Nullability and optionality

Faber distinguishes absence in a value from optional provision at a declaration site.

Nullable values — T ∪ nihil

Use T ∪ nihil when the value can be absent:

functio find(textus key) → numerus ∪ nihil {
    redde nihil
}

functio divide(numerus a, numerus b) → numerus ∪ nihil {
    si b ≡ 0 ∴ redde nihil
    redde a / b
}

Optional declaration slots — sponte

Use sponte after the name when a parameter or field may be omitted by the caller or constructor:

functio connect(textus host, numerus port sponte) → vacuum { }

genus User {
    textus email sponte
}

Borrow markers can combine with optional parameters:

functio process(de numerus depth sponte) → vacuum { }

Non-null assertion — !

Use !., ![, !( to assert a nullable value is not nihil:

genus Box { numerus ∪ nihil val }
fixum Box ∪ nihil maybe_name ← Box { val = 7 }
fixum _ name ← maybe_name!.val

A non-null assertion on nihil aborts at runtime.

Nullish coalescing — vel

fixum textus ∪ nihil provided ← nihil
fixum _ name ← provided vel "default"

ignotum

ignotum is the top-level unknown type for escape hatches and incomplete knowledge. It is not a nullability mechanism.