GALA-E0079 — Invalid sealed-view match

When it fires. A type with a method MatchView() S, where S is a sealed type, can be matched with S’s variants: when a case names one, the match reads its subject through MatchView() and matches the view. Every case of that match must then apply to the view. The error fires for a case that does not:

an extractor, struct pattern or literal of another type — the subject’s own type included;
a typed pattern (x: T) whose type is not S; or
a binding of the whole subject (case other =>), which would bind the view, not the subject.

The same holds for a partial function { case ... } whose argument has a view, and for a sub-pattern that reads its value’s view: each alternative of Some(None | Slot(_)) must be a variant of the view or _.

Minimal repro.

package main

struct Slot(raw int)

func (s Slot) MatchView() Option[int] = When(s.raw >= 0, s.raw)

func describe(s Slot) string = s match {
    case Some(n) => s"slot $n"
    case other   => "empty"
}

func main() {
    Println(describe(Slot(3)))
}

Error output.

error[GALA-E0079]: `case other` would bind the Option[int] view, not the Slot itself; this match reads Slot through MatchView() as Option[int]
  --> main.gala:9:10
  |
9 |     case other   => "empty"
  |          ^^^^^ match only Option[int]'s variants and `_` here
  |
  = hint: match only Option[int]'s variants and `_` here; match the Slot itself in a separate match

Fix. Match the remaining variant, or use _ for the rest:

package main

struct Slot(raw int)

func (s Slot) MatchView() Option[int] = When(s.raw >= 0, s.raw)

func describe(s Slot) string = s match {
    case Some(n) => s"slot $n"
    case None()  => "empty"
}

func main() {
    Println(describe(Slot(3)))
}

Inside a case, the subject is still in scope by its own name (s above). To test the subject with its own patterns, write a separate match without the view’s variants.

Rationale. A match over a view is a match over S: its cases, binding types and exhaustiveness are those of S. A case that only applies to the subject’s own type would make one match test two different values, and a binding of the whole subject would silently hold the view instead of the value written before match. See Sealed Views.