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feat: label with fallthrough default value (#6163)#6174

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feat: label with fallthrough default value (#6163)#6174
ggreif wants to merge 3 commits into
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gabor/label-defaults

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@ggreif ggreif commented Jun 4, 2026 •
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Resolves #6163 — a labeled early-exit with a fallthrough default, so

label found : Bool = false
  for (c in values.vals()) { if (cmp(v, #eq, c)) break found true }

replaces the mechanical

label found : Bool do {
  for (c in values.vals()) { if (cmp(v, #eq, c)) break found true };
  false
}

Desugaring (parser only)

label x [: T] = <default> <body> ⟿ label x [: T] do { <body>; <default> }

The default is the value when no break x fires. Being the block's tail it is evaluated only on demand — skipped once a break exits the label, exactly like the else branch of if.

Default operand level

Parsed at exp_nullary (literals, identifiers, parenthesised expressions). A higher level juxtaposed with the body is LR-ambiguous (unary/binary operator overlap — exp_bin/exp_un both reintroduce menhir conflicts). Complex defaults are written parenthesised: (#err "not found"), (?x).

Typing: which forms work

The label's type still comes from the annotation T (the body — hence the default and any break value — is checked against it). There is no inference of the label type from the body, so:

non-bottom default (false) bottom default (return …/trap, None)
annotated label x : T = D … ✅ ✅ (None <: T)
compact label x = D … ❌ M0050 (Bool vs ()) ✅ (None <: ())

Without an annotation the label type is (), so the compact form only typechecks when the default fits () (a bottom default). The compact-with-non-bottom form (type inferred from the default, as floated in the issue) would need a typechecker change and is not part of this parser-only PR.

Tests

  • test/run/label-default.mo — annotated × {non-bottom, bottom} and compact × bottom, plus a check that the default is evaluated lazily. Passes [tc] [run] [run-ir] [run-low] [comp] [valid] [wasm-run].
  • test/fail/label-default-compact.mo — pins compact + non-bottom as a type error (M0050/M0096).

printers.ml handles the menhir-generated optional-default nonterminal in syntax-error messages.

🤖 Generated with Claude Code

@ggreif @claude
feat(parser): label with fallthrough default value (#6163)
3d2a9c9 
Desugar `label x [: T] = <default> <body>` to
`label x [: T] do { <body>; <default> }`.  The default is the value when no
`break x` fires; being the block's tail it is evaluated only on demand, like
the `else` branch of `if`.  Replaces the mechanical `do { <body>; <default> }`
boilerplate.

The default operand is parsed at `exp_nullary` (literals, identifiers,
parenthesised expressions) — a higher level juxtaposed with the body is
LR-ambiguous (unary/binary operator overlap).  Complex defaults such as
`#err "x"` or `?x` are written parenthesised.

The label's type still comes from the annotation T (the body, hence the
default and any `break` value, is checked against it).  So:
  - `label x : T = <default> ...`  works for any default (`None <: T`);
  - `label x = <default> ...`      (compact, no annotation) gives the label
    type (), so it typechecks only when the default fits () — i.e. a bottom
    default like `return`/trap, not a non-bottom one.

Tests:
  - test/run/label-default.mo: annotated x {non-bottom, bottom} and
    compact x bottom, plus a check that the default is evaluated lazily.
  - test/fail/label-default-compact.mo: compact + non-bottom default is a
    type error (M0050/M0096).

printers.ml: handle the menhir-generated optional-default nonterminal in
syntax-error messages.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
@ggreif ggreif requested a review from a team as a code owner June 4, 2026 08:13
@ggreif ggreif self-assigned this Jun 4, 2026
@github-actions

github-actions Bot commented Jun 4, 2026

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Cursor AI review

👍 APPROVE — looks safe to merge

Category Assessment Details
Summary ✅ Adds parser sugar label x [: T] = <default> <body> desugaring to label x [: T] do { <body>; <default> }, with syntax-error printer support and run/fail tests for annotated, compact-bottom, laziness, and compact+non-bottom rejection.
Code Quality ✅ Reuses existing LabelE/BlockE desugaring and preceded(EQ, …) pattern; no redundant helpers.
Consistency ✅ Matches existing label/for/while rewriting and BlockE/ExpD construction style in parser.mly.
Correctness ✅ Desugaring preserves on-demand default evaluation and existing label typing; default operand level avoids documented menhir conflicts.
Tests ✅ test/run/label-default.mo covers annotated/compact-bottom/laziness; test/fail/label-default-compact.mo has matching .tc.ok, .tc-human.ok, and .ret.ok files.
Changelog ⚠️ New user-visible syntax has no top-of-file changelog bullet (repo convention places unreleased items before version headers).

Verdict

Decision: APPROVE
Risk: Low
Reason: Parser-only desugar to existing AST forms with thorough tests and no codegen/typechecker changes; missing changelog is a minor documentation gap, not a correctness defect.

Generated for commit 3d2a9c9

@ggreif ggreif marked this pull request as draft June 4, 2026 08:16
@ggreif ggreif changed the title feat(parser): label with fallthrough default value (#6163) feat: label with fallthrough default value (#6163) Jun 4, 2026
@ggreif @claude
docs(grammar): regenerate for label fallthrough default (#6163)
fae550a 
The `label` production gained an optional `('=' <exp_nullary>)?` default; update
the committed grammar.txt so the check-grammar CI step matches.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
@github-actions

github-actions Bot commented Jun 4, 2026 •
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Comparing from 56ddce8 to b7f2d4d:
In terms of gas, no changes are observed in 5 tests.
In terms of size, no changes are observed in 5 tests.

@ggreif @claude
docs(changelog): label with a fallthrough default (#6163)
b7f2d4d 
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
@ggreif ggreif mentioned this pull request Jun 4, 2026
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Allow label with fallthrough default value

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