Remove parser modes by encoding constructor arity explicitly by cknitt · Pull Request #8610 · rescript-lang/rescript

cknitt · 2026-09-03T17:40:45Z

Motivation

The ReScript parser previously supported two modes:

type mode =
  | ParseForTypeChecker
  | Default

These modes allowed the parser to produce different trees depending on whether its output was intended for the type checker or the printer.

In particular, the type-checker representation collapsed the distinction between multiple constructor arguments and a tuple passed as a single constructor argument. The printer needed a different, source-preserving representation so it could retain the extra parentheses.

The parser mode was also used for differences in string-literal representation. #8606 normalized string literals across the compiler and was the first step toward removing the mode entirely. Constructor arity was the main remaining reason for it.

This change

Constructor arity is now represented explicitly in the parsetree:

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

The same approach is used for polymorphic variant expressions, patterns, and type payload groups.

For example:

type binary = Binary(int, int)
type unaryTuple = UnaryTuple((int, int))

Their applications have unambiguous parsetree representations:

Binary(x, y)
→ Pexp_construct("Binary", [x; y])

UnaryTuple((x, y))
→ Pexp_construct("UnaryTuple", [Pexp_tuple [x; y]])

The type checker can therefore compare the number of syntactic arguments directly with the constructor's declared arity.

This also means that a constructor with one tuple payload must use the corresponding nested parentheses:

Some((x, y))

Writing Some(x, y) now correctly reports an arity mismatch.

Alignment with subsequent compiler layers

The new representation is more closely aligned with subsequent compiler layers. Nominal constructors are already represented as argument lists in the typedtree, so the type checker no longer needs to infer their arity by inspecting tuple nodes.

Polymorphic variants continue to be normalized to their semantic single-payload representation when entering the type checker. Their source-level arity remains explicit in the parsetree for printing and tooling.

There is therefore no need to change the typedtree or later compiler representations.

Cleanup enabled by the new representation

With both source shape and constructor arity available in one parsetree, this PR removes:

ParseForTypeChecker and Default from the parser
the for_printer argument throughout parser APIs and their callers
the developer-only -typechecker parser flag
parser branches that produced different trees for printing and type checking
type-checker logic that inferred constructor arity by inspecting tuple nodes
redundant tuple unpacking in analysis and compiler traversals

It also:

consolidates constructor argument parsing and printing
centralizes AST0 constructor argument encoding and decoding
confines legacy ocaml.explicit_arity handling to the AST0 compatibility boundary
makes constructor argument naming consistently reflect that the parsetree contains a list

PPX compatibility

The frozen parsetree0 representation remains unchanged.

When converting multiple constructor arguments to AST0, the bridge encodes them as a tuple and attaches internal _res.constructor_args metadata. Conversion back uses that metadata to restore the argument list and removes the internal attribute before returning to the current parsetree.

This preserves the distinction between multiple arguments and a single tuple argument across PPX round trips without requiring changes to the frozen PPX-facing AST.

Breaking change

Code that relied on the previous ambiguity must add explicit tuple parentheses.

For example:

Some(x, y)

must become:

Some((x, y))

when the constructor has one tuple payload.

Testing

The change includes coverage for:

nominal constructor expressions and patterns
polymorphic variant expressions, patterns, and type payloads
unary tuple payloads versus multiple arguments
constructor arity mismatch diagnostics
AST0 and PPX round trips
syntax printing and round trips
analysis features and completion locations
end-to-end compilation and runtime behavior

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

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

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-03T18:50:23.455914Z	`b374049`	Manual request

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

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Reviewed commit: ba681ec574

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

codecov · 2026-09-03T17:54:51Z

Codecov Report

❌ Patch coverage is 85.78199% with 90 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.43%. Comparing base (bc382a4) to head (cbf4a97).

Files with missing lines	Patch %	Lines
tests/ounit_tests/ounit_ast_mapper0_tests.ml	86.71%	19 Missing ⚠️
analysis/src/dump_ast.ml	0.00%	12 Missing ⚠️
compiler/ml/printast.ml	0.00%	9 Missing ⚠️
compiler/ml/ast_mapper_from0.ml	91.17%	6 Missing ⚠️
compiler/frontend/bs_builtin_ppx.ml	55.55%	4 Missing ⚠️
compiler/ml/ast_mapper.ml	76.47%	4 Missing ⚠️
analysis/src/completion_expressions.ml	76.92%	3 Missing ⚠️
analysis/src/type_utils.ml	40.00%	3 Missing ⚠️
compiler/ml/ast_helper.ml	40.00%	3 Missing ⚠️
compiler/ml/ast_mapper_to0.ml	93.75%	3 Missing ⚠️
... and 16 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8610      +/-   ##
==========================================
+ Coverage   77.32%   77.43%   +0.10%     
==========================================
  Files         467      467              
  Lines       63342    63491     +149     
==========================================
+ Hits        48982    49166     +184     
+ Misses      14360    14325      -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_patterns.ml	`86.40% <100.00%> (-1.32%)`	⬇️
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/xform.ml	`88.20% <100.00%> (+0.19%)`	⬆️
compiler/bsc/rescript_compiler_main.ml	`71.49% <ø> (ø)`
compiler/common/pattern_printer.ml	`87.75% <100.00%> (-0.25%)`	⬇️
compiler/frontend/ast_derive_projector.ml	`90.16% <100.00%> (-0.47%)`	⬇️
... and 42 more

... and 2 files with indirect coverage changes

🚀 New features to boost your workflow:

❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
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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/linux-arm64@8610

@rescript/linux-x64

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

@rescript/runtime

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

@rescript/win32-x64

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

commit: cbf4a97

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

cknitt · 2026-09-03T18:41:57Z

@codex review

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Reviewed commit: b374049de4

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

cknitt added 7 commits September 3, 2026 19:01

Refactor constructor arity in parsetree

917530a

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

Remove obsolete parser printer flag

843776d

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

Share constructor pattern argument parsing

066e90d

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

Share constructor argument printing

1f5f01e

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

Centralize AST0 constructor argument bridging

aa8fd17

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

Localize legacy explicit arity handling

9edec64

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

Use plural names for constructor source arguments

ba681ec

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

Add constructor arity changelog entry

758d9b1

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

chatgpt-codex-connector Bot reviewed Sep 3, 2026

View reviewed changes

Comment thread compiler/ml/ast_mapper_from0.ml

Comment thread compiler/ml/typecore.ml

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

cknitt added 2 commits September 3, 2026 19:51

Preserve fresh AST0 constructor arity

0fe7287

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

Partition polymorphic variant argument comments

244813e

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

Fix AST0 constructor payload locations and printing

b374049

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

chatgpt-codex-connector Bot reviewed Sep 3, 2026

View reviewed changes

Comment thread compiler/ext/config.ml

Bump compiled artifact versions for constructor AST changes

6e1f279

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

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

Deduplicate AST0 bridge marker removal

cbf4a97

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

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Remove parser modes by encoding constructor arity explicitly - #8610

Remove parser modes by encoding constructor arity explicitly#8610
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cknitt commented Sep 3, 2026

Motivation

This change

Alignment with subsequent compiler layers

Cleanup enabled by the new representation

PPX compatibility

Breaking change

Testing

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