Match (updated)

[rossirpaulo]

Important

Supersedes #2825
I renamed the paulo/map branch to paulo/match and had to recreate this PR. The original is referenced above to preserve the original comments.

This is the first pass on matchs implementation. I've tried to make sure there are plenty of comments referencing the BEP shared yesterday on critical moments; expect a cleanup later on.

There are somethings to keep in mind:

Completed Work

Core Infrastructure

Component	Status	Location
Lexer tokens	Done	TokenKind::Match, FatArrow, Pipe
Syntax kinds	Done	syntax_kind.rs
Parser	Done	parser.rs - parse_match_expr, parse_match_arm, etc.
HIR	Done	body.rs - Expr::Match, MatchArm, Pattern variants
AST helpers	Done	ast.rs

Type Checking

Feature	Status	Location
Pattern variable binding	Done	lib.rs:781-785 - ctx.define(name, narrowed_ty)
Type narrowing	Done	lib.rs:834-855 - extract_pattern_binding returns narrowed type
Exhaustiveness checking	Done	lib.rs:1011-1121 - check_match_exhaustiveness
Unreachable arm detection	Done	Same function, detects arms after catch-all

Phase 3: Code Generation

Feature	Status	Location
instanceof codegen	Done	compiler.rs:913-943 - compile_match_pattern emits CmpOp::InstanceOf
Pattern binding	Done	compiler.rs:1047-1061 - bind_pattern_variable
Guard handling	Done	compiler.rs:518-535
If-else desugaring	Done	Full match-to-if-else chain implemented

Tests (PARTIAL)

Test Type	Status	Location
VM integration tests	Done	match_expr.rs - 20+ tests covering bindings, literals, typed patterns, guards
Exhaustiveness tests	Done	match_exhaustiveness/

What's Next

Type System Improvements

1. Type Alias Expansion
Current: Type aliases like type Result = Success | Failure are treated as opaque Ty::Named("Result").
Required: Expand type aliases to their definitions for proper union exhaustiveness checking.

Without this, exhaustiveness checking doesn't work for type alias unions.

2. Enum Variant Exhaustiveness
Current: Enum variant patterns (Status.Active) are treated as literals, not contributing to exhaustiveness.

Required: Track which enum variants are covered and error if variants are missing.

enum Status { Active, Inactive, Pending }
match (s) {
  Status.Active => "active"
  // ERROR: Inactive and Pending not handled
}

3. Literal Type Exhaustiveness
Current: Literal patterns don't contribute to type-level exhaustiveness.

Required: For literal union types (200 | 201 | 204), track which values are covered.

Refinements

1. Overlapping Pattern Verification
The BEP says this should work (second arm only matches B, not unreachable):

match (x) {
  _: A => "a"
  _: A | B => "ab"  // Should NOT be flagged as unreachable
}

Status: Likely works, but needs explicit test coverage.

2. Better Error Spans
Current: Error spans point to the match expression, not the specific arm.

Required: Point to the specific arm that's unreachable or the missing cases.

[vercel]

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Project	Deployment	Review	Updated (UTC)
promptfiddle	Ready	Preview, Comment	Dec 17, 2025 9:47pm

[github-actions]

✅ Latest build complete • View logs

[github-actions]

🐑 BEPs Preview Ready

Preview URL: https://dj7ggjkp4tlhz.cloudfront.net/paulo-match/

_{Commit: f2f902ffdbed401156175e021d12c5893400036c • Workflow run}

[codspeed-hq]

CodSpeed Performance Report

Merging #2828 will not alter performance

_{Comparing paulo/match (f2f902f) with canary (54559e1)}

Summary

✅ 15 untouched
⏩ 14 skipped¹

Footnotes

1. 14 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[antoniosarosi]

Can you add some of these tests to baml_codegen/tests/ as well?

[antoniosarosi]

Here I would add AST accessors to make lowering higher level (pun intended), not direct syntax nodes access. See this comment in PR #2838

[antoniosarosi]

Same as above, I'd add AST accessors.

[antoniosarosi]

CST -> AST -> HIR

[antoniosarosi]

I think this is no longer necessary, we merged match-3 branch