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Fix fatal error for external with @as #7978

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Merged
merged 4 commits into from
Oct 23, 2025
Merged

Fix fatal error for external with @as #7978

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4e6f176
Fix fatal error for external with @as
cknitt Oct 22, 2025
43761e1
Add a synthetic unit argument when all external parameters collapse t…
cknitt Oct 23, 2025
878179b
Simplify
cknitt Oct 23, 2025
1badbec
CHANGELOG
cknitt Oct 23, 2025
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1 change: 1 addition & 0 deletions CHANGELOG.md
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Expand Up @@ -21,6 +21,7 @@
#### :bug: Bug fix

- Fix @directive on function level with async and multiple parameters. https://github.com/rescript-lang/rescript/pull/7977
- Fix fatal error for external with @as. https://github.com/rescript-lang/rescript/pull/7978

#### :memo: Documentation

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21 changes: 19 additions & 2 deletions compiler/frontend/ast_external_process.ml
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Expand Up @@ -997,6 +997,24 @@ let handle_attributes (loc : Bs_loc.t) (type_annotation : Parsetree.core_type)
new_arg_types,
if arg_type = Ignore then i else i + 1 ))
in
(* If every original argument was erased (e.g. all `@as(json ...) _`),
keep the external binding callable by threading a final `unit`
parameter through the type and arg specs. *)
let args, arg_type_specs =
match (args, arg_type_specs_length) with
| [], n when n > 0 ->
let unit_type =
Ast_helper.Typ.constr ~loc
(Location.mkloc (Longident.Lident "unit") loc)
[]
in
let unit_arg = {Parsetree.attrs = []; lbl = Nolabel; typ = unit_type} in
( [unit_arg],
arg_type_specs
@ [{External_arg_spec.arg_label = Arg_empty; arg_type = Extern_unit}]
)
| _ -> (args, arg_type_specs)
in
let ffi : External_ffi_types.external_spec =
external_desc_of_non_obj loc external_desc prim_name_with_source
arg_type_specs_length arg_types_ty arg_type_specs
Expand All @@ -1008,8 +1026,7 @@ let handle_attributes (loc : Bs_loc.t) (type_annotation : Parsetree.core_type)
let return_wrapper =
check_return_wrapper loc external_desc.return_wrapper result_type
in
let fn_type = Ast_helper.Typ.arrows ~loc args result_type in
( build_uncurried_type ~arity:(List.length args) fn_type,
( Ast_helper.Typ.arrows ~loc args result_type,
External_ffi_types.ffi_bs arg_type_specs return_wrapper ffi,
unused_attrs,
Comment on lines +1029 to 1031
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P1 Badge Preserve uncurried arity when rebuilding external type

Previously this code rebuilt the return type with build_uncurried_type ~arity:(List.length args) so externals declared with an uncurried arrow retained their arity metadata. After the change it returns Ast_helper.Typ.arrows directly and the build_uncurried_type wrapper is never applied in the non-object path, so externals such as external add: (int, int) => int are now typed as int -> int -> int. Calls written with uncurried syntax (add(1, 2)) will start raising Apply_non_function/arity mismatches during type checking because the arity flag is gone. The new code should still apply build_uncurried_type to the reconstructed arrow to keep uncurried externals working.

Useful? React with 👍 / 👎.

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@codex But Ast_helper.Typ.arrows does already set the arity for the outer function to List.length args.

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To use Codex here, create an environment for this repo.

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Definitely not true, external add: (int, int) => int still works correctly.

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How comes codex runs now? That changed?
It used to get stuck trying to install ocaml.

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I think it didn't actually run anything, it just reviews the code.

relative )
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5 changes: 4 additions & 1 deletion tests/tests/src/AsInUncurriedExternals.mjs
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Expand Up @@ -19,9 +19,12 @@ function shouldNotFail(objectMode, name) {
return 3;
}

let x = somescope.somefn({foo:true});

export {
mo,
options,
shouldNotFail,
x,
}
/* No side effect */
/* x Not a pure module */
5 changes: 5 additions & 0 deletions tests/tests/src/AsInUncurriedExternals.res
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Expand Up @@ -13,3 +13,8 @@ let mo = makeOptions
let options = mo(~name="foo", ())

let shouldNotFail: (~objectMode: _, ~name: string) => int = (~objectMode, ~name) => 3

@scope("somescope")
external constantArgOnly: @as(json`{foo:true}`) _ => string = "somefn"

let x = constantArgOnly()
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