[cknitt]

Typed arrays' copyWithin and copyAllWithin return the typed array they mutate, but the stdlib typed them as returning an array, so Int32Array.copyAllWithin(v, ~target=1)->Array.push(5) compiled and threw r.push is not a function. The generic TypedArray externals had this signature, and #9050 copied it into the Impl functor that every typed-array module includes. Both now return the typed array (t<'a> in TypedArray, T.t in the modules), and the duplicated @send @send on TypedArray.copyWithin is gone.

TypedArray.map and mapWithIndex were typed (t<'a>, 'a => 'b) => t<'b>, but a typed array's map creates a typed array of the same kind and converts each result to its element type, so mapping a t<int> to strings gave a t<string> holding zeros. They are now (t<'a>, 'a => 'a) => t<'a>, like the per-module versions already were. This is a breaking change for code that used them to change the element type, so it has its own breaking changelog fragment; the copyWithin fix is a fix fragment.

The docstrings now say that copyWithin/copyAllWithin return the typed array and that map returns a typed array of the same kind.

Tests:

• tests/tests/src/stdlib/Stdlib_TypedArrayTests.res: copyWithin/copyAllWithin on Int32Array and on TypedArray return the very array they mutated, with the right type annotation, and TypedArray.map/mapWithIndex with an element-type-preserving function.
• tests/build_tests/super_errors/fixtures/TypedArrayMapElementType.res: TypedArray.map with a function returning string on a t<int> is now a type error.
• The completion snapshot CompletionTypedArrays.res.txt shows the new return types and docstrings.

Breaking change: making TypedArray.map type-preserving breaks code that used it to change the element type (which only worked by accident, since the values are converted). This ships as a breaking change in v13, without a deprecation period.

Fixes #9103

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https://claude.ai/code/session_01Kbf3gGkPma9LAZimuQjnrP

[github-actions]

Changelog

💥 Breaking Change

• TypedArray.map and TypedArray.mapWithIndex now keep the element type, because a typed array converts the results of map to its own element type.

🐛 Bug fix

• Type copyWithin and copyAllWithin of typed arrays as returning the typed array instead of an array.

Semantic Diff

1 commit · Full page

Type typed-array copyWithin as returning the typed array, and map as keeping the element type

_{489f49e · cknitt + Claude · Oct 28, 2026, 15:26 UTC}

Why

copyWithin and copyAllWithin return the typed array they mutated, but were typed as returning an array, so Array.push on the result compiled and threw. A typed array's map converts each result to its own element type, but TypedArray.map let f change the type, so a TypedArray.t<string> held numbers.

Typing modified (TypedArray and per-module externals)

rescript

-external copyAllWithin: (t<'a>, ~target: int) => array<'a>
-external map: (t<'a>, 'a => 'b) => t<'b>
-external mapWithIndex: (t<'a>, ('a, int) => 'b) => t<'b>
+external copyAllWithin: (t<'a>, ~target: int) => t<'a>
+external map: (t<'a>, 'a => 'a) => t<'a>
+external mapWithIndex: (t<'a>, ('a, int) => 'a) => t<'a>
 // Int32Array and the other modules
-external copyAllWithin: (t, ~target: int) => array<int>
+external copyAllWithin: (t, ~target: int) => t

copyWithin changes like copyAllWithin, and loses a duplicated @send.

⚠️ Breaking: TypedArray.map and mapWithIndex no longer change the element type; to get values of another type, build an array from the elements.

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commit: 489f49e

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[cknitt]

@codex review

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Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-28T17:49:14.216212Z	489f49e	Manual request

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[cknitt]

@codex review

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Codex Review: Didn't find any major issues. What shall we delve into next?

Reviewed commit: 489f49e09a

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