explicit for loop condition

Author: zth

compiler/ml/error_message_utils.ml

@@ -88,6 +88,7 @@ type type_clash_context =
     }
   | FunctionArgument
   | Statement of type_clash_statement
+  | ForLoopCondition
 
 let fprintf = Format.fprintf
 
@@ -113,6 +114,8 @@ let error_expected_type_text ppf type_clash_context =
   | Some TryReturn -> fprintf ppf "But this try/catch is expected to return:"
   | Some WhileCondition ->
     fprintf ppf "But a @{<info>while@} loop condition must always be of type:"
+  | Some ForLoopCondition ->
+    fprintf ppf "But a @{<info>for@} loop bounds must always be of type:"
   | Some IfCondition ->
     fprintf ppf "But @{<info>if@} conditions must always be of type:"
   | Some IfReturn ->

compiler/ml/typecore.ml

@@ -2858,9 +2858,12 @@ and type_expect_ ~context ?in_function ?(recarg = Rejected) env sexp ty_expected
         exp_env = env;
       }
   | Pexp_for (param, slow, shigh, dir, sbody) ->
-    (* TODO: Add explicit ForCondition *)
-    let low = type_expect ~context:None env slow Predef.type_int in
-    let high = type_expect ~context:None env shigh Predef.type_int in
+    let low =
+      type_expect ~context:(Some ForLoopCondition) env slow Predef.type_int
+    in
+    let high =
+      type_expect ~context:(Some ForLoopCondition) env shigh Predef.type_int
+    in
     let id, new_env =
       match param.ppat_desc with
       | Ppat_any -> (Ident.create "_for", env)

tests/build_tests/super_errors/expected/for_loop_condition.res.expected

@@ -0,0 +1,12 @@
+
+  We've found a bug for you!
+  /.../fixtures/for_loop_condition.res:1:16-19
+
+  1 │ for (x in 0 to "10") {
+  2 │   Console.log(x)
+  3 │ }
+
+  This has type: string
+  But a for loop bounds must always be of type: int
+  
+  You can convert string to int with Int.fromString.

tests/build_tests/super_errors/fixtures/for_loop_condition.res

@@ -0,0 +1,3 @@
+for x in 0 to "10" {
+  Console.log(x)
+}