GALA-E0004.md

GALA-E0004 — Sealed variant arity mismatch

When it fires. A pattern in a sealed-type match binds a different number of fields than the variant declares. Using _ for unused fields is fine; silently truncating the tail is not.

Minimal repro.

sealed type Shape {
    case Rect(W int, H int, Label string)
    case Circle(R int)
}

func area(s Shape) int = s match {
    case Rect(w, h)  => w * h       // Rect has 3 fields, pattern binds 2
    case Circle(r)   => r * r * 3
}

Error output.

[SemanticError GALA-E0004] main.gala:6:25 sealed variant "Rect" pattern binds 2 field(s) but declares 3 (hint: use `_` for unused fields)

Fix. Bind every field — use _ for the ones you don't care about:

case Rect(w, h, _) => w * h

Rationale. A pattern like Rect(w, h) looks like it matches a two-field struct, but Rect has a third Label field. Without the arity check, the generated code would either misalign the field bindings (silently assigning h to Label etc.) or fail at runtime. Making arity mismatches a compile error at the pattern site means the author sees the problem while writing the pattern, not when the test suite diverges.

The _ placeholder is the explicit "I know this field exists and I don't need it" acknowledgement — it looks different from forgetting the field entirely, which is exactly the distinction that matters.

Related work. Introduced as B6 in PR #166 via validateSealedVariantArity in match.go. The arity check runs alongside the exhaustiveness check (GALA-E0002) during transformMatchClauses.