Add provisional %raw syntax for fixed external arguments by cknitt · Pull Request #8876 · rescript-lang/rescript

cknitt · 2026-10-16T08:03:10Z

Second part of splitting #8609, stacked on #8879. The %raw syntax here is provisional and up for discussion with the community.

Summary

Fixed external arguments can be written with the %raw extension point:

@val
external unsafeAddStyle: (%raw("{}"), t, {..}) => t = "Object.assign"

external createElement: (%raw(`"img"`), document) => element = "createElement"

The payload is a string with JavaScript source, which must be exactly one JavaScript literal. Trailing code and comments are rejected because the source is copied into the output.

The @as(json...) _, @as("...") _ and @as(1) _ forms still compile but emit a deprecation warning, and the formatter rewrites them to %raw(...). Integers are rewritten as plain JavaScript integers (@as(007) becomes %raw("7")).
The fixed value is stored as a raw_source from Represent %raw, %ffi and %%raw with parsetree nodes #8879 (Pfixed_raw of raw_source), so template payloads have their escapes decoded the same way as in %raw expressions, with or without a PPX.
Attributes on a fixed argument are checked as for @as(value) _: @ignore, @int etc. next to the value have no effect and are reported as unused. Since the formatter prints all of an argument's attributes before its label, this now includes an attribute written before the label (@ignore ~a: @as(1) _), which used to be ignored silently.
%raw is only allowed as a direct external argument, and not as an optional one.
The stdlib, Belt and the test suites use the new syntax. dev-playground builds with the released compiler, so its bindings avoid fixed arguments.

Tests

Parser errors for %raw in other positions, non-string payloads, and two adjacent %raw arguments.
The printer fixture from Remove Pconst_json and represent fixed external arguments in the parsetree #8875 now shows the migration to %raw, plus comments around and inside %raw(...).
Error fixtures for a non-literal payload, an optional %raw argument, and an unused attribute next to %raw.
ounit tests for the literal check and for how fixed values are spelled as JavaScript.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NP47qMfjMbjUTKZDsLthDo

codecov · 2026-10-16T08:09:51Z

Codecov Report

❌ Patch coverage is 86.82635% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.98%. Comparing base (b3c4184) to head (133fd8e).

Files with missing lines	Patch %	Lines
compiler/frontend/ast_external_process.ml	75.00%	5 Missing ⚠️
compiler/ml/ast_fixed_arg.ml	84.37%	5 Missing ⚠️
compiler/ml/printast.ml	0.00%	5 Missing ⚠️
tests/ounit_tests/ounit_ast_mapper0_tests.ml	84.21%	3 Missing ⚠️
compiler/ml/ast_mapper_to0.ml	75.00%	2 Missing ⚠️
compiler/ml/classify_function.ml	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8876      +/-   ##
==========================================
- Coverage   80.98%   80.98%   -0.01%     
==========================================
  Files         455      455              
  Lines       63390    63510     +120     
==========================================
+ Hits        51339    51436      +97     
- Misses      12051    12074      +23

Files with missing lines	Coverage Δ
compiler/ext/config.ml	`100.00% <ø> (ø)`
compiler/frontend/ast_attributes.ml	`89.04% <100.00%> (-2.74%)`	⬇️
compiler/ml/ast_iterator.ml	`93.33% <100.00%> (+0.62%)`	⬆️
compiler/ml/ast_mapper.ml	`98.45% <100.00%> (+<0.01%)`	⬆️
compiler/ml/pprintast.ml	`91.93% <100.00%> (+0.37%)`	⬆️
compiler/syntax/src/res_comments_table.ml	`90.97% <100.00%> (-0.01%)`	⬇️
compiler/syntax/src/res_core.ml	`92.03% <100.00%> (+0.04%)`	⬆️
compiler/syntax/src/res_printer.ml	`94.03% <100.00%> (-0.01%)`	⬇️
tests/ounit_tests/ounit_flow_parser_tests.ml	`95.00% <100.00%> (+1.06%)`	⬆️
tests/ounit_tests/ounit_string_literal_tests.ml	`90.90% <100.00%> (+0.35%)`	⬆️
... and 6 more

... and 3 files with indirect coverage changes

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pkg-pr-new · 2026-10-16T08:19:03Z

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commit: 133fd8e

cknitt · 2026-10-16T12:35:48Z

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chatgpt-codex-connector · 2026-10-16T12:36:07Z

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Codex Review: Didn't find any major issues. Can't wait for the next one!

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`%raw("{}")` writes a fixed external argument as a JavaScript literal. The `@as(value) _` form is deprecated and formats as `%raw(...)`. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NP47qMfjMbjUTKZDsLthDo Signed-off-by: Christoph Knittel <ck@cca.io>

Signed-off-by: Christoph Knittel <ck@cca.io>

A computed object key such as `[f()]` runs code, so it is not a literal. Also test that a `%raw` fixed argument survives the v0 bridge. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NP47qMfjMbjUTKZDsLthDo Signed-off-by: Christoph Knittel <ck@cca.io>

The output is strict, so legacy octal literals such as `007` and octal escapes are rejected. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NP47qMfjMbjUTKZDsLthDo Signed-off-by: Christoph Knittel <ck@cca.io>

@as

As for @as(value) _, an attribute such as @ignore next to %raw has no effect. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NP47qMfjMbjUTKZDsLthDo Signed-off-by: Christoph Knittel <ck@cca.io>

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NP47qMfjMbjUTKZDsLthDo Signed-off-by: Christoph Knittel <ck@cca.io>

Negating an array, object or regexp can run a valueOf method. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NP47qMfjMbjUTKZDsLthDo Signed-off-by: Christoph Knittel <ck@cca.io>

github-actions · 2026-10-17T07:19:13Z

Semantic diff: https://cdn.rescript-lang.org/semantic-diff/pr-8876/index.html?v=133fd8e28

github-actions · 2026-10-17T07:34:55Z

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-8876

cknitt force-pushed the raw-external-args-syntax branch from 4110a6e to a3120b2 Compare October 16, 2026 08:03

cknitt added this pull request to stack #8877 October 16, 2026 08:03

cknitt force-pushed the raw-external-args-syntax branch from a3120b2 to b31d9f9 Compare October 16, 2026 09:01

cknitt mentioned this pull request Oct 16, 2026

Add %raw syntax for fixed external arguments #8609

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Comment thread compiler/ml/classify_function.ml

cknitt mentioned this pull request Oct 16, 2026

RFC: New syntax for fixed external arguments (replacing @as(...) _) #8878

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cknitt force-pushed the raw-external-args-syntax branch from b31d9f9 to 370b817 Compare October 16, 2026 12:57

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cknitt force-pushed the raw-external-args-syntax branch from 370b817 to 0c75297 Compare October 16, 2026 13:21

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

Base automatically changed from fixed-external-args-parsetree to master October 16, 2026 14:49

cknitt force-pushed the raw-external-args-syntax branch 2 times, most recently from 4d07bc5 to 876498c Compare October 16, 2026 15:02

cknitt removed this pull request from stack #8877 October 16, 2026 15:02

cknitt changed the base branch from master to raw-extension-nodes October 16, 2026 15:02

cknitt added this pull request to stack #8880 October 16, 2026 15:02

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cknitt force-pushed the raw-external-args-syntax branch from 876498c to 134919c Compare October 16, 2026 15:45

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cknitt force-pushed the raw-external-args-syntax branch from 134919c to 415e2ec Compare October 16, 2026 19:13

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cknitt mentioned this pull request Oct 17, 2026

Draw commit records as semantic diffs of the compiler's objects #8882

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Base automatically changed from raw-extension-nodes to master October 17, 2026 07:18

cknitt and others added 7 commits October 17, 2026 09:18

Allow slow CI builds in watch recompilation test

70da6ea

Signed-off-by: Christoph Knittel <ck@cca.io>

Test migrating fixed values with template syntax

1bd4f35

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NP47qMfjMbjUTKZDsLthDo Signed-off-by: Christoph Knittel <ck@cca.io>

Allow unary minus only on numbers in fixed literals

133fd8e

Negating an array, object or regexp can run a valueOf method. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NP47qMfjMbjUTKZDsLthDo Signed-off-by: Christoph Knittel <ck@cca.io>

cknitt force-pushed the raw-external-args-syntax branch from ad2b5b3 to 133fd8e Compare October 17, 2026 07:18

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