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Clarify arity mismatch message #7709

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Merged
merged 3 commits into from
Jul 24, 2025
Merged

Clarify arity mismatch message #7709

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cf96360
clarify arity mismatch message
zth Jul 24, 2025
7929b79
explicitly call out supplying fewer arguments
zth Jul 24, 2025
4624817
changelog
zth Jul 24, 2025
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1 change: 1 addition & 0 deletions CHANGELOG.md
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Expand Up @@ -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

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4 changes: 3 additions & 1 deletion compiler/ml/typecore.ml
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Expand Up @@ -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 *)
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2 changes: 1 addition & 1 deletion tests/build_tests/super_errors/expected/arity_mismatch3.res.expected
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Expand Up @@ -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
10 changes: 10 additions & 0 deletions tests/build_tests/super_errors/expected/arity_mismatch_fewer.res.expected
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@@ -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
2 changes: 1 addition & 1 deletion tests/build_tests/super_errors/expected/curried_expected.res.expected
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Expand Up @@ -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
6 changes: 6 additions & 0 deletions tests/build_tests/super_errors/fixtures/arity_mismatch_fewer.res
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@@ -0,0 +1,6 @@
let f = (cb: (int, int) => int): string => {
ignore(cb)
"hello"
}

let x = f(_a => "hello")