core: match guards through typing + interp + wasm (#67)#74
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End-to-end implementation of `MatchArm::guard` (the `if e` clause that
runs after the pattern matches and before the arm body). The
typechecker enforces `Ty::Bool`, the interpreter falls through on
`false`, codegen emits an `i32.eqz + br_if $arm_i_fail` after the
destructure.
## Typing — `ephapax-typing`
* `check_match` now type-checks the guard under the arm's pattern
bindings; the guard's type is unified with `Bool` at the guard's
span. Non-Bool guards produce `TypeMismatch`.
* `check_exhaustiveness` skips guarded arms when building the
matrix — guards can refute at runtime, so they don't contribute
to coverage. `Some(v) if v > 0 -> ... | None -> ...` is
non-exhaustive (missing the Some-non-positive case); only
`Some(v) -> ... | None -> ...` (no guard) covers Some entirely.
## Interp — `ephapax-interp`
* `eval_match` evaluates the guard after applying pattern bindings.
If the guard returns `false`, the bindings are restored to their
prior state and the next arm is tried. Failed guards must not
leak bindings — confirmed by a regression test where an outer
`v=999` survives a refuted inner `Some(v)` arm.
* Non-Bool guard values raise `RuntimeError::TypeError { expected:
"Bool", found: ... }` — should be unreachable for well-typed
programs.
## Wasm — `ephapax-wasm`
* `compile_match` emits the guard expression after the pattern
destructure. The guard pushes an `i32` onto the stack; `i32.eqz`
+ `br_if 0` falls through to the surrounding `$arm_i_fail` block
when the guard is false, exactly like a refuted nested pattern.
* The fall-through machinery established by #68 is reused — no new
control-flow structure needed.
## Linear — `ephapax-linear`
* No changes. Both `linear.rs::walk_expr` and `affine.rs::walk_expr`
already walked into `arm.guard` when computing per-arm snapshots
(in place since the variant landed); the existing N-arm branch-
agreement check correctly enforces consumption parity across
guarded and non-guarded arms.
## Tests
* `cargo test -p ephapax-typing --lib` → 73 pass (was 70), +3:
- `test_match_guard_typechecks_with_pattern_binding` — guard
reads `v` bound by `Some(v)`.
- `test_match_guard_non_bool_rejected` — guard of type `I32`
rejected as `TypeMismatch`.
- `test_match_guarded_arm_not_counted_for_exhaustiveness` —
`Some(v) if v > 0 | None` produces `NonExhaustiveMatch`.
* `cargo test -p ephapax-interp --lib` → 17 pass (was 14), +3:
- `test_eval_match_guard_pass` — `Some(7) if v > 0 => v` ⇒ 7.
- `test_eval_match_guard_fail_falls_through` — `Some(-3) if v > 0`
falls through to wildcard ⇒ 0.
- `test_eval_match_failed_guard_does_not_leak_bindings` —
refuted arm's binding is reverted before the next arm runs.
* `cargo test -p ephapax-wasm --lib` → 79 pass (was 77), +2:
- `compile_module_match_guard_simple` — `Some(v) if v > 0 | Some(_)
| None`, validates via `wasmparser`.
- `compile_module_match_guard_on_wildcard` — guard on a wildcard
catch-all, validates via `wasmparser`.
## Out of scope
* Or-patterns inside a single arm — separate feature.
* Guard expressions that themselves contain `match` — should fall
out for free; no targeted test added.
Closes #67.
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Closes #67.
What changed
Match guards (
if eafter a pattern) now work end-to-end throughephapax-typing,ephapax-interp, andephapax-wasm. The grammar already carriesMatchArm::guard: Option<Box<Expr>>from #61 — this PR replaces theNotYetSupportedInCorestub in typing, theUnimplementedstub in interp, and adds thei32.eqz + br_ifemission in wasm.Per-crate changes
ephapax-typingcheck_matchtype-checks the guard under the arm's pattern bindings; unified againstBoolat the guard's span. Non-Bool guards returnTypeMismatch.check_exhaustivenessnow skips guarded arms when building the Maranget matrix — guards can refute at runtime so they don't contribute to coverage.Some(v) if v > 0 -> ... | None -> ...is correctly reported as non-exhaustive.ephapax-interpeval_matchevaluates the guard after applying the pattern bindings.false⇒ restore bindings, continue to next arm.true⇒ run body.v=999survives a refutedSome(v)arm.ephapax-wasmcompile_matchemits the guard expression after the destructure.i32.eqz + br_if 0falls through to the surrounding$arm_i_failblock, reusing the per-arm fall-through machinery from wasm: nested constructor patterns inside ExprKind::Match arm args #68.ephapax-lineararm.guard, so guarded-arm linearity is enforced automatically.Behaviour table
match x { Some(v) if v > 0 => v | Some(v) => v | None => 0 }match x { Some(v) if v > 0 => v | None => 0 }NonExhaustiveMatch { missing: "Some(_)" }.Some(_) if 42 => ...TypeMismatch(guard isI32, expectedBool).Some(7) if v > 0 => vSome(-3) if v > 0 => v | _ => 0Test plan
cargo test -p ephapax-typing --lib→ 73 pass (was 70), +3 guard tests.cargo test -p ephapax-interp --lib→ 17 pass (was 14), +3 guard tests.cargo test -p ephapax-wasm --lib→ 79 pass (was 77), +2 guard tests (validated viawasmparser).Out of scope
match— should fall out for free; no targeted test added.🤖 Generated with Claude Code