Home / Docs / Error Codes / GALA-E0074

GALA-E0074 — Invalid default case

When it fires. A sealed type’s default case is invalid:

• more than one case is marked default. The default case is the type’s zero value — what a value nothing constructed holds — so a sealed type has at most one; or
• the default case has a field of the sealed type itself. Such a field is held through a pointer, which a zero value leaves nil, so the zero value could not be read as that case.

Minimal repro.

package main

sealed type Light {
    default case Off()
    case Dim(Level int)
    default case Unknown()
}

func main() {
    Println(Off())
}

Error output.

error[GALA-E0074]: sealed type "Light" marks both "Off" and "Unknown" as its default case
  --> main.gala:6:5
  |
6 |     default case Unknown()
  |     ^^^^^^^ keep `default` on one case: the default case is the type's z…
  |
  = hint: keep `default` on one case: the default case is the type's zero value

Fix. Keep default on the case a value should be when nothing set it, and make it a case without a field of the type itself:

package main

sealed type Light {
    default case Off()
    case Dim(Level int)
    case Unknown()
}

func main() {
    Println(Off())
}

Rationale. A Go zero value of a sealed type — an uninitialized var, a field a struct literal leaves out, a slot of a fresh slice — is its case with tag 0, and the default case takes that tag. Two default cases would need the same tag. A sealed type with no default case has no zero value: its cases are numbered from 1, and a match on a zero value panics instead of reading it as one of the cases. See Sealed Types: Default Case.