Clarify arity mismatch message (#7709)

* clarify arity mismatch message

* explicitly call out supplying fewer arguments

* changelog

Author: zth

CHANGELOG.md

@@ -25,6 +25,7 @@
 
 - Configuration fields `bs-dependencies`, `bs-dev-dependencies` and `bsc-flags` are now deprecated in favor of `dependencies`, `dev-dependencies` and `compiler-flags`. https://github.com/rescript-lang/rescript/pull/7658
 - Better error message if platform binaries package is not found. https://github.com/rescript-lang/rescript/pull/7698
+- Polish arity mismatch error message a bit. https://github.com/rescript-lang/rescript/pull/7709
 
 #### :house: Internal
 

compiler/ml/typecore.ml

@@ -794,9 +794,11 @@ let print_expr_type_clash ~context env loc trace ppf =
     show_extra_help ppf env trace
 
 let report_arity_mismatch ~arity_a ~arity_b ppf =
-  fprintf ppf "This function expected @{<info>%s@} %s, but got @{<error>%s@}"
+  fprintf ppf
+    "This function is expected to have @{<info>%s@} %s, but%s has @{<error>%s@}"
     arity_b
     (if arity_b = "1" then "argument" else "arguments")
+    (if int_of_string arity_a < int_of_string arity_b then " only" else "")
     arity_a
 
 (* Records *)

tests/build_tests/super_errors/expected/arity_mismatch3.res.expected

@@ -5,4 +5,4 @@
   1 │ Belt.Array.map([], (a, b) => 1)
   2 │ 
 
-  This function expected 1 argument, but got 2
+  This function is expected to have 1 argument, but has 2

tests/build_tests/super_errors/expected/arity_mismatch_fewer.res.expected

@@ -0,0 +1,10 @@
+
+  We've found a bug for you!
+  /.../fixtures/arity_mismatch_fewer.res:6:11-23
+
+  4 │ }
+  5 │ 
+  6 │ let x = f(_a => "hello")
+  7 │ 
+
+  This function is expected to have 2 arguments, but only has 1

tests/build_tests/super_errors/expected/curried_expected.res.expected

@@ -7,4 +7,4 @@
   3 │ let z1 = expectCurried((x, y) => x + y)
   4 │ 
 
-  This function expected 1 argument, but got 2
+  This function is expected to have 1 argument, but has 2

tests/build_tests/super_errors/fixtures/arity_mismatch_fewer.res

@@ -0,0 +1,6 @@
+let f = (cb: (int, int) => int): string => {
+  ignore(cb)
+  "hello"
+}
+
+let x = f(_a => "hello")