Section 9: Match Expressions

Goal: Full pattern matching support

SPEC: spec/15-patterns.md

Proposals:

proposals/approved/match-expression-syntax-proposal.md — Match expression and pattern syntax
proposals/approved/pattern-matching-exhaustiveness-proposal.md — Exhaustiveness checking
proposals/approved/range-patterns-char-byte-proposal.md — Range patterns on char and byte

9.0 Match Expression Syntax

Proposal: proposals/approved/match-expression-syntax-proposal.md

Documents the existing implementation of match expressions. Key specifications:

match expr { pattern -> expression, ... } syntax (comma-separated arms)
Guard syntax if condition
Pattern types: literal, binding, wildcard, variant, struct, tuple, list, range, or-pattern, at-pattern
Top-to-bottom, first-match-wins evaluation
Integer-only literal patterns (no float patterns)

Status: IMPLEMENTED — This proposal formalizes existing behavior.

9.0.1 Comma-Separated Match Arms

Proposal: proposals/approved/match-arm-comma-separator-proposal.md

Changes match arm separators from newlines to commas (trailing commas optional). Also formally adopts if guard syntax replacing .match(condition). Key changes:

match_arms = [ match_arm { "," match_arm } [ "," ] ] — comma-separated
match_arm = match_pattern [ "if" expression ] "->" expression — if guards
.match() now exclusively means method-style pattern matching
Formatter allows single-line matches for short, simple arms

Status: APPROVED — Parser migration part of block-expression-syntax implementation.

Implementation

Implement: Parser — comma-separated match arms in match expr { } block syntax
- Rust Tests: ori_parse/src/tests/ — comma-separated arm parsing
- Ori Tests: tests/spec/patterns/match.ori — update to comma syntax
Implement: Parser — if guard syntax replacing .match(condition)
- Rust Tests: ori_parse/src/tests/ — if guard parsing
- Ori Tests: tests/spec/patterns/match.ori — update guard tests
Implement: Formatter — emit commas, support single-line short matches
- Rust Tests: ori_fmt/src/formatter/ — comma emission tests

9.1 match Expression

9.2 Pattern Types

9.3 Pattern Guards

9.4 Exhaustiveness Checking

Proposal: proposals/approved/pattern-matching-exhaustiveness-proposal.md

Pattern matrix decomposition algorithm (Maranget’s algorithm) for exhaustiveness verification.

9.4.1 Core Algorithm

Implement: Pattern matrix decomposition — proposals/approved/pattern-matching-exhaustiveness-proposal.md § Algorithm
- Rust Tests: ori_types/src/check/exhaustiveness/tests.rs — matrix decomposition
- Ori Tests: tests/spec/patterns/match_exhaustive.ori
Implement: Constructor enumeration for types — proposals/approved/pattern-matching-exhaustiveness-proposal.md § Algorithm
- Rust Tests: ori_types/src/check/exhaustiveness/tests.rs — type constructors
- Ori Tests: tests/spec/patterns/match_exhaustive.ori

9.4.2 Exhaustiveness Errors

Implement: Match expressions must be exhaustive (E0123) — proposals/approved/pattern-matching-exhaustiveness-proposal.md § Error Policy
- Rust Tests: ori_types/src/check/exhaustiveness/tests.rs — exhaustiveness checking
- Ori Tests: tests/spec/patterns/match_exhaustive.ori
Implement: Let binding refutability check — proposals/approved/pattern-matching-exhaustiveness-proposal.md § Refutability Requirements
- Rust Tests: ori_types/src/check/exhaustiveness/tests.rs — refutability errors
- Ori Tests: tests/spec/patterns/match_exhaustive.ori
Implement: Function clause exhaustiveness — proposals/approved/pattern-matching-exhaustiveness-proposal.md § Error Policy
- Rust Tests: ori_types/src/check/exhaustiveness/tests.rs — clause exhaustiveness
- Ori Tests: tests/spec/patterns/function_clauses_exhaustive.ori

9.4.3 Guard Handling

Implement: Guards not considered for exhaustiveness — proposals/approved/pattern-matching-exhaustiveness-proposal.md § Guards
- Rust Tests: ori_types/src/check/exhaustiveness/tests.rs — guard handling
- Ori Tests: tests/spec/patterns/match_guards_exhaustive.ori
Implement: Guards require catch-all pattern (E0124) — proposals/approved/pattern-matching-exhaustiveness-proposal.md § Guards
- Rust Tests: ori_types/src/check/exhaustiveness/tests.rs — guard catch-all requirement
- Ori Tests: tests/spec/patterns/match_guards_exhaustive.ori

9.4.4 Pattern Coverage

9.4.5 Unreachable Pattern Detection

Implement: Detect completely unreachable patterns (W0456) — proposals/approved/pattern-matching-exhaustiveness-proposal.md § Unreachable Pattern Detection
- Rust Tests: ori_types/src/check/exhaustiveness/tests.rs — unreachable detection
- Ori Tests: tests/spec/patterns/match_unreachable.ori
Implement: Detect overlapping range patterns (W0457) — proposals/approved/pattern-matching-exhaustiveness-proposal.md § Range Overlap
- Rust Tests: ori_types/src/check/exhaustiveness/tests.rs — range overlap detection
- Ori Tests: tests/spec/patterns/match_range_overlap.ori
Implement: Suggest missing patterns in error messages — proposals/approved/pattern-matching-exhaustiveness-proposal.md § Error Messages
- Rust Tests: ori_types/src/check/exhaustiveness/tests.rs — suggestions
- Ori Tests: tests/spec/patterns/match_exhaustive.ori

9.5 Named Variant Pattern Fields (Argument Punning)

Proposal: proposals/approved/argument-punning-proposal.md

Allow named field syntax in variant patterns, with punning when the binding variable matches the field name: Circle(radius:) for Circle(radius: radius).

type Shape = Circle(radius: float) | Rectangle(width: float, height: float)

// Before (positional only):
match shape {
    Circle(r) -> r * r * 3.14
    Rectangle(w, h) -> w * h
}

// After (named + punned):
match shape {
    Circle(radius:) -> radius * radius * 3.14
    Rectangle(width:, height:) -> width * height
}

Parser

Implement: Support name: and name: pattern in variant pattern fields
- Rust Tests: ori_parse/src/grammar/pattern/tests.rs — named variant field parsing
- Ori Tests: tests/spec/patterns/variant_punning.ori
Implement: Mixed named and positional fields in same variant pattern
- Ori Tests: tests/spec/patterns/variant_punning_mixed.ori
Implement: Positional variant patterns unchanged (no regression)
- Ori Tests: tests/spec/patterns/variant_positional_regression.ori

IR

Implement: Extend variant pattern representation to support named fields
- Rust Tests: ori_ir/src/ast/pattern/tests.rs — named variant field IR

Type Checker

Implement: Validate named fields match variant definition
- Rust Tests: ori_types/src/check/ — variant field name validation
- Ori Tests: tests/compile-fail/variant_punning_unknown_field.ori
Implement: Named fields can appear in any order
- Ori Tests: tests/spec/patterns/variant_punning_reorder.ori

Evaluator

Implement: Match named variant fields by name (reorder to definition order)
- Rust Tests: ori_eval/src/interpreter/ — named variant field matching
- Ori Tests: tests/spec/patterns/variant_punning.ori

LLVM

Implement: LLVM codegen for named variant field patterns
- LLVM Rust Tests: ori_llvm/tests/matching_tests.rs — named variant field codegen
- AOT Tests: No AOT coverage yet

Formatter

Implement: Detect name: name in variant patterns and emit name: form
- Rust Tests: ori_fmt/src/formatter/ — variant pattern punning canonicalization

Documentation

Implement: Update spec 10-patterns.md with named variant field patterns
Implement: Update grammar.ebnf with variant_field production
Implement: Update .claude/rules/ori-syntax.md with pattern punning syntax

9.6 Section Completion Checklist

All items above have all three checkboxes marked [ ]
Spec updated: spec/15-patterns.md reflects implementation
CLAUDE.md updated if syntax/behavior changed
80+% test coverage
Run full test suite: ./test-all.sh

Exit Criteria: Match expressions work like spec