Implement `for...of` and `for await...of` loops by cknitt · Pull Request #7887 · rescript-lang/rescript

cknitt · 2025-09-13T06:56:24Z

Summary

This PR adds native for...of and for await...of loop support to ReScript across the full compiler pipeline.

It introduces parser, AST, typedtree, lambda IR, and JS backend support for both loop forms, preserves break / continue semantics inside nested control-flow, and adds syntax, type-error, analysis, and end-to-end test coverage for the new feature.

What changed

Add parsing and printing support for:
- for item of items { ... }
- for await item of items { ... }
Extend the parsetree, typedtree, lambda IR, and JS IR with dedicated for...of / for await...of nodes instead of lowering them too early.
Type-check for...of against iterable<'a> with a convenience fallback for raw array<'a> values, and type-check for await...of against asyncIterable<'a>.
Restrict loop bindings to variables or _, and report a targeted error for unsupported destructuring patterns on the left-hand side.
Compile the new loop forms to native JavaScript for...of and for await...of statements.
Preserve labeled loop behavior so break and continue still target the enclosing loop correctly, including when emitted through nested JS switch statements.
Update reanalyze and dependency traversal so the new loop forms participate correctly in dead-code and side-effect analysis.

Coverage

Syntax tests for parsing, printing, AST mapping, and loop-control formatting.
Build tests for type errors, including non-iterables, strings, invalid async iteration targets, and element type mismatches.
Integration tests covering:
- basic iteration
- empty and single-element loops
- wildcard and unused bindings
- shadowing inside loop bodies
- break / continue in plain loops and nested switches
- realistic async iteration scenarios
Analysis tests covering dead-code handling for for...of and for await...of.

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commit: c1baab7

cristianoc · 2025-09-18T23:58:25Z

The big questions were language design related ones, rather than implementation.
There are many ways to go about this.
@cometkim still got some design notes from the retreat? This could be the right time to do some more text-based discussion before the code-based one.

cometkim · 2025-09-19T00:37:16Z

Frankly, importing the JavaScript for-of syntax as-is doesn't sound all that appealing. It's not much different from embedding %raw.

Here are my thoughts on designing for loops:

Since iterators are essential in JavaScript, we need to define iterator semantics before design the synyax. The syntax follows.
Without control flow support, the for-of syntax itself isn't very useful. With good control semantics, the simple iterator protocol and even the MoonBit style loop syntax become useful.
The current for x into y syntax is valid for index-based access, but the JS' for-of doesn't. To combine the benefits of both style, generator (for sequencing) support will be needed. Zig's for loop design is a good example.
Are for statements enough? To maintain a robust type-based approach, it would be better to design them based on expressions.

cometkim · 2025-09-19T00:42:11Z

@cometkim still got some design notes from the retreat?

https://github.com/JonoPrest/iterator-rescript/blob/retreat-note/RETREAT_NOTE.md

On the last retreat, we only discussed iterator semantics and simulating break/continue; we also need to deal with the early-return or jump (for escaping nested loops) semantics.

cristianoc · 2025-09-19T02:32:28Z

Early return keeps on coming up as a topic.
Perhaps it makes sense to explore that as a prerequisite to this.

cknitt · 2025-09-19T06:25:56Z

For me, the for..of feature as implemented here would already be useful in and of itself because it

allows the usage of await like in this example from the tests which would not be possible using arr->Array.forEach(...)

let asyncProcess = async () => {
  let results = []
  for item of arr {
    let result = await processData(item)
    results->Array.push(result)
  }
  results
}

is more concise

for item for arr {
# vs.
arr->Array.forEach(item =>

is more performant
Increases our coverage of JS features that JS developer may expect to have

I agree that early return would be nice to have, but not sure if we should tie this to for..of.

I also agree that it would be a good idea to look into how iterators should work.
OTOH having this for arrays would be a low-hanging fruit that could hopefully be extended to iterators later.

cometkim · 2025-09-22T07:01:42Z

OTOH having this for arrays would be a low-hanging fruit that could hopefully be extended to iterators later.

This is already a dangerous assumption. for-of loops are all about iterators, not arrays.

It should be designed to interact with iterator types. When it works with array types, it leads users to copy arbitrary iterators into arrays unnecessarily.

cknitt · 2026-04-17T12:20:54Z

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cknitt · 2026-04-18T15:49:16Z

It should be designed to interact with iterator types. When it works with array types, it leads users to copy arbitrary iterators into arrays unnecessarily.

Done! 😄

cknitt · 2026-04-18T15:49:45Z

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chatgpt-codex-connector · 2026-04-18T16:02:35Z

+        | Int_switch _ | String_switch _ | ForRange _ | ForOf _ | ForAwaitOf _
+        | If _ | While _ | Block _ | Return _ | Try _ ->


Treat iterator loops as side-effecting in JS analyzer

no_side_effect_obj.statement now classifies ForOf and ForAwaitOf via structural traversal, so loops with a pure-looking iterable expression/body are considered removable. Unlike ForRange, these loops invoke iterator protocol methods (Symbol.iterator/next or async variants), which can execute arbitrary user code even when the iterable is just an identifier; this feeds into js_shake's statement pruning and can incorrectly drop runtime-visible effects.

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chatgpt-codex-connector · 2026-04-18T16:02:35Z

+  | Texp_for_of (_id, _pat, e1, e2) ->
+    e1 |> exprNoSideEffects && e2 |> exprNoSideEffects
+  | Texp_for_await_of (_id, _pat, e1, e2) ->
+    e1 |> exprNoSideEffects && e2 |> exprNoSideEffects


Mark iterator loops effectful in reanalyze side-effects

exprNoSideEffects now returns true for Texp_for_of/Texp_for_await_of when only the loop expression/body look pure, but iteration itself can run effectful iterator callbacks on custom iterables. Since SideEffects.checkExpr is used to set declaration side-effect metadata in dead-value reporting, this can misclassify effectful values as effect-free and produce incorrect dead-code diagnostics.

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cknitt · 2026-04-18T17:03:23Z

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chatgpt-codex-connector · 2026-04-18T17:10:50Z

Codex Review: Didn't find any major issues. Bravo.

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cknitt marked this pull request as draft September 13, 2025 06:56

cknitt commented Sep 13, 2025

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cknitt commented Sep 13, 2025

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Comment thread tests/syntax_tests/data/parsing/errors/other/expected/for.res.txt Outdated

Comment thread tests/syntax_tests/data/parsing/errors/pattern/expected/missing.res.txt

cknitt force-pushed the for-of branch from 9824f08 to 7866081 Compare September 13, 2025 08:03

cknitt requested a review from cristianoc September 13, 2025 12:51

cknitt changed the title ~~Very basic implementation of for..of loops~~ [Poc] for..of loops Sep 17, 2025

cknitt changed the title ~~[Poc] for..of loops~~ [PoC] for..of loops Sep 17, 2025

cknitt force-pushed the for-of branch 3 times, most recently from 77f91d8 to 8b779af Compare September 18, 2025 19:07

cknitt force-pushed the for-of branch from 8b779af to 2a96c79 Compare April 17, 2026 12:20

github-code-quality bot found potential problems Apr 17, 2026

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Comment thread tests/tests/src/test_for_of.mjs Fixed

Comment thread tests/tests/src/test_for_of.mjs Fixed

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Comment thread compiler/core/js_dump.ml Outdated

Comment thread compiler/syntax/src/res_core.ml Outdated

Implement for..of and for await..of loops

4389e9c

cknitt force-pushed the for-of branch from 2a96c79 to 4389e9c Compare April 18, 2026 15:46

cknitt changed the title ~~[PoC] for..of loops~~ Implement for...of and for await...of loops Apr 18, 2026

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cknitt force-pushed the for-of branch from 1c897b9 to 58d4736 Compare April 18, 2026 17:03

fixes

55c3de7

cknitt force-pushed the for-of branch from 58d4736 to 55c3de7 Compare April 18, 2026 17:13

cknitt marked this pull request as ready for review April 18, 2026 17:27

CHANGELOG

c1baab7

cknitt assigned cristianoc and zth Apr 18, 2026

zth approved these changes Apr 18, 2026

View reviewed changes

		\| Int_switch _ \| String_switch _ \| ForRange _ \| ForOf _ \| ForAwaitOf _
		\| If _ \| While _ \| Block _ \| Return _ \| Try _ ->

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