PoC %typeof

compiler/ml/typetexp.ml

@@ -565,8 +565,26 @@ and transl_type_aux env policy styp =
            pack_txt = p;
          })
       ty
-  | Ptyp_extension ext ->
-    raise (Error_forward (Builtin_attributes.error_of_extension ext))
+  | Ptyp_extension ext -> (
+    match ext with
+    | {txt = "typeof"; loc = ext_loc}, payload -> (
+      (* %typeof payload must be a single identifier *)
+      match Ast_payload.as_ident payload with
+      | Some ({txt = lid; loc = lid_loc} as _ident) ->
+        (* Lookup the value and embed a generic instance of its type.
+           Using a generic instance avoids capturing weak (non-generalized)
+           type variables from the value into a type position. *)
+        let _path, desc = find_value env lid_loc lid in
+        let ty = Ctype.generic_instance env desc.val_type in
+        (* Build a core_type node carrying the looked up type; we mark the
+           desc as any since downstream only consults ctyp_type for typing. *)
+        ctyp Ttyp_any ty
+      | None ->
+        let msg =
+          "%%typeof expects an identifier. Example: type t = %typeof(x)"
+        in
+        raise (Error_forward (Location.error ~loc:ext_loc msg)))
+    | _ -> raise (Error_forward (Builtin_attributes.error_of_extension ext)))
 
 and transl_poly_type env policy t =
   transl_type env policy (Ast_helper.Typ.force_poly t)

tests/analysis_tests/tests/src/Hover.res

@@ -281,3 +281,13 @@ module Arr = Belt.Array
 
 type aliased = variant
 //    ^hov
+
+let myFn = (a, b) => a ++ b->Int.toString
+
+type fnType = %typeof(myFn)
+//    ^hov
+
+let myFnPartial = myFn("hello", ...)
+
+type fnTypePartial = %typeof(myFnPartial)
+//    ^hov

tests/analysis_tests/tests/src/expected/Hover.res.txt

@@ -348,3 +348,9 @@ Hover src/Hover.res 278:8
 Hover src/Hover.res 281:6
 {"contents": {"kind": "markdown", "value": "```rescript\ntype aliased = variant\n```\n\n---\n\n```\n \n```\n```rescript\ntype variant = CoolVariant | OtherCoolVariant\n```\nGo to: [Type definition](command:rescript-vscode.go_to_location?%5B%22Hover.res%22%2C251%2C0%5D)\n"}}
 
+Hover src/Hover.res 286:6
+{"contents": {"kind": "markdown", "value": "```rescript\ntype fnType = (string, int) => string\n```"}}
+
+Hover src/Hover.res 291:6
+{"contents": {"kind": "markdown", "value": "```rescript\ntype fnTypePartial = int => string\n```"}}
+

tests/build_tests/super_errors/expected/typeof_mismatch.res.expected

@@ -0,0 +1,13 @@
+
+  We've found a bug for you!
+  /.../fixtures/typeof_mismatch.res:6:28
+
+  4 │ 
+  5 │ let f: fnType = myFn
+  6 │ let ff: fnType = (a, b) => a->Int.toString + b
+  7 │ 
+
+  This has type: string
+  But this function argument is expecting: int
+
+  You can convert string to int with Int.fromString.

tests/build_tests/super_errors/fixtures/typeof_mismatch.res

@@ -0,0 +1,6 @@
+let myFn = (a, b) => a ++ b->Int.toString
+
+type fnType = %typeof(myFn)
+
+let f: fnType = myFn
+let ff: fnType = (a, b) => a->Int.toString + b

tests/tests/src/Typeof.mjs

@@ -0,0 +1,14 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+
+
+function myFn(a, b) {
+  return a + b.toString();
+}
+
+let f = myFn;
+
+export {
+  myFn,
+  f,
+}
+/* No side effect */

tests/tests/src/Typeof.res

@@ -0,0 +1,5 @@
+let myFn = (a, b) => a ++ b->Int.toString
+
+type fnType = %typeof(myFn)
+
+let f: fnType = myFn