diff --git a/lib/std/core.kk b/lib/std/core.kk
index e47034a6a..1a8ae1969 100644
--- a/lib/std/core.kk
+++ b/lib/std/core.kk
@@ -160,6 +160,7 @@ type exception
 type open exception-info {
   Error
   Assert
+  Unreachable
   Todo
   Range
   Pattern( location : string, definition : string )
@@ -2583,7 +2584,11 @@ fun assert( message : string, condition : bool ) : () {
   if (!condition) unsafe-noexn{ throw(message,Assert) }
 }
 
-fun todo( message : string ) : () {
+fun unreachable() : a {
+  unsafe-noexn{ throw("BUG: this should never happen",Unreachable) }
+}
+
+fun todo( message : string ) : a {
   unsafe-noexn{ throw(message,Todo) }
 }
 
diff --git a/lib/std/data/red-black-tree.kk b/lib/std/data/red-black-tree.kk
new file mode 100644
index 000000000..ec09dff8c
--- /dev/null
+++ b/lib/std/data/red-black-tree.kk
@@ -0,0 +1,256 @@
+/*---------------------------------------------------------------------------
+  Copyright 2017 Microsoft Corporation.
+
+  This is free software; you can redistribute it and/or modify it under the
+  terms of the Apache License, Version 2.0. A copy of the License can be
+  found in the file "license.txt" at the root of this distribution.
+---------------------------------------------------------------------------*/
+
+// Adapted from https://github.com/sweirich/dth/blob/b888942f33425a871d46105862683e8950d15b23/examples/red-black/RedBlack.lhs
+// TODO unit tests
+module std/data/red-black-tree
+
+type color {
+  Red
+  Black
+}
+
+abstract type tree<a> {
+  Empty
+  Node(left: tree<a>, value: a, right: tree<a>, color: color)
+}
+
+
+public val empty: forall<a> tree<a> = Empty
+
+
+public fun single(value: a): tree<a> {
+  Node(empty, value, empty, Black)
+}
+
+
+// This is unsafe because it's possible to use different compare functions on the same tree
+public fun unsafe-lookup(tree: tree<a>, key: b, compare: (b, a) -> e order): e maybe<a> {
+  match (tree) {
+    Node(left, value, right, _) ->
+      match (compare(key, value)) {
+        Lt -> unsafe-lookup(left, key, compare)
+        Eq -> Just(value)
+        Gt -> unsafe-lookup(right, key, compare)
+      }
+
+    Empty ->
+      Nothing
+  }
+}
+
+
+// TODO implement this more efficiently
+public fun find-first(tree: tree<a>, fn: (a) -> e maybe<b>): e maybe<b> {
+  match (tree) {
+    Node(left, value, right, _) -> match (find-first(left, fn)) {
+      Nothing -> match (fn(value)) {
+        Nothing -> find-first(right, fn)
+        a -> a
+      }
+      a -> a
+    }
+    Empty -> Nothing
+  }
+}
+
+
+public fun foldl(tree: tree<a>, initial: b, fn: (b, a) -> e b): e b {
+  match (tree) {
+    Node(left, value, right, _) -> foldl(right, fn(foldl(left, initial, fn), value), fn)
+    Empty -> initial
+  }
+}
+
+
+public fun foldr(tree: tree<a>, initial: b, fn: (a, b) -> e b): e b {
+  match (tree) {
+    Node(left, value, right, _) -> foldr(left, fn(value, foldr(right, initial, fn)), fn)
+    Empty -> initial
+  }
+}
+
+
+public fun map(tree: tree<a>, fn: (a) -> e b): e tree<b> {
+  match (tree) {
+    // TODO this can be parallelized
+    Node(left, value, right, color) -> Node(map(left, fn), fn(value), map(right, fn), color)
+    Empty -> empty
+  }
+}
+
+
+public fun to-list(tree: tree<a>): list<a> {
+  tree.foldr([], Cons)
+}
+
+
+// TODO should this be called unsafe ?
+public fun from-list(list: list<a>, map: (a) -> e b, compare: (b, a) -> e order): e tree<a> {
+  list.foldl(empty) fun(l, r) {
+    unsafe-insert(l, map(r), r, compare, True)
+  }
+}
+
+
+fun balance(tree: tree<a>): tree<a> {
+  match (tree) {
+    Node(Node(Node(a, x, b, Red), y, c, Red), z, d, Black) ->
+    Node(Node(a, x, b, Black), y, Node(c, z, d, Black), Red)
+
+    Node(Node(a, x, Node(b, y, c, Red), Red), z, d, Black) ->
+    Node(Node(a, x, b, Black), y, Node(c, z, d, Black), Red)
+
+    Node(a, x, Node(Node(b, y, c, Red), z, d, Red), Black) ->
+    Node(Node(a, x, b, Black), y, Node(c, z, d, Black), Red)
+
+    Node(a, x, Node(b, y, Node(c, z, d, Red), Red), Black) ->
+    Node(Node(a, x, b, Black), y, Node(c, z, d, Black), Red)
+
+    a -> a
+  }
+}
+
+fun balance-left(left: tree<a>, value: a, right: tree<a>): tree<a> {
+  match (left) {
+    Node(a, x, b, Red) ->
+      Node(Node(a, x, b, Black), value, right, Red)
+
+    _ -> match (right) {
+      Node(a, y, b, Black) ->
+        balance(Node(left, value, Node(a, y, b, Red), Black))
+
+      Node(Node(a, y, b, Black), z, c, Red) ->
+        Node(Node(left, value, a, Black), y, balance(Node(b, z, redden(c), Black)), Red)
+
+      _ -> unreachable()
+    }
+  }
+}
+
+fun balance-right(left: tree<a>, value: a, right: tree<a>): tree<a> {
+  match (right) {
+    Node(b, y, c, Red) ->
+      Node(left, value, Node(b, y, c, Black), Red)
+
+    _ -> match (left) {
+      Node(a, x, b, Black) ->
+        balance(Node(Node(a, x, b, Red), value, right, Black))
+
+      Node(a, x, Node(b, y, c, Black), Red) ->
+        Node(balance(Node(redden(a), x, b, Black)), y, Node(c, value, right, Black), Red)
+
+      _ -> unreachable()
+    }
+  }
+}
+
+fun unsafe-merge(left: tree<a>, right: tree<a>): tree<a> {
+  // TODO replace this with multi-pattern matching ?
+  match (left) {
+    Empty -> right
+
+    Node(a, x, b, Red) -> match (right) {
+      Empty -> left
+
+      Node(c, y, d, Red) ->
+        match (unsafe-merge(b, c)) {
+          Node(b1, z, c1, Red) -> Node(Node(a, x, b1, Red), z, Node(c1, y, d, Red), Red)
+          bc -> Node(a, x, Node(bc, y, d, Red), Red)
+        }
+
+      _ -> Node(a, x, unsafe-merge(b, right), Red)
+    }
+
+    Node(a, x, b, Black) -> match (right) {
+      Empty -> left
+
+      Node(c, y, d, Black) ->
+        match (unsafe-merge(b, c)) {
+          Node(b1, z, c1, Red) -> Node(Node(a, x, b1, Black), z, Node(c1, y, d, Black), Red)
+          bc -> balance-left(a, x, Node(bc, y, d, Black))
+        }
+
+      Node(b1, x1, c, Red) ->
+        Node(unsafe-merge(left, b1), x1, c, Red)
+
+      _ -> unreachable() // TODO remove this after Koka's exhaustiveness checker is fixed
+    }
+
+    _ -> unreachable() // TODO remove this after Koka's exhaustiveness checker is fixed
+  }
+}
+
+
+fun blacken(tree: tree<a>): tree<a> {
+  match (tree) {
+    Node(left, value, right, Red) -> Node(left, value, right, Black)
+    a -> a
+  }
+}
+
+fun redden(tree: tree<a>): tree<a> {
+  match (tree) {
+    Node(left, value, right, Black) -> Node(left, value, right, Red)
+    _ -> unreachable()
+  }
+}
+
+
+fun unsafe-insert1(tree: tree<a>, key: b, value: a, compare: (b, a) -> e order, replace: bool): e tree<a> {
+  match (tree) {
+    Node(left, middle, right, color) ->
+      match (compare(key, middle)) {
+        Lt -> balance(Node(unsafe-insert1(left, key, value, compare, replace), middle, right, color))
+
+        Gt -> balance(Node(left, middle, unsafe-insert1(right, key, value, compare, replace), color))
+
+        Eq -> if (replace) {
+          // TODO in this situation we should avoid balancing, because the structure hasn't changed
+          Node(left, value, right, color) // TODO return the Node unchanged if value is equal to middle
+        } else {
+          // TODO in this situation we should immediately return all the way to the root, because nothing has changed
+          tree
+        }
+      }
+
+    Empty -> Node(empty, value, empty, Red)
+  }
+}
+
+// This is unsafe because it's possible to use different compare functions on the same tree
+public fun unsafe-insert(tree: tree<a>, key: b, value: a, compare: (b, a) -> e order, replace: bool): e tree<a> {
+  blacken(unsafe-insert1(tree, key, value, compare, replace))
+}
+
+
+fun unsafe-remove1(tree: tree<a>, key: b, compare: (b, a) -> e order): e tree<a> {
+  match (tree) {
+    Node(left, middle, right, _) ->
+      match (compare(key, middle)) {
+        Lt -> match (left) {
+          Node(_, _, _, Black) -> balance-left(unsafe-remove1(left, key, compare), middle, right)
+          _ -> Node(unsafe-remove1(left, key, compare), middle, right, Red)
+        }
+
+        Eq -> unsafe-merge(left, right)
+
+        Gt -> match (right) {
+          Node(_, _, _, Black) -> balance-right(left, middle, unsafe-remove1(right, key, compare))
+          _ -> Node(left, middle, unsafe-remove1(right, key, compare), Red)
+        }
+      }
+
+    Empty -> tree
+  }
+}
+
+// This is unsafe because it's possible to use different compare functions on the same tree
+public fun unsafe-remove(tree: tree<a>, key: b, compare: (b, a) -> e order): e tree<a> {
+  blacken(unsafe-remove1(tree, key, compare))
+}
diff --git a/lib/std/data/sorted-dict.kk b/lib/std/data/sorted-dict.kk
new file mode 100644
index 000000000..fcffeddc3
--- /dev/null
+++ b/lib/std/data/sorted-dict.kk
@@ -0,0 +1,100 @@
+/*---------------------------------------------------------------------------
+  Copyright 2017 Microsoft Corporation.
+
+  This is free software; you can redistribute it and/or modify it under the
+  terms of the Apache License, Version 2.0. A copy of the License can be
+  found in the file "license.txt" at the root of this distribution.
+---------------------------------------------------------------------------*/
+
+// TODO unit tests
+// TODO maybe allow for compare functions with effects
+// TODO maybe add in the other functions from sorted-set
+module std/data/sorted-dict
+
+import red-black-tree
+
+
+abstract struct sorted-dict<k, a>(
+  compare: (k, (k, a)) -> order,
+  tree: tree<(k, a)>
+)
+
+fun from-compare(compare: (k, k) -> order): ((k, (k, a)) -> order) {
+  (fun(k1, v2) {
+    match (v2) {
+      (k2, _) -> compare(k1, k2)
+    }
+  })
+}
+
+public fun from-list(list: list<(k, a)>, compare: (k, k) -> order): sorted-dict<k, a> {
+  val compare2 = from-compare(compare)
+  Sorted-dict(compare2, from-list(list, fst, compare2))
+}
+
+public fun to-list(dict: sorted-dict<k, a>): list<(k, a)> {
+  to-list(dict.tree)
+}
+
+public fun empty(compare: (k, k) -> order): sorted-dict<k, a> {
+  Sorted-dict(from-compare(compare), empty)
+}
+
+public fun [](dict: sorted-dict<k, a>, key: k): maybe<a> {
+  unsafe-lookup(dict.tree, key, dict.compare).map(snd)
+}
+
+// TODO inline this
+// TODO maybe it shouldn't include this ?
+public fun [](dict: sorted-dict<k, a>, key: k, value: a): sorted-dict<k, a> {
+  set(dict, key, value)
+}
+
+public fun has?(dict: sorted-dict<k, a>, key: k): bool {
+  // TODO this can be implemented faster
+  bool(dict[key])
+}
+
+public fun set(dict: sorted-dict<k, a>, key: k, value: a): sorted-dict<k, a> {
+  dict(tree = unsafe-insert(dict.tree, key, (key, value), dict.compare, True))
+}
+
+public fun remove(dict: sorted-dict<k, a>, key: k): sorted-dict<k, a> {
+  dict(tree = unsafe-remove(dict.tree, key, dict.compare))
+}
+
+public fun merge-left(left: sorted-dict<k, a>, right: sorted-dict<k, a>): sorted-dict<k, a> {
+  val compare = left.compare
+  left(
+    tree = right.tree.foldl(left.tree) fun(old, a) {
+      match (a) {
+        // If a value exists in both `left` and `right` it will prefer the value from `left`
+        (key, _) -> unsafe-insert(old, key, a, compare, False)
+      }
+    }
+  )
+}
+
+public fun merge-right(left: sorted-dict<k, a>, right: sorted-dict<k, a>): sorted-dict<k, a> {
+  val compare = right.compare
+  right(
+    tree = left.tree.foldl(right.tree) fun(old, a) {
+      match (a) {
+        // If a value exists in both `left` and `right` it will prefer the value from `right`
+        (key, _) -> unsafe-insert(old, key, a, compare, False)
+      }
+    }
+  )
+}
+
+public fun subset-of?(smaller: sorted-dict<k, a>, bigger: sorted-dict<k, a>): bool {
+  smaller.tree.find-first fun(a) {
+    match (a) {
+      (key, _) -> if (bigger.has?(key)) {
+        Nothing
+      } else {
+        Just(False)
+      }
+    }
+  }.default(True)
+}
diff --git a/lib/std/data/sorted-set.kk b/lib/std/data/sorted-set.kk
new file mode 100644
index 000000000..19fd1c5be
--- /dev/null
+++ b/lib/std/data/sorted-set.kk
@@ -0,0 +1,113 @@
+/*---------------------------------------------------------------------------
+  Copyright 2017 Microsoft Corporation.
+
+  This is free software; you can redistribute it and/or modify it under the
+  terms of the Apache License, Version 2.0. A copy of the License can be
+  found in the file "license.txt" at the root of this distribution.
+---------------------------------------------------------------------------*/
+
+// TODO unit tests
+// TODO disjoint unions
+// TODO power set
+// TODO cartesian product
+// TODO maybe allow for compare functions with effects
+module std/data/sorted-set
+
+import red-black-tree
+
+
+abstract struct sorted-set<a>(
+  compare: (a, a) -> order,
+  tree: tree<a>
+)
+
+public fun from-list(list: list<a>, compare: (a, a) -> order): sorted-set<a> {
+  Sorted-set(compare, from-list(list, id, compare))
+}
+
+public fun to-list(set: sorted-set<a>): list<a> {
+  to-list(set.tree)
+}
+
+public fun empty(compare: (a, a) -> order): sorted-set<a> {
+  Sorted-set(compare, empty)
+}
+
+public fun has?(set: sorted-set<a>, value: a): bool {
+  // TODO this can be implemented faster
+  bool(unsafe-lookup(set.tree, value, set.compare))
+}
+
+public fun insert(set: sorted-set<a>, value: a): sorted-set<a> {
+  set(tree = unsafe-insert(set.tree, value, value, set.compare, True))
+}
+
+public fun remove(set: sorted-set<a>, value: a): sorted-set<a> {
+  set(tree = unsafe-remove(set.tree, value, set.compare))
+}
+
+public fun exclude(set: sorted-set<a>, exclude: sorted-set<a>): sorted-set<a> {
+  val compare = set.compare
+  set(
+    tree = exclude.tree.foldl(set.tree) fun(old, value) {
+      unsafe-remove(old, value, compare)
+    }
+  )
+}
+
+public fun intersection-right(left: sorted-set<a>, right: sorted-set<a>): sorted-set<a> {
+  val compare = right.compare
+  right(
+    tree = right.tree.foldl(empty) fun(old, value) {
+      if (left.has?(value)) {
+        unsafe-insert(old, value, value, compare, True)
+      } else {
+        old
+      }
+    }
+  )
+}
+
+public fun intersection-left(left: sorted-set<a>, right: sorted-set<a>): sorted-set<a> {
+  intersection-right(right, left)
+}
+
+public fun difference-left(left: sorted-set<a>, right: sorted-set<a>): sorted-set<a> {
+  // TODO implement this more efficiently
+  union-left(exclude(left, right), exclude(right, left))
+}
+
+public fun difference-right(left: sorted-set<a>, right: sorted-set<a>): sorted-set<a> {
+  // TODO implement this more efficiently
+  union-right(exclude(left, right), exclude(right, left))
+}
+
+public fun union-left(left: sorted-set<a>, right: sorted-set<a>): sorted-set<a> {
+  val compare = left.compare
+  left(
+    tree = right.tree.foldl(left.tree) fun(old, value) {
+      // If a value exists in both `left` and `right` it will prefer the value from `left`
+      unsafe-insert(old, value, value, compare, False)
+    }
+  )
+}
+
+public fun union-right(left: sorted-set<a>, right: sorted-set<a>): sorted-set<a> {
+  val compare = right.compare
+  right(
+    tree = left.tree.foldl(right.tree) fun(old, value) {
+      // If a value exists in both `left` and `right` it will prefer the value from `right`
+      unsafe-insert(old, value, value, compare, False)
+    }
+  )
+}
+
+public fun subset-of?(smaller: sorted-set<a>, bigger: sorted-set<a>): bool {
+  smaller.tree.find-first fun(value) {
+    if (bigger.has?(value)) {
+      Nothing
+    } else {
+      Just(False)
+    }
+  }.default(True)
+}