diff --git a/birdie_snapshots/snap_dummy_src_no_trailing_underscore_gleam.accepted b/birdie_snapshots/snap_dummy_src_no_trailing_underscore_gleam.accepted
new file mode 100644
index 0000000..2ec6f52
--- /dev/null
+++ b/birdie_snapshots/snap_dummy_src_no_trailing_underscore_gleam.accepted
@@ -0,0 +1,10 @@
+---
+version: 1.1.2
+title: ./snap_dummy/src/no_trailing_underscore.gleam
+---
+Path: ./snap_dummy/src/no_trailing_underscore.gleam
+
+Location Identifier: with_trailing_
+Rule: NoTrailingUnderscore
+Error: Trailing underscore in function name
+Details: We don't like no trailing underscores.
\ No newline at end of file
diff --git a/snap_dummy/src/no_trailing_underscore.gleam b/snap_dummy/src/no_trailing_underscore.gleam
new file mode 100644
index 0000000..129170c
--- /dev/null
+++ b/snap_dummy/src/no_trailing_underscore.gleam
@@ -0,0 +1,7 @@
+pub fn with_trailing_() {
+  1
+}
+
+pub fn without_trailing() {
+  1
+}
diff --git a/src/code_review.gleam b/src/code_review.gleam
index 7309092..5c014af 100644
--- a/src/code_review.gleam
+++ b/src/code_review.gleam
@@ -143,84 +143,98 @@ fn read_project(project_root_path: String) -> Result(KnowledgeBase, WhingeError)
 
 fn visit_knowledge_base(kb: KnowledgeBase, rules: List(Rule)) -> List(RuleError) {
   use acc, Module(path, module) <- list.fold(kb.src_modules, [])
-  visit_module(path, rules, module)
+  visit_module(module, rules)
+  |> list.map(fn(error) { RuleError(..error, path: path) })
   |> list.append(acc)
 }
 
-fn visit_module(
-  path: String,
-  rules: List(Rule),
-  input_module: glance.Module,
-) -> List(RuleError) {
-  visit_expressions(input_module, rules)
-  |> list.map(fn(error) { RuleError(..error, path: path) })
+fn visit_module(input: glance.Module, rules: List(Rule)) -> List(RuleError) {
+  let glance.Module(constants: constants, functions: functions, ..) = input
+
+  // Visit all constants
+  let results_after_const: List(RuleError) = visit_constants(constants, rules)
+  let results_after_functions: List(RuleError) =
+    visit_functions(functions, rules, results_after_const)
+
+  results_after_functions
 }
 
-// Extracts all the top level functions out of a glance module.
-fn extract_functions(from input: glance.Module) -> List(glance.Function) {
-  let glance.Module(functions: function_defs, ..) = input
-  let _functions =
-    list.map(function_defs, fn(def) {
-      let glance.Definition(_, func) = def
-      func
+fn visit_constants(
+  constants: List(glance.Definition(glance.Constant)),
+  rules: List(Rule),
+) {
+  let f = fn(location_identifier, expr) {
+    apply_visitor(expr, rules, fn(rule) { rule.expression_visitors })
+    |> list.map(fn(error) {
+      RuleError(..error, location_identifier: location_identifier)
     })
-}
+  }
 
-fn extract_constants(from input: glance.Module) -> List(glance.Constant) {
-  let glance.Module(constants: consts, ..) = input
-  list.map(consts, fn(const_) {
-    let glance.Definition(_, c) = const_
-    c
+  list.fold(constants, [], fn(const_acc, constant_with_definition) {
+    let glance.Definition(_, c) = constant_with_definition
+    do_visit_expressions(c.value, const_acc, fn(expr) { f(c.name, expr) })
   })
 }
 
-fn visit_expressions(input: glance.Module, rules: List(Rule)) -> List(RuleError) {
-  let funcs = extract_functions(input)
-  let consts = extract_constants(input)
+fn visit_functions(
+  functions: List(glance.Definition(glance.Function)),
+  rules: List(Rule),
+  acc: List(RuleError),
+) {
+  use acc0: List(RuleError), glance.Definition(_, func) <- list.fold(
+    functions,
+    acc,
+  )
 
-  let f = fn(location_identifier, expr) {
-    list.flat_map(rules, fn(rule) {
-      list.flat_map(rule.expression_visitors, fn(visitor) { visitor(expr) })
-      |> list.map(fn(error) {
-        RuleError(
-          ..error,
-          rule: rule.name,
-          location_identifier: location_identifier,
-        )
-      })
-    })
-  }
+  let errors_for_func: List(RuleError) =
+    apply_visitor(func, rules, fn(rule) { rule.function_visitors })
+    |> list.map(fn(error) { RuleError(..error, location_identifier: func.name) })
 
-  // Visit all the expressions in top level functions
-  let func_results: List(RuleError) = {
-    use func <- list.flat_map(funcs)
-    use stmt <- list.flat_map(func.body)
+  use acc1: List(RuleError), stmt <- list.fold(
+    func.body,
+    list.append(errors_for_func, acc0),
+  )
 
-    let expr = case stmt {
-      glance.Use(_, expr) -> expr
-      glance.Assignment(value: val, ..) -> val
-      glance.Expression(expr) -> expr
-    }
+  list.append(
+    visit_statement(stmt, rules)
+      |> list.map(fn(error) {
+      RuleError(..error, location_identifier: func.name)
+    }),
+    acc1,
+  )
+}
 
-    do_visit_expressions(expr, [], fn(expr) { f(func.name, expr) })
-    |> list.flatten
+fn visit_statement(
+  statement: glance.Statement,
+  rules: List(Rule),
+) -> List(RuleError) {
+  let expr: glance.Expression = case statement {
+    glance.Use(_, expr) -> expr
+    glance.Assignment(value: val, ..) -> val
+    glance.Expression(expr) -> expr
   }
+  do_visit_expressions(expr, [], fn(expr) {
+    apply_visitor(expr, rules, fn(rule) { rule.expression_visitors })
+  })
+}
 
-  // Visit all the expressions in constants
-  let const_results: List(RuleError) =
-    list.flat_map(consts, fn(c) {
-      do_visit_expressions(c.value, [], fn(expr) { f(c.name, expr) })
-    })
-    |> list.flatten
-  list.append(func_results, const_results)
+fn apply_visitor(
+  a: a,
+  rules: List(Rule),
+  visitor_fn: fn(Rule) -> List(fn(a) -> List(RuleError)),
+) {
+  list.flat_map(rules, fn(rule) {
+    list.flat_map(visitor_fn(rule), fn(visitor) { visitor(a) })
+    |> list.map(fn(error) { RuleError(..error, rule: rule.name) })
+  })
 }
 
 fn do_visit_expressions(
   input: glance.Expression,
-  acc: List(a),
-  do f: fn(glance.Expression) -> a,
-) -> List(a) {
-  let acc = [f(input), ..acc]
+  acc: List(RuleError),
+  do f: fn(glance.Expression) -> List(RuleError),
+) -> List(RuleError) {
+  let acc: List(RuleError) = list.append(f(input), acc)
   case input {
     glance.Todo(_)
     | glance.Panic(_)
diff --git a/src/review_config.gleam b/src/review_config.gleam
index d8e24cb..c5be35c 100644
--- a/src/review_config.gleam
+++ b/src/review_config.gleam
@@ -1,7 +1,12 @@
 import rule.{type Rule, Rule}
 import rules/no_panic
+import rules/no_trailing_underscore
 import rules/no_unnecessary_string_concatenation
 
 pub fn config() -> List(Rule) {
-  [no_panic.rule(), no_unnecessary_string_concatenation.rule()]
+  [
+    no_panic.rule(),
+    no_unnecessary_string_concatenation.rule(),
+    no_trailing_underscore.rule(),
+  ]
 }
diff --git a/src/rule.gleam b/src/rule.gleam
index 5590c75..089ccd7 100644
--- a/src/rule.gleam
+++ b/src/rule.gleam
@@ -1,21 +1,49 @@
 import glance
+import gleam/list
 
 pub type Rule {
   Rule(
     name: String,
+    function_visitors: List(fn(glance.Function) -> List(RuleError)),
     expression_visitors: List(fn(glance.Expression) -> List(RuleError)),
   )
 }
 
 pub fn new(name: String) {
-  Rule(name: name, expression_visitors: [])
+  Rule(name: name, function_visitors: [], expression_visitors: [])
+}
+
+pub fn with_function_visitor(
+  rule: Rule,
+  visitor: fn(glance.Function) -> List(RuleError),
+) {
+  Rule(
+    ..rule,
+    function_visitors: [
+      set_rule_name_on_errors(rule.name, visitor),
+      ..rule.function_visitors
+    ],
+  )
 }
 
 pub fn with_expression_visitor(
   rule: Rule,
   visitor: fn(glance.Expression) -> List(RuleError),
 ) {
-  Rule(..rule, expression_visitors: [visitor, ..rule.expression_visitors])
+  Rule(
+    ..rule,
+    expression_visitors: [
+      set_rule_name_on_errors(rule.name, visitor),
+      ..rule.expression_visitors
+    ],
+  )
+}
+
+fn set_rule_name_on_errors(name: String, visitor: fn(a) -> List(RuleError)) {
+  fn(a: a) {
+    visitor(a)
+    |> list.map(fn(error) { RuleError(..error, rule: name) })
+  }
 }
 
 // Represents an error reported by a rule.
diff --git a/src/rules/no_trailing_underscore.gleam b/src/rules/no_trailing_underscore.gleam
new file mode 100644
index 0000000..58bde1f
--- /dev/null
+++ b/src/rules/no_trailing_underscore.gleam
@@ -0,0 +1,19 @@
+import glance
+import gleam/string
+import rule.{type Rule, type RuleError}
+
+pub fn rule() -> Rule {
+  rule.new("NoTrailingUnderscore")
+  |> rule.with_function_visitor(function_visitor)
+}
+
+pub fn function_visitor(function: glance.Function) -> List(RuleError) {
+  case string.ends_with(function.name, "_") {
+    True -> [
+      rule.error(message: "Trailing underscore in function name", details: [
+        "We don't like no trailing underscores.",
+      ]),
+    ]
+    False -> []
+  }
+}
diff --git a/test/code_review_test.gleam b/test/code_review_test.gleam
index ad607a7..8c51871 100644
--- a/test/code_review_test.gleam
+++ b/test/code_review_test.gleam
@@ -2,6 +2,7 @@ import birdie
 import code_review
 import gleam/list
 import gleeunit
+import rule
 
 pub fn main() {
   gleeunit.main()
@@ -11,8 +12,8 @@ pub fn main() {
 // while there are lots of moving pieces.
 // 
 pub fn smoke_test() {
-  let assert Ok(rules) = code_review.run(on: "./snap_dummy")
-  use rule <- list.each(rules)
+  let assert Ok(rule_errors) = code_review.run(on: "./snap_dummy")
+  use rule: rule.RuleError <- list.each(rule_errors)
 
   rule
   |> code_review.display_rule_error