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GALA-E0003 — Match expression must have a default case

What it means. A match expression over a non-sealed type — a primitive, a plain struct, anything with an unbounded instance set — has no default branch. The compiler cannot prove exhaustiveness for open types, so a default is mandatory.

Code that triggers it

package main

func name(n int) string = n match {
    case 1 => "one"
    case 2 => "two"
    // missing: case _ => ...
}

func main() {
    Println(name(1))
}

Compiler message

error[GALA-E0003]: match expression must have a default case
  --> e0003.gala:4:5
  |
4 |     case 1 => "one"
  |     ^^^^ add `case _ => ...`
  |
  = hint: add `case _ => ...`

How to fix it

Add a default that produces a sensible fallback:

func name(n int) string = n match {
    case 1 => "one"
    case 2 => "two"
    case _ => "other"
}

If you genuinely want the program to fail on unknown values, say so explicitly. A bare panic(...) is GALA-E0035 — GALA’s form is go_builtins.Panic:

package main

import "martianoff/gala/go_builtins"

func name(n int) string = n match {
    case 1 => "one"
    case 2 => "two"
    case _ => go_builtins.Panic(s"unexpected n: $n")
}

func main() {
    Println(name(1))
}

Why the rule exists

Non-sealed types have unbounded instance sets, so the transpiler cannot verify every value is covered. Without a default, the generated Go would need a synthesized one (usually a runtime panic), hiding the missing branch from the author. Requiring the default in source surfaces the decision where it is made.

If you want the compiler to prove exhaustiveness for you instead, model the value as a sealed type — then you get GALA-E0002 coverage checking and no default is needed.