A pattern for dealing with unreachable conditions in Pony.

unreachable.pony

class Unreachable
  new create(value: (Stringable | None) = None, loc: SourceLoc = __loc) =>
    @printf[I32](
      "ABORT: Unreachable condition at %s:%zu (in %s method)\n".cstring(),
      loc.file().cstring(), loc.line(), loc.method().cstring())
    
    if value isnt None then
      @printf[I32]("%s\n".cstring(), value.string().cstring())
    end
    
    @exit[None](I32(1))

unreachable_example.pony

fun apply(): I32 =>
  match x
  | let x': Foo => handle_foo(x')
  | let x': Bar => handle_bar(x')
  | let x': Baz => handle_baz(x')
  else Unreachable(x); 0
  end