hamilton-core

Language Match Semantics — Feature Expert

Ownership

• Frontend patterns may preserve a bare identifier as a binding-shaped HIR node when its value/category is unavailable until MIR lowering.
• MIR must resolve immutable current-module scalar val names before enum or capture fallback. Mutable var names and genuinely unbound identifiers stay capture patterns.
• Dispatch must consume normalized arms. Integer-only cases retain the switch path; text and bool cases use ordered scalar equality, with text compared by content through rt_text_eq_any.

Regression contract

test/01_unit/compiler/codegen/match_bare_val_constant_spec.spl is the exact and adjacent contract: two text constants, two integer constants, bool values, wildcard reachability, mutable-name capture, and genuine capture.

Evidence honesty

Source and static contracts do not close native behavior. Require a provenance-admitted pure-Simple CLI to execute the focused spec before changing the BugDB row from verification-pending to fixed.

Guards are evaluated on every arm shape (3a)

case P if g: runs its body only when P matches and g holds, for every pattern kind. Two lowerings carry guards and both are counted:

• enum arms and guarded catch-alls: lower_enum_match_flat; guarded int / or-of-int / binding / wildcard arms: lower_guarded_scalar_match (50.mir/_MirLoweringExpr/switch_operators_calls.spl);
• tuple, text, range, struct and array arms: build_if_chain (20.hir/hir_lowering/_Expressions/match_desugaring.spl), which tests pattern and { destructure prelude; guard }.

The guard-count invariant (E-HIR-MATCH-GUARD / E-MIR-MATCH-GUARD, level via SIMPLE_MIR_VERIFY_LEVELS=E-MIR-MATCH-GUARD=...) reports a guard that was parsed but not lowered. Guide: doc/07_guide/compiler/robustness/match-guard-invariant.md. Specs: test/01_unit/compiler/50.mir/match_guard/. The seed run engine ignores a guard on case _ if c:; simple test evaluates it.