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Why optionals (a real crash)

2026-07-14

Most languages get optionals from type theory fashion. wick got them from a crash on frame one of a game we had already shown people.

The Lua bug

Lantern Night stores a hi-score. The Lua build did the usual thing:

local best = tonumber(lt.load_save("koranight_best") or "0")

If the save file is missing, load_save fails and or "0" works. If the save file is present but empty (interrupted write, 0 bytes), load_save returned "". In Lua, "" is truthy. The fallback never ran. tonumber("") is nil. The next arithmetic blew up — not in a unit test, on the title screen of a real session.

That is not a hypothetical. It is why optionals are not optional in wick.

The wick shape

let best = num(lt.load_save("koranight_wick_best") ?? "") ?? 0
  • lt.load_save returns str? — missing save is nil.
  • ?? "" forces a string into num(...).
  • num returns num? — parse failure (including "") is nil.
  • ?? 0 is the only way the type becomes num.

There is no other program the compiler accepts for this idea. You cannot “forget” the empty-string case. That is the whole design.

What we refused

  • Making load_save return "" on missing and empty — hides the two cases.
  • Runtime null checks only — we already had those; they were skippable.
  • A linter rule — linters are optional; crashes are not.

Narrowing

let s = lt.load_save("hi")
if s != nil {
  lt.print(s, 4, 4, 1, 1, 1, 1)  // s: str here
}

Narrowing is local-only in the current compiler (one-pass). Globals use ??. That is a trade for implementation size, not a philosophical ban.

Takeaway

Optionals in wick are not “we wanted sum types.” They are the empty save file, made unrepresentable as a silent crash. Every other type feature will have to clear the same bar.

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