Remove parser modes by cknitt · Pull Request #8610 · rescript-lang/rescript

cknitt · 2026-09-03T17:40:45Z

Motivation

The parser previously had two modes:

type mode = ParseForTypeChecker | Default

These produced different trees for printing and type checking. The printer needed to preserve constructor argument grouping and parentheses, while the type-checker representation collapsed those distinctions.

The modes also handled differences in string-literal representation. [#8606](#8606) unified that representation and was the first step toward removing the modes. Constructor argument grouping was the main remaining obstacle.

This PR replaces the two representations with one source-preserving parsetree, while retaining existing source and PPX compatibility.

Source syntax and semantic arity

Constructor expressions and patterns now carry located argument lists:

Pexp_construct of Longident.t loc * expression list loc
Ppat_construct of Longident.t loc * pattern list loc

The same approach applies to polymorphic variant expressions, patterns, and type payload groups.

These lists describe what was written, not the constructor’s declared arity. For example, omitting locations:

C(x, y)   → [x; y]
C((x, y)) → [Pexp_tuple [x; y]]

The printer can preserve either spelling. Once the type checker resolves an ordinary constructor’s declaration, it normalizes the arguments accordingly: unpacking a tuple for a multi-argument constructor or packing multiple syntactic arguments into a single tuple payload.

Existing spellings such as Some(x, y) and Some((x, y)) therefore remain supported. This does not introduce stricter arity rules or require source migration.

Normalization happens within the existing type-checking flow, without an additional AST pass. Ordinary constructors continue into the typedtree as semantic argument lists; polymorphic variants retain their optional single-payload representation. No typedtree or later-layer representation changes are needed.

Argument locations

Argument lists carry their own source span, including the surrounding parentheses. Consumers no longer need to reconstruct that span from individual arguments or encode it in attributes.

This simplifies printing and comment handling and fixes constructor signature help appearing in whitespace or comments between the constructor name and its arguments. Signature help now uses the argument-list boundaries.

Cleanup

The unified representation removes:

ParseForTypeChecker and Default
for_printer parameters and the developer-only -typechecker parser flag
mode-dependent tuple wrapping in the parser
redundant tuple unpacking and source-shape reconstruction in printers, traversals, and editor tooling

It also consolidates constructor argument parsing and printing, makes AST helpers accept located argument lists directly, and confines legacy explicit-arity attribute handling to the AST0 compatibility boundary.

PPX compatibility

The frozen parsetree0 representation remains unchanged.

The bridge encodes multiple syntactic arguments as a tuple with internal _res.constructor_args metadata. Conversion back consumes that metadata and restores the argument list. A single tuple argument needs no marker.

Unmarked tuples generated by existing PPXs remain supported: their semantic grouping is resolved during type checking. Legacy explicit_arity and ocaml.explicit_arity attributes are handled at the bridge boundary.

Multi-argument locations survive through the encoded tuple. Where AST0 cannot represent a separate outer argument-list span for a single payload, conversion falls back to the payload’s location.

Testing

Coverage includes constructor and polymorphic variant expressions, patterns, and type payloads; tuple grouping compatibility; arity diagnostics; AST0 round trips and legacy PPX output; argument locations and parser recovery; printing and comment preservation; signature-help boundaries; and end-to-end compilation and runtime behavior.

chatgpt-codex-connector · 2026-09-03T17:40:56Z

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-05T20:21:16.942816Z	`425e204`	Manual request

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codecov · 2026-09-03T17:54:51Z

Codecov Report

❌ Patch coverage is 87.81838% with 110 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.55%. Comparing base (0aa58d8) to head (425e204).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
tests/ounit_tests/ounit_ast_mapper0_tests.ml	89.02%	26 Missing ⚠️
analysis/src/dump_ast.ml	0.00%	12 Missing ⚠️
analysis/src/type_utils.ml	77.77%	12 Missing ⚠️
compiler/ml/printast.ml	0.00%	11 Missing ⚠️
...s/ounit_tests/ounit_constructor_arguments_tests.ml	86.30%	10 Missing ⚠️
compiler/ml/ast_mapper.ml	81.81%	4 Missing ⚠️
compiler/ml/typecore.ml	89.74%	4 Missing ⚠️
compiler/syntax/src/res_core.ml	90.24%	4 Missing ⚠️
compiler/frontend/bs_builtin_ppx.ml	66.66%	3 Missing ⚠️
compiler/ml/ast_helper.ml	57.14%	3 Missing ⚠️
... and 14 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8610      +/-   ##
==========================================
+ Coverage   77.38%   77.55%   +0.17%     
==========================================
  Files         474      475       +1     
  Lines       63524    63851     +327     
==========================================
+ Hits        49156    49518     +362     
+ Misses      14368    14333      -35

Files with missing lines	Coverage Δ
analysis/src/codemod.ml	`68.96% <ø> (ø)`
analysis/src/commands.ml	`63.82% <ø> (ø)`
analysis/src/completion_expressions.ml	`85.45% <100.00%> (+5.60%)`	⬆️
analysis/src/completion_patterns.ml	`86.59% <100.00%> (-1.13%)`	⬇️
analysis/src/diagnostics.ml	`75.00% <ø> (ø)`
analysis/src/hint.ml	`75.00% <100.00%> (ø)`
analysis/src/process_attributes.ml	`83.78% <100.00%> (ø)`
analysis/src/shared_types.ml	`70.42% <ø> (ø)`
analysis/src/xform.ml	`88.26% <100.00%> (+0.25%)`	⬆️
compiler/bsc/rescript_compiler_main.ml	`71.49% <ø> (ø)`
... and 45 more

... and 4 files with indirect coverage changes

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pkg-pr-new · 2026-09-03T18:36:10Z

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npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/darwin-arm64@8610

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npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/linux-arm64@8610

@rescript/linux-x64

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@rescript/win32-x64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/win32-x64@8610

commit: 425e204

cknitt · 2026-09-03T18:41:57Z

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cknitt · 2026-09-04T05:12:15Z

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cristianoc

what happens in the bridge should stat in the bridge: ast should be restored and any encoding trick should be gone and not leaked in to the type checker

@cristianoc

Apply the normalization approach proposed by @cristianoc in PR rescript-lang#8610. Keep source argument lists for printing and resolve semantic grouping after constructor disambiguation, without legacy PPX marker handling in the type checker or printer. Signed-off-by: Christoph Knittel <ck@cca.io>

cknitt · 2026-09-04T16:08:31Z

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cknitt · 2026-09-04T20:00:21Z

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cknitt · 2026-09-05T04:59:08Z

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

cknitt · 2026-09-05T17:02:00Z

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cknitt · 2026-09-05T18:09:13Z

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cknitt · 2026-09-05T20:15:54Z

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chatgpt-codex-connector · 2026-09-05T20:21:14Z

+    match tuple_args arg with
+    | Some args when split_tuple -> List.map map args
+    | _ -> [map arg])


Preserve attributes on marked constructor payload tuples

When a v0 PPX adds an attribute to the synthesized tuple for a marked multi-argument constructor or polymorphic-variant expression/pattern, this branch unwraps the tuple by mapping only its children, silently discarding the tuple's pexp_attributes or ppat_attributes. This breaks chained PPXs that communicate through payload attributes; as with the type-payload handling, keep an attributed tuple wrapper while consuming _res.constructor_args.

AGENTS.md reference: AGENTS.md:L37-L39

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cknitt requested a review from cristianoc September 3, 2026 19:16

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cknitt force-pushed the codex/remove-parser-mode branch from cbf4a97 to a6d5edc Compare September 4, 2026 16:00

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cknitt changed the title ~~Remove parser modes by encoding constructor arity explicitly~~ Remove parser modes Sep 4, 2026

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cknitt added 11 commits September 5, 2026 18:48

Refactor constructor arity in parsetree

c00d32e

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

Remove obsolete parser printer flag

2201335

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

Share constructor pattern argument parsing

89684c2

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

Share constructor argument printing

a2e5db1

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

Centralize AST0 constructor argument bridging

bc05fa5

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

Localize legacy explicit arity handling

fe2cdde

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

Use plural names for constructor source arguments

3469bc8

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

Add constructor arity changelog entry

f442710

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

Preserve fresh AST0 constructor arity

ef87eba

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

Partition polymorphic variant argument comments

62d26cc

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

Fix AST0 constructor payload locations and printing

a9d34a7

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

cknitt added 19 commits September 5, 2026 18:48

Simplify constructor bridge metadata handling and assertions

23602f2

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

Clarify constructor argument naming and simplify traversal

3dd09af

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

Rename constructor argument compatibility fixture

da1d7e0

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

Preserve constructor argument-list locations explicitly

d4babe2

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

Reuse argument-list locations in parsing and type checking

3b74c81

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

Use argument-list boundaries for constructor signature help

b226130

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

Accept located constructor arguments in AST helpers

976c632

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

Separate constructor argument tests from AST0 bridge coverage

0fb0355

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

Document the constructor argument AST0 bridge contract

b8368e9

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

Use explicit locations at constructor helper call sites

a6352c8

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

Normalize constructor tuple paths in completion

fb1e677

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

Select constructor tuple parameters in signature help

f537d77

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

Preserve constructor source arity in completion paths

6ff0d42

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

Use shared argument traversal for constructor completion gaps

37dda63

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

Reuse resolved inline-record constructor fields in completion

68d6884

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

Preserve list constructor attributes on the AST0 wire

7411334

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

Simplify AST0 argument tuple callbacks

60adb6f

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

Preserve attributed list payload tuples across AST0 conversions

cdb15da

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

Adapt constructor tag helper to argument lists

045c62d

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

cknitt force-pushed the codex/remove-parser-mode branch from 6d16043 to 045c62d Compare September 5, 2026 16:59

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cknitt added 2 commits September 5, 2026 19:19

Normalize constructor payload grouping in completion paths

87349f5

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

Keep unary tuple signature help on its only parameter

531a5fd

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

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Preserve attributes on marked polymorphic variant type tuples

425e204

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

cknitt force-pushed the codex/remove-parser-mode branch from a7a9475 to 425e204 Compare September 5, 2026 18:47

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Motivation

Source syntax and semantic arity

Argument locations

Cleanup

PPX compatibility

Testing

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