives:
           );
           turn back; move; move; move; move; turn left
         );
-        ifC (has "tree") {return false} {return true}
-      } { return false }
+        ifC (has "tree") {pure false} {pure true}
+      } { pure false }
 solution: |
   def x4 = \c. c;c;c;c end;
   def mp = move; place "rock" end;
diff --git a/data/scenarios/Tutorials/scan.yaml b/data/scenarios/Tutorials/scan.yaml
index 24f5814aa..d2c3fee51 100644
--- a/data/scenarios/Tutorials/scan.yaml
+++ b/data/scenarios/Tutorials/scan.yaml
@@ -12,8 +12,8 @@ objectives:
         bm <- as base {knows "mountain"};
         bt <- as base {knows "tree"};
         bw <- as base {knows "wavy water"};
-        return (bm || bt || bw)
-      } { return false }
+        pure (bm || bt || bw)
+      } { pure false }
 solution: |
   build {
     turn east; move; move; move;
diff --git a/data/scenarios/Tutorials/type-errors.yaml b/data/scenarios/Tutorials/type-errors.yaml
index 75ae57a61..eb3d4dddc 100644
--- a/data/scenarios/Tutorials/type-errors.yaml
+++ b/data/scenarios/Tutorials/type-errors.yaml
@@ -21,8 +21,8 @@ objectives:
     condition: |
       try {
         w <- as base {has "Win"};
-        return (not w);
-      } { return false }
+        pure (not w);
+      } { pure false }
 entities:
   - name: Win
     display:
diff --git a/data/scenarios/Tutorials/types.yaml b/data/scenarios/Tutorials/types.yaml
index 22456996b..78dc4eec3 100644
--- a/data/scenarios/Tutorials/types.yaml
+++ b/data/scenarios/Tutorials/types.yaml
@@ -25,8 +25,8 @@ objectives:
     condition: |
       try {
         w <- as base {has "Win"};
-        return (not w);
-      } { return false }
+        pure (not w);
+      } { pure false }
 entities:
   - name: Win
     display:
diff --git a/data/scenarios/Tutorials/world101.yaml b/data/scenarios/Tutorials/world101.yaml
index 4fb5fed65..25463d0cf 100644
--- a/data/scenarios/Tutorials/world101.yaml
+++ b/data/scenarios/Tutorials/world101.yaml
@@ -13,14 +13,14 @@ objectives:
     condition: |
       try {
         n <- as base {count "tree"};
-        return (n >= 3)
-      } { return false }
+        pure (n >= 3)
+      } { pure false }
   - id: get_harvester
     teaser: Make a harvester
     goal:
       - Nice work!  Now, use the `tree`{=entity}s to make a `harvester`{=entity} device. This will require several intermediate products; try making various things, and take a look at your available recipes (**F3**) and at the recipes listed for items in your inventory.  Of course, you may end up needing some additional trees.
     condition: |
-      try { as base {has "harvester"} } {return false}
+      try { as base {has "harvester"} } {pure false}
     prerequisite: get_trees
   - goal:
       - Now that you have a `harvester`{=entity}, you can use `harvest` instead of `grab` whenever you pick up a growing item (check for the word "growing" at the top of the item description), to leave behind a seed that will regrow.
@@ -28,7 +28,7 @@ objectives:
       - One of the next things you will probably want is a `lambda`{=entity}, so you can define and use parameterized commands.  Scan some things and use the process of elimination to find one.  Since lambdas regrow, once you find one, try getting it with `harvest`.
       - "**TIP:** remember that you can click on cells in the world to see their coordinates."
     condition: |
-      try { as base {has "lambda"} } {return false}
+      try { as base {has "lambda"} } {pure false}
     prerequisite: get_harvester
 solution: |
   run "scenarios/Tutorials/world101.sw"
diff --git a/data/scenarios/Vignettes/_roadway/coordinator.sw b/data/scenarios/Vignettes/_roadway/coordinator.sw
index 78d7451d9..3a9053721 100644
--- a/data/scenarios/Vignettes/_roadway/coordinator.sw
+++ b/data/scenarios/Vignettes/_roadway/coordinator.sw
@@ -9,13 +9,13 @@ def atLocation = \newLoc. \f.
     teleport self newLoc;
     retval <- f;
     teleport self prevLoc;
-    return retval;
+    pure retval;
     end;
 
 def swapItem = \ent.
   create ent;
   emptyHere <- isempty;
-  if emptyHere {} {grab; return ()};
+  if emptyHere {} {grab; pure ()};
   place ent;
   end;
 
diff --git a/data/scenarios/Vignettes/_roadway/drone.sw b/data/scenarios/Vignettes/_roadway/drone.sw
index 9022825e0..dff622755 100644
--- a/data/scenarios/Vignettes/_roadway/drone.sw
+++ b/data/scenarios/Vignettes/_roadway/drone.sw
@@ -58,13 +58,13 @@ def init :
     } in
     turn $ fst locdir;
     teleport self $ snd locdir;
-    return (isLongitudinal, idx);
+    pure (isLongitudinal, idx);
     end;
 
 def isGreenLight = \isLongitudinal.
     r <- robotnamed "stoplight";
     isGreen <- as r {has "bit (1)"};
-    return $ isLongitudinal != isGreen;
+    pure $ isLongitudinal != isGreen;
     end;
 
 def getCanMove :
@@ -89,30 +89,30 @@ def getCanMove :
     eitherNeighbor <- meet;
     // TODO: Make sure we only consider the neighbor directly in front of us.
     neighborIsStopped <- case eitherNeighbor
-        (\_. return false)
+        (\_. pure false)
         (\r. as r {has "bit (0)"}); // zero-bit means stopped
 
-    return $ hasGreenLight || not (atStopLine || neighborIsStopped);
+    pure $ hasGreenLight || not (atStopLine || neighborIsStopped);
     end;
 
 def doTunnelWrap : [xMin : Int, xMax : Int, yMin : Int, yMax : Int] -> Cmd Bool = \extents.
     myloc <- whereami;
     didWrap <- if (fst myloc < extents.xMin) {
         teleport self (extents.xMax, snd myloc);
-        return true;
+        pure true;
     } $ elif (fst myloc > extents.xMax) {
         teleport self (extents.xMin, snd myloc);
-        return true;
+        pure true;
     } $ elif (snd myloc < extents.yMin) {
         teleport self (fst myloc, extents.yMax);
-        return true;
+        pure true;
     } $ elif (snd myloc > extents.yMax) {
         teleport self (fst myloc, extents.yMin);
-        return true;
+        pure true;
     } $ else {
-        return false;
+        pure false;
     };
-    return didWrap;
+    pure didWrap;
     end;
 
 def moveWithWrap :
@@ -129,7 +129,7 @@ def moveWithWrap :
         move;
         doTunnelWrap extents;
     } {
-        return false;
+        pure false;
     };
 
     try {
@@ -137,7 +137,7 @@ def moveWithWrap :
         if canMove {make "bit (1)"} {make "bit (0)"}
     } {};
 
-    return (canMove, wentThroughTunnel);
+    pure (canMove, wentThroughTunnel);
     end;
 
 def getNewDelayState :
diff --git a/data/scenarios/Vignettes/progress-bar.yaml b/data/scenarios/Vignettes/progress-bar.yaml
index 1a0b62f99..658b396d7 100644
--- a/data/scenarios/Vignettes/progress-bar.yaml
+++ b/data/scenarios/Vignettes/progress-bar.yaml
@@ -12,7 +12,7 @@ objectives:
       teleport self (1,-1); c; move; c; move; c; move; c; move; c; move; c; move; c; move; c; move; c; move; c;
       teleport self (1,-5); c; move; c; move; c; move; c; move; c; move; c; move; c; move; c; move; c; move; c;
       teleport self (1,-9); c; move; c; move; c; move; c; move; c; move; c; move; c; move; c; move; c; move; c;
-      return true
+      pure true
 solution: |
   wait 100;
 robots:
diff --git a/editors/emacs/swarm-mode.el b/editors/emacs/swarm-mode.el
index e7d58f913..9cb989177 100644
--- a/editors/emacs/swarm-mode.el
+++ b/editors/emacs/swarm-mode.el
@@ -113,7 +113,7 @@
       "setname"
       "random"
       "run"
-      "return"
+      "pure"
       "try"
       "print"
       "erase"
diff --git a/editors/vim/swarm.vim b/editors/vim/swarm.vim
index 4dc0e76a0..52deb4e2a 100644
--- a/editors/vim/swarm.vim
+++ b/editors/vim/swarm.vim
@@ -1,6 +1,6 @@
 syn keyword Keyword def tydef rec end let in require
 syn keyword Builtins waypoint waypoints hastag tagmembers self parent base if inl inr case fst snd force undefined fail not format read chars split charat tochar key
-syn keyword Command noop wait selfdestruct move backup volume path push stride turn grab harvest sow ignite place ping give equip unequip make has equipped count drill use build salvage reprogram say listen log view appear create halt time scout whereami locateme structures floorplan detect resonate density sniff chirp watch surveil heading blocked scan upload ishere isempty meet meetall whoami setname random run return try print erase swap atomic instant installkeyhandler teleport warp as robotnamed robotnumbered knows
+syn keyword Command noop wait selfdestruct move backup volume path push stride turn grab harvest sow ignite place ping give equip unequip make has equipped count drill use build salvage reprogram say listen log view appear create halt time scout whereami locateme structures floorplan detect resonate density sniff chirp watch surveil heading blocked scan upload ishere isempty meet meetall whoami setname random run pure try print erase swap atomic instant installkeyhandler teleport warp as robotnamed robotnumbered knows
 syn keyword Direction east north west south down forward left back right
 syn match Type "\<[A-Z][a-zA-Z_]*\>"
 syn match Operators "[-=!<>|&+*/^$:]"
diff --git a/editors/vscode/syntaxes/swarm.tmLanguage.yaml b/editors/vscode/syntaxes/swarm.tmLanguage.yaml
index 89d1a7dff..f0d884bfc 100644
--- a/editors/vscode/syntaxes/swarm.tmLanguage.yaml
+++ b/editors/vscode/syntaxes/swarm.tmLanguage.yaml
@@ -46,7 +46,7 @@ repository:
       keyword:
         name: keyword.other
         match: >-
-          \b(waypoint|waypoints|hastag|tagmembers|self|parent|base|if|inl|inr|case|fst|snd|force|undefined|fail|not|format|read|chars|split|charat|tochar|key|noop|wait|selfdestruct|move|backup|volume|path|push|stride|turn|grab|harvest|sow|ignite|place|ping|give|equip|unequip|make|has|equipped|count|drill|use|build|salvage|reprogram|say|listen|log|view|appear|create|halt|time|scout|whereami|locateme|structures|floorplan|detect|resonate|density|sniff|chirp|watch|surveil|heading|blocked|scan|upload|ishere|isempty|meet|meetall|whoami|setname|random|run|return|try|print|erase|swap|atomic|instant|installkeyhandler|teleport|warp|as|robotnamed|robotnumbered|knows)\b
+          \b(waypoint|waypoints|hastag|tagmembers|self|parent|base|if|inl|inr|case|fst|snd|force|undefined|fail|not|format|read|chars|split|charat|tochar|key|noop|wait|selfdestruct|move|backup|volume|path|push|stride|turn|grab|harvest|sow|ignite|place|ping|give|equip|unequip|make|has|equipped|count|drill|use|build|salvage|reprogram|say|listen|log|view|appear|create|halt|time|scout|whereami|locateme|structures|floorplan|detect|resonate|density|sniff|chirp|watch|surveil|heading|blocked|scan|upload|ishere|isempty|meet|meetall|whoami|setname|random|run|pure|try|print|erase|swap|atomic|instant|installkeyhandler|teleport|warp|as|robotnamed|robotnumbered|knows)\b
       require:
         name: keyword.control.require
         match: \b(require)\b
diff --git a/editors/vscode/test/hello.sw b/editors/vscode/test/hello.sw
index db9c9e5f6..cc9b6ad7c 100644
--- a/editors/vscode/test/hello.sw
+++ b/editors/vscode/test/hello.sw
@@ -2,4 +2,4 @@
 
 def hello: Text = "Hello World" end;
 
-return hello;
+pure hello;
diff --git a/editors/vscode/test/hello.sw.snap b/editors/vscode/test/hello.sw.snap
index 4a25ccf5f..29343162a 100644
--- a/editors/vscode/test/hello.sw.snap
+++ b/editors/vscode/test/hello.sw.snap
@@ -18,9 +18,9 @@
 #                                ^^^ source.swarm keyword.control.dictionary.def.end
 #                                   ^ source.swarm keyword.operator.chain
 >
->return hello;
-#^^^^^^ source.swarm keyword.other
-#      ^ source.swarm
-#       ^^^^^ source.swarm variable.parameter
-#            ^ source.swarm keyword.operator.chain
+>pure hello;
+#^^^^ source.swarm keyword.other
+#    ^ source.swarm
+#     ^^^^^ source.swarm variable.parameter
+#          ^ source.swarm keyword.operator.chain
 >
\ No newline at end of file
diff --git a/example/BFS-clear.sw b/example/BFS-clear.sw
index 9448a1cd2..5ceffea77 100644
--- a/example/BFS-clear.sw
+++ b/example/BFS-clear.sw
@@ -13,11 +13,11 @@ def while : Cmd Bool -> Cmd Unit -> Cmd Unit = \test.\c.
 end;
 def getX : Cmd Int =
   pos <- whereami;
-  return (fst pos);
+  pure (fst pos);
 end;
 def getY : Cmd Int =
   pos <- whereami;
-  return (snd pos);
+  pure (snd pos);
 end;
 def gotoX : Int -> Cmd Unit = \tgt.
   cur <- getX;
@@ -47,7 +47,7 @@ def spawnfwd : {Cmd Unit} -> Cmd Unit = \c.
      move;
      b <- isHere "tree";
      if b
-       { build c; return () }
+       { build c; pure () }
        {};
      turn back;
      move
diff --git a/example/cat.sw b/example/cat.sw
index 7d603a7fe..2fab62149 100644
--- a/example/cat.sw
+++ b/example/cat.sw
@@ -6,7 +6,7 @@ let repeat : Int -> Cmd Unit -> Cmd Unit =
   \n. \c. if (n == 0) {} {c ; repeat (n-1) c} in
 let randdir : Cmd Dir =
   d <- random 4;
-  return (
+  pure (
     if (d == 0) {north}
     {if (d == 1) {east}
     {if (d == 2) {south} {west}}})
diff --git a/example/multi-key-handler.sw b/example/multi-key-handler.sw
index 712a6e290..0f47dcec9 100644
--- a/example/multi-key-handler.sw
+++ b/example/multi-key-handler.sw
@@ -5,7 +5,7 @@ def cons : a * b -> (a -> b) -> (a -> b) = \p. \k. \a.
   if (a == fst p) {snd p} {k a}
 end
 
-def nil : a -> Cmd Unit = \a. return () end
+def nil : a -> Cmd Unit = \a. pure () end
 
 // The delay around the first argument is necessary to prevent
 // infinite recursion
diff --git a/example/pilotmode.sw b/example/pilotmode.sw
index 7d19fb184..befa29c42 100644
--- a/example/pilotmode.sw
+++ b/example/pilotmode.sw
@@ -2,7 +2,7 @@ def cons : a * b -> (a -> b) -> (a -> b) = \p. \k. \a.
   if (a == fst p) {snd p} {k a}
 end
 
-def nil : a -> Cmd Unit = \a. return () end
+def nil : a -> Cmd Unit = \a. pure () end
 
 // Suitable to use as e.g.
 //   installKeyHandler "(S-)←↓↑→ [Del] [g]rab [h]arvest [d]rill [s]can [b]locked [u]pload" pilot
@@ -18,8 +18,8 @@ def pilot : Key -> Cmd Unit =
   cons (key "Del",     selfdestruct) $
   cons (key "g",       res <- grab; log res) $
   cons (key "h",       res <- harvest; log res) $
-  cons (key "d",       res <- drill forward; case res (\_. return ()) log) $
-  cons (key "s",       res <- scan forward; case res (\_. return ()) log) $
+  cons (key "d",       res <- drill forward; case res (\_. pure ()) log) $
+  cons (key "s",       res <- scan forward; case res (\_. pure ()) log) $
   cons (key "b",       b <- blocked; if b {log "blocked"} {log "not blocked"}) $
   cons (key "u",       upload base) $
   nil
diff --git a/example/recursive-containers.sw b/example/recursive-containers.sw
index 697cccd21..121ee6d30 100644
--- a/example/recursive-containers.sw
+++ b/example/recursive-containers.sw
@@ -535,7 +535,7 @@ end
 def group = \i. \m. \tests.
   log $ indent i ++ "START " ++ m ++ ":";
   tests $ i + 1;
-  return ()
+  pure ()
 end
 
 def test_empty: Int -> Cmd Unit = \i.
@@ -585,7 +585,7 @@ def test_insert: Int -> Cmd Unit = \i.
 end
 
 def randomTestS: Int -> Set Int -> (Set Int -> Cmd a) -> Cmd (Set Int) = \i.\s.\test.
-  if (i <= 0) {return s} {
+  if (i <= 0) {pure s} {
     x <- random 20;
     let ns = tree_set.insert x s in
     test ns;
@@ -650,7 +650,7 @@ def benchmark: Int -> s -> (s -> s) -> Cmd (Int * (Int * Int)) = \times.\s.\act.
   let max = \x.\y. if (x > y) {x} {y} in
   let runM: (Int * Maybe (Int * Int)) -> s -> Int -> Cmd (Int * Maybe (Int * Int)) = \acc.\s.\n.
     if (n <= 0) {
-      return acc
+      pure acc
     } {
       t0 <- time;
       //log $ "START " ++ format t0;
@@ -667,7 +667,7 @@ def benchmark: Int -> s -> (s -> s) -> Cmd (Int * (Int * Int)) = \times.\s.\act.
   //log "end run";
   let avg = fst res / times in
   let lim = case (snd res) (\_. fail "BENCHMARK NOT RUN") (\l.l) in
-  return (avg, lim)
+  pure (avg, lim)
 end
 
 def cmp_bench : Int -> Text -> (Int * (Int * Int)) -> Text -> (Int * (Int * Int)) -> Cmd Unit
@@ -691,7 +691,7 @@ end
 // Get a list of random integers of given length and maximum element number
 def gen_random_list: Int -> Int -> Cmd (List Int) = 
   let gen = \acc.\n.\rlim.
-    if (n <= 0) { return acc } {
+    if (n <= 0) { pure acc } {
       x <- random rlim;
       gen (cons x acc) (n - 1) rlim
     }
@@ -701,7 +701,7 @@ end
 // Get a number of lists of random integers of given length and maximum element number
 def gen_random_lists: Int -> Int -> Int -> Cmd (List (List Int)) = \m.\n.\rlim.
   let gen = \acc.\m.
-    if (m <= 0) { return acc } {
+    if (m <= 0) { pure acc } {
       l <- gen_random_list n rlim;
       gen (cons l acc) (m - 1)
     }
@@ -775,4 +775,4 @@ def benchmark_tree: Cmd Unit =
     bench_contains i;
   );
   log "ALL BENCHMARKS DONE!"
-end
\ No newline at end of file
+end
diff --git a/src/swarm-doc/Swarm/Doc/Pedagogy.hs b/src/swarm-doc/Swarm/Doc/Pedagogy.hs
index ae61d27fe..ec85aecdd 100644
--- a/src/swarm-doc/Swarm/Doc/Pedagogy.hs
+++ b/src/swarm-doc/Swarm/Doc/Pedagogy.hs
@@ -120,7 +120,7 @@ getDescCommands s = S.fromList $ concatMap filterConst allCode
 isConsidered :: Const -> Bool
 isConsidered c = isUserFunc c && c `S.notMember` ignoredCommands
  where
-  ignoredCommands = S.fromList [Run, Return, Noop, Force]
+  ignoredCommands = S.fromList [Run, Pure, Noop, Force]
 
 -- | Extract the command names from the source code of the solution.
 --
diff --git a/src/swarm-engine/Swarm/Game/CESK.hs b/src/swarm-engine/Swarm/Game/CESK.hs
index aa4abbca5..4ae0369e8 100644
--- a/src/swarm-engine/Swarm/Game/CESK.hs
+++ b/src/swarm-engine/Swarm/Game/CESK.hs
@@ -320,7 +320,7 @@ cont = lens get set
 -- | Create a brand new CESK machine, with empty environment and
 --   store, to evaluate a given term.  We always initialize the
 --   machine with a single FExec frame as the continuation; if the
---   given term does not have a command type, we wrap it in @return@.
+--   given term does not have a command type, we wrap it in @pure@.
 initMachine :: TSyntax -> CESK
 initMachine t = In (prepareTerm mempty t) mempty emptyStore [FExec]
 
@@ -353,14 +353,14 @@ continue t = \case
 
 -- | Prepare a term for evaluation by a CESK machine in the given
 --   environment: erase all type annotations, and optionally wrap it
---   in @return@ if it does not have a command type.  Note that since
+--   in @pure@ if it does not have a command type.  Note that since
 --   the environment might contain type aliases, we have to be careful
 --   to expand them before concluding whether the term has a command
 --   type or not.
 prepareTerm :: Env -> TSyntax -> Term
 prepareTerm e t = case whnfType (e ^. envTydefs) (ptBody (t ^. sType)) of
   TyCmd _ -> t'
-  _ -> TApp (TConst Return) t'
+  _ -> TApp (TConst Pure) t'
  where
   t' = eraseS t
 
diff --git a/src/swarm-engine/Swarm/Game/Step/Const.hs b/src/swarm-engine/Swarm/Game/Step/Const.hs
index 28b096f99..0bce30dcb 100644
--- a/src/swarm-engine/Swarm/Game/Step/Const.hs
+++ b/src/swarm-engine/Swarm/Game/Step/Const.hs
@@ -143,7 +143,7 @@ execConst runChildProg c vs s k = do
   -- Now proceed to actually carry out the operation.
   case c of
     Noop -> return $ mkReturn ()
-    Return -> case vs of
+    Pure -> case vs of
       [v] -> return $ Out v s k
       _ -> badConst
     Wait -> case vs of
diff --git a/src/swarm-lang/Swarm/Language/Capability.hs b/src/swarm-lang/Swarm/Language/Capability.hs
index 62380b40c..cb88283c8 100644
--- a/src/swarm-lang/Swarm/Language/Capability.hs
+++ b/src/swarm-lang/Swarm/Language/Capability.hs
@@ -120,7 +120,7 @@ constCaps = \case
   Knows -> Nothing
   Noop -> Nothing
   Parent -> Nothing
-  Return -> Nothing
+  Pure -> Nothing
   Say -> Nothing -- speaking is natural to robots (unlike listening)
   Setname -> Nothing
   Undefined -> Nothing
diff --git a/src/swarm-lang/Swarm/Language/Elaborate.hs b/src/swarm-lang/Swarm/Language/Elaborate.hs
index c63d2f2cc..954f28391 100644
--- a/src/swarm-lang/Swarm/Language/Elaborate.hs
+++ b/src/swarm-lang/Swarm/Language/Elaborate.hs
@@ -74,8 +74,8 @@ insertSuspend t = case t of
   TTydef x pty mtd t1 -> TTydef x pty mtd (insertSuspend t1)
   TBind mx mty mreq c1 c2 -> TBind mx mty mreq c1 (insertSuspend c2)
   TAnnotate t1 ty -> TAnnotate (insertSuspend t1) ty
-  -- Replace return or noop with suspend
-  TApp (TConst Return) t1 -> TSuspend t1
+  -- Replace pure or noop with suspend
+  TApp (TConst Pure) t1 -> TSuspend t1
   TConst Noop -> TSuspend TUnit
   -- Anything else: p => (__res__ <- p; suspend __res__)
   --
diff --git a/src/swarm-lang/Swarm/Language/Parser/Core.hs b/src/swarm-lang/Swarm/Language/Parser/Core.hs
index c82797090..3f688b268 100644
--- a/src/swarm-lang/Swarm/Language/Parser/Core.hs
+++ b/src/swarm-lang/Swarm/Language/Parser/Core.hs
@@ -57,7 +57,7 @@ data Antiquoting = AllowAntiquoting | DisallowAntiquoting
 -- | Which version of the Swarm language are we parsing?  As a general
 --   rule, we want to support one older version in addition to the
 --   current version, to allow for upgrading code via @swarm format@.
-data LanguageVersion = SwarmLang0_5 | SwarmLangLatest
+data LanguageVersion = SwarmLang0_6 | SwarmLangLatest
   deriving (Eq, Ord, Show, Enum, Bounded)
 
 -- | Read-only parser configuration.
diff --git a/src/swarm-lang/Swarm/Language/Parser/Lex.hs b/src/swarm-lang/Swarm/Language/Parser/Lex.hs
index 1568fb878..34113de5e 100644
--- a/src/swarm-lang/Swarm/Language/Parser/Lex.hs
+++ b/src/swarm-lang/Swarm/Language/Parser/Lex.hs
@@ -45,7 +45,7 @@ module Swarm.Language.Parser.Lex (
   brackets,
 ) where
 
-import Control.Lens (use, view, (%=), (.=))
+import Control.Lens (use, (%=), (.=))
 import Control.Monad (void)
 import Data.Char (isLower, isUpper)
 import Data.Containers.ListUtils (nubOrd)
@@ -176,12 +176,6 @@ reservedWords =
       ++ primitiveTypeNames
       ++ keywords
 
--- | Cached version of the reserved words list with everything
---   lowercase, for use in parsing version 0.5 of the language, where
---   types were lowercase instead of uppercase.
-lowerReservedWords :: Set Text
-lowerReservedWords = S.map T.toLower reservedWords
-
 -- | Parse a reserved word, given a string recognizer (which can
 --   /e.g./ be case sensitive or not), making sure it is not a prefix
 --   of a longer variable name, and allowing the parser to backtrack
@@ -203,32 +197,26 @@ data IdentifierType = IDTyVar | IDTyName | IDTmVar
 
 -- | Parse an identifier together with its source location info.
 locIdentifier :: IdentifierType -> Parser LocVar
-locIdentifier idTy = do
-  ver <- view languageVersion
-  uncurry LV <$> parseLocG ((lexeme . try) (p >>= check ver) <?> "variable name")
+locIdentifier idTy =
+  uncurry LV <$> parseLocG ((lexeme . try) (p >>= check) <?> "variable name")
  where
   p = (:) <$> (letterChar <|> char '_') <*> many (alphaNumChar <|> char '_' <|> char '\'')
-  check ver (into @Text -> t) = case ver of
-    SwarmLang0_5
-      | T.toLower t `S.member` lowerReservedWords ->
-          failT ["reserved word", squote t, "cannot be used as variable name"]
-      | otherwise -> return t
-    SwarmLangLatest
-      | IDTyVar <- idTy
-      , T.toTitle t `S.member` reservedWords ->
-          failT ["Reserved type name", squote t, "cannot be used as a type variable name; perhaps you meant", squote (T.toTitle t) <> "?"]
-      | IDTyName <- idTy
-      , t `S.member` reservedWords ->
-          failT ["Reserved type name", squote t, "cannot be redefined."]
-      | t `S.member` reservedWords || T.toLower t `S.member` reservedWords ->
-          failT ["Reserved word", squote t, "cannot be used as a variable name"]
-      | IDTyName <- idTy
-      , isLower (T.head t) ->
-          failT ["Type synonym names must start with an uppercase letter"]
-      | IDTyVar <- idTy
-      , isUpper (T.head t) ->
-          failT ["Type variable names must start with a lowercase letter"]
-      | otherwise -> return t
+  check (into @Text -> t)
+    | IDTyVar <- idTy
+    , T.toTitle t `S.member` reservedWords =
+        failT ["Reserved type name", squote t, "cannot be used as a type variable name; perhaps you meant", squote (T.toTitle t) <> "?"]
+    | IDTyName <- idTy
+    , t `S.member` reservedWords =
+        failT ["Reserved type name", squote t, "cannot be redefined."]
+    | t `S.member` reservedWords || T.toLower t `S.member` reservedWords =
+        failT ["Reserved word", squote t, "cannot be used as a variable name"]
+    | IDTyName <- idTy
+    , isLower (T.head t) =
+        failT ["Type synonym names must start with an uppercase letter"]
+    | IDTyVar <- idTy
+    , isUpper (T.head t) =
+        failT ["Type variable names must start with a lowercase letter"]
+    | otherwise = return t
 
 -- | Parse a term variable together with its source location info.
 locTmVar :: Parser LocVar
diff --git a/src/swarm-lang/Swarm/Language/Parser/Term.hs b/src/swarm-lang/Swarm/Language/Parser/Term.hs
index c71e72dbb..1d8affd21 100644
--- a/src/swarm-lang/Swarm/Language/Parser/Term.hs
+++ b/src/swarm-lang/Swarm/Language/Parser/Term.hs
@@ -43,10 +43,17 @@ parseDirection = asum (map alternative allDirs) <?> "direction constant"
 
 -- | Parse Const as reserved words (e.g. @Fail <$ reserved "fail"@)
 parseConst :: Parser Const
-parseConst = asum (map alternative consts) <?> "built-in user function"
+parseConst = do
+  ver <- view languageVersion
+  asum (map (alternative ver) consts) <?> "built-in user function"
  where
   consts = filter isUserFunc allConst
-  alternative c = c <$ reserved (syntax $ constInfo c)
+  alternative ver c = c <$ reserved (syntax $ constInfo' ver c)
+
+  -- Version 0.6 of the language had a constant named @return@, which
+  -- is now renamed to @pure@
+  constInfo' SwarmLang0_6 Pure = (constInfo Pure) {syntax = "return"}
+  constInfo' _ c = constInfo c
 
 -- | Parse an atomic term, optionally trailed by record projections like @t.x.y.z@.
 --   Record projection binds more tightly than function application.
@@ -136,7 +143,7 @@ parseTerm = sepEndBy1 parseStmt (symbol ";") >>= mkBindChain
 
 mkBindChain :: [Stmt] -> Parser Syntax
 mkBindChain stmts = case last stmts of
-  Binder x _ -> return $ foldr mkBind (STerm (TApp (TConst Return) (TVar (lvVar x)))) stmts
+  Binder x _ -> return $ foldr mkBind (STerm (TApp (TConst Pure) (TVar (lvVar x)))) stmts
   BareTerm t -> return $ foldr mkBind t (init stmts)
  where
   mkBind (BareTerm t1) t2 = loc Nothing t1 t2 $ SBind Nothing Nothing Nothing Nothing t1 t2
diff --git a/src/swarm-lang/Swarm/Language/Parser/Type.hs b/src/swarm-lang/Swarm/Language/Parser/Type.hs
index 1ba7cb5b6..6c6e38cb0 100644
--- a/src/swarm-lang/Swarm/Language/Parser/Type.hs
+++ b/src/swarm-lang/Swarm/Language/Parser/Type.hs
@@ -13,13 +13,12 @@ module Swarm.Language.Parser.Type (
   parseTyCon,
 ) where
 
-import Control.Lens (view)
 import Control.Monad.Combinators (many)
 import Control.Monad.Combinators.Expr (Operator (..), makeExprParser)
 import Data.Fix (Fix (..), foldFix)
 import Data.List.Extra (enumerate)
 import Data.Maybe (fromMaybe)
-import Swarm.Language.Parser.Core (LanguageVersion (..), Parser, languageVersion)
+import Swarm.Language.Parser.Core (Parser)
 import Swarm.Language.Parser.Lex (
   braces,
   brackets,
@@ -78,14 +77,8 @@ parseTypeAtom =
 -- | A type constructor.
 parseTyCon :: Parser TyCon
 parseTyCon = do
-  ver <- view languageVersion
-  let reservedCase = case ver of
-        -- Version 0.5 of the language accepted type names in any case
-        SwarmLang0_5 -> reserved
-        -- The latest version requires them to be uppercase
-        SwarmLangLatest -> reservedCS
-  choice (map (\b -> TCBase b <$ reservedCase (baseTyName b)) enumerate)
-    <|> TCCmd <$ reservedCase "Cmd"
+  choice (map (\b -> TCBase b <$ reservedCS (baseTyName b)) enumerate)
+    <|> TCCmd <$ reservedCS "Cmd"
     <|> TCUser <$> tyName
 
 -- | Close over a recursive type, replacing any bound occurrences
diff --git a/src/swarm-lang/Swarm/Language/Syntax/Constants.hs b/src/swarm-lang/Swarm/Language/Syntax/Constants.hs
index 9e4b813f3..b732e16a1 100644
--- a/src/swarm-lang/Swarm/Language/Syntax/Constants.hs
+++ b/src/swarm-lang/Swarm/Language/Syntax/Constants.hs
@@ -226,8 +226,8 @@ data Const
     Snd
   | -- | Force a delayed evaluation.
     Force
-  | -- | Return for the cmd monad.
-    Return
+  | -- | Pure for the cmd monad.
+    Pure
   | -- | Try/catch block
     Try
   | -- | Undefined
@@ -790,7 +790,7 @@ constInfo c = case c of
     command 1 Intangible . doc (Set.singleton $ Query PRNG) "Get a uniformly random integer." $
       ["The random integer will be chosen from the range 0 to n-1, exclusive of the argument."]
   Run -> command 1 long $ shortDoc (Set.singleton $ Mutation $ RobotChange BehaviorChange) "Run a program loaded from a file."
-  Return -> command 1 Intangible $ shortDoc Set.empty "Make the value a result in `cmd`."
+  Pure -> command 1 Intangible $ shortDoc Set.empty "Create a pure `Cmd a`{=type} computation that yields the given value."
   Try -> command 2 Intangible $ shortDoc Set.empty "Execute a command, catching errors."
   Undefined -> function 0 $ shortDoc Set.empty "A value of any type, that is evaluated as error."
   Fail -> function 1 $ shortDoc Set.empty "A value of any type, that is evaluated as error with message."
diff --git a/src/swarm-lang/Swarm/Language/Typecheck.hs b/src/swarm-lang/Swarm/Language/Typecheck.hs
index 0f08dece5..109993f64 100644
--- a/src/swarm-lang/Swarm/Language/Typecheck.hs
+++ b/src/swarm-lang/Swarm/Language/Typecheck.hs
@@ -58,7 +58,7 @@ import Control.Effect.Throw
 import Control.Lens (view, (^.))
 import Control.Lens.Indexed (itraverse)
 import Control.Monad (forM_, void, when, (<=<), (>=>))
-import Control.Monad.Free (Free (..))
+import Control.Monad.Free qualified as Free
 import Data.Data (Data, gmapM)
 import Data.Foldable (fold, traverse_)
 import Data.Functor.Identity
@@ -289,7 +289,7 @@ instance FreeUVars UCtx where
 
 -- | Generate a fresh unification variable.
 fresh :: Has Unification sig m => m UType
-fresh = Pure <$> U.freshIntVar
+fresh = Free.Pure <$> U.freshIntVar
 
 -- | Perform a substitution over a 'UType', substituting for both type
 --   and unification variables.  Note that since 'UType's do not have
@@ -298,10 +298,10 @@ fresh = Pure <$> U.freshIntVar
 substU :: Map (Either Var IntVar) UType -> UType -> UType
 substU m =
   ucata
-    (\v -> fromMaybe (Pure v) (M.lookup (Right v) m))
+    (\v -> fromMaybe (Free.Pure v) (M.lookup (Right v) m))
     ( \case
         TyVarF v -> fromMaybe (UTyVar v) (M.lookup (Left v) m)
-        f -> Free f
+        f -> Free.Free f
     )
 
 -- | Make sure none of the given skolem variables have escaped.
@@ -1080,7 +1080,7 @@ inferConst c = run . runReader @TVCtx Ctx.empty . quantify $ case c of
   Fst -> [tyQ| a * b -> a |]
   Snd -> [tyQ| a * b -> b |]
   Force -> [tyQ| {a} -> a |]
-  Return -> [tyQ| a -> Cmd a |]
+  Pure -> [tyQ| a -> Cmd a |]
   Try -> [tyQ| {Cmd a} -> {Cmd a} -> Cmd a |]
   Undefined -> [tyQ| a |]
   Fail -> [tyQ| Text -> a |]
@@ -1226,8 +1226,8 @@ check s@(CSyntax l t cs) expected = addLocToTypeErr l $ case t of
 
     -- If we are checking a 'def', ensure t2 has a command type.  This ensures that
     -- something like 'def ... end; x + 3' is not allowed, since this
-    -- would result in the whole thing being wrapped in return, like
-    -- 'return (def ... end; x + 3)', which means the def would be local and
+    -- would result in the whole thing being wrapped in pure, like
+    -- 'pure (def ... end; x + 3)', which means the def would be local and
     -- not persist to the next REPL input, which could be surprising.
     --
     -- On the other hand, 'let x = y in x + 3' is perfectly fine.
@@ -1250,7 +1250,7 @@ check s@(CSyntax l t cs) expected = addLocToTypeErr l $ case t of
     -- terms if the environment holds not only a value but also a type
     -- + requirements for them.  For example:
     --
-    -- > def x : Int = 3 end; return (x + 2)
+    -- > def x : Int = 3 end; pure (x + 2)
     -- 5
     -- > x
     -- 3
@@ -1258,7 +1258,7 @@ check s@(CSyntax l t cs) expected = addLocToTypeErr l $ case t of
     -- 5
     -- > y
     -- 1:1: Unbound variable y
-    -- > let y = 3 in def x = 5 end; return (x + y)
+    -- > let y = 3 in def x = 5 end; pure (x + y)
     -- 8
     -- > y
     -- 1:1: Unbound variable y
@@ -1490,5 +1490,5 @@ isSimpleUType = \case
   UTyCmd {} -> False
   UTyDelay {} -> False
   -- Make the pattern-match coverage checker happy
-  Pure {} -> False
-  Free {} -> False
+  Free.Pure {} -> False
+  Free.Free {} -> False
diff --git a/test/unit/TestEval.hs b/test/unit/TestEval.hs
index 3ae51b058..9632020cf 100644
--- a/test/unit/TestEval.hs
+++ b/test/unit/TestEval.hs
@@ -206,19 +206,19 @@ testEval g =
             ("fail \"foo\"" `throwsError` ("foo" `T.isInfixOf`))
         , testCase
             "try / no exception 1"
-            ("try {return 1} {return 2}" `evaluatesTo` VInt 1)
+            ("try {pure 1} {pure 2}" `evaluatesTo` VInt 1)
         , testCase
             "try / no exception 2"
-            ("try {return 1} {let x = x in x}" `evaluatesTo` VInt 1)
+            ("try {pure 1} {let x = x in x}" `evaluatesTo` VInt 1)
         , testCase
             "try / fail"
-            ("try {fail \"foo\"} {return 3}" `evaluatesTo` VInt 3)
+            ("try {fail \"foo\"} {pure 3}" `evaluatesTo` VInt 3)
         , testCase
             "try / fail / fail"
             ("try {fail \"foo\"} {fail \"bar\"}" `throwsError` ("bar" `T.isInfixOf`))
         , testCase
             "try / div by 0"
-            ("try {return (1/0)} {return 3}" `evaluatesTo` VInt 3)
+            ("try {pure (1/0)} {pure 3}" `evaluatesTo` VInt 3)
         ]
     , testGroup
         "text"
@@ -446,33 +446,33 @@ testEval g =
         "scope - #681"
         [ testCase
             "binder in local scope"
-            ("def f = a <- scan down end; let a = 2 in f; return (a+1)" `evaluatesTo` VInt 3)
+            ("def f = a <- scan down end; let a = 2 in f; pure (a+1)" `evaluatesTo` VInt 3)
         , testCase
             "binder in local scope, no type change"
-            ("def f = a <- return 1 end; let a = 2 in f; return a" `evaluatesTo` VInt 2)
+            ("def f = a <- pure 1 end; let a = 2 in f; pure a" `evaluatesTo` VInt 2)
         , testCase
             "repeat with scan"
-            ("def x = \\n. \\c. if (n==0) {} {c; x (n-1) c} end; x 10 ( c <- scan down; case c (\\_. say \"Hi\") (\\_. return ()))" `evaluatesTo` VUnit)
+            ("def x = \\n. \\c. if (n==0) {} {c; x (n-1) c} end; x 10 ( c <- scan down; case c (\\_. say \"Hi\") (\\_. pure ()))" `evaluatesTo` VUnit)
         , testCase
             "nested recursion with binder - #1032"
-            ("def go = \\n. if (n > 0) {i <- return n; s <- go (n-1); return (s+i)} {return 0} end; go 4" `evaluatesTo` VInt 10)
+            ("def go = \\n. if (n > 0) {i <- pure n; s <- go (n-1); pure (s+i)} {pure 0} end; go 4" `evaluatesTo` VInt 10)
         , testCase
             "binder in local scope - #1796"
-            ("def x = \\x.x end; def foo = x <- return 0 end; foo; return (x 42)" `evaluatesTo` VInt 42)
+            ("def x = \\x.x end; def foo = x <- pure 0 end; foo; pure (x 42)" `evaluatesTo` VInt 42)
         ]
     , testGroup
         "nesting"
         [ testCase
             "def nested in def"
-            ("def x : Cmd Int = def y : Int = 3 end; return (y + 2) end; x" `evaluatesTo` VInt 5)
+            ("def x : Cmd Int = def y : Int = 3 end; pure (y + 2) end; x" `evaluatesTo` VInt 5)
         , testCase
             "nested def does not escape"
-            ( "def z = 1 end; def x = def z = 3 end; return (z + 2) end; n <- x; return (n + z)"
+            ( "def z = 1 end; def x = def z = 3 end; pure (z + 2) end; n <- x; pure (n + z)"
                 `evaluatesTo` VInt 6
             )
         , testCase
             "nested tydef"
-            ( "def x = (tydef X = Int end; def z : X = 3 end; return (z + 2)) end; x"
+            ( "def x = (tydef X = Int end; def z : X = 3 end; pure (z + 2)) end; x"
                 `evaluatesTo` VInt 5
             )
         ]
diff --git a/test/unit/TestLanguagePipeline.hs b/test/unit/TestLanguagePipeline.hs
index 75404dac3..a6015738f 100644
--- a/test/unit/TestLanguagePipeline.hs
+++ b/test/unit/TestLanguagePipeline.hs
@@ -220,10 +220,10 @@ testLanguagePipeline =
             )
         , testCase
             "grabif"
-            (valid "def grabif : Text -> Cmd Unit = \\x. atomic (b <- ishere x; if b {grab; return ()} {}) end")
+            (valid "def grabif : Text -> Cmd Unit = \\x. atomic (b <- ishere x; if b {grab; pure ()} {}) end")
         , testCase
             "placeif"
-            (valid "def placeif : Text -> Cmd Bool = \\thing. atomic (res <- scan down; if (res == inl ()) {place thing; return true} {return false}) end")
+            (valid "def placeif : Text -> Cmd Bool = \\thing. atomic (res <- scan down; if (res == inl ()) {place thing; pure true} {pure false}) end")
         , testCase
             "atomic move+move"
             ( process
@@ -568,13 +568,13 @@ testLanguagePipeline =
         "generalize top-level binds #351 #1501"
         [ testCase
             "top-level polymorphic bind is OK"
-            (valid "r <- return (\\x.x)")
+            (valid "r <- pure (\\x.x)")
         , testCase
             "top-level bind is polymorphic"
-            (valid "f <- return (\\x.x); return (f 3, f \"hi\")")
+            (valid "f <- pure (\\x.x); pure (f 3, f \"hi\")")
         , testCase
             "local bind is polymorphic"
-            (valid "def foo : Cmd (Int * Text) = f <- return (\\x.x); return (f 3, f \"hi\") end")
+            (valid "def foo : Cmd (Int * Text) = f <- pure (\\x.x); pure (f 3, f \"hi\") end")
         ]
     , testGroup
         "type synonyms"
@@ -703,7 +703,7 @@ testLanguagePipeline =
             (valid "let x = 3 in x + 2")
         , testCase
             "let at cmd type"
-            (valid "let x = 3 in move; return (x+2)")
+            (valid "let x = 3 in move; pure (x+2)")
         , testCase
             "def at non-cmd type"
             ( process
@@ -712,7 +712,7 @@ testLanguagePipeline =
             )
         , testCase
             "def at cmd type"
-            (valid "def x = 3 end; move; return (x+2)")
+            (valid "def x = 3 end; move; pure (x+2)")
         ]
     , testGroup
         "nested let/def/annot #2101"