dhall-1.26.0: dhall-lang/Prelude/Optional/fold

{-
`fold` is the primitive function for consuming `Optional` values
-}
let fold
    :   ∀(a : Type)
      → Optional a
      → ∀(optional : Type)
      → ∀(some : a → optional)
      → ∀(none : optional)
      → optional
    = Optional/fold

let example0 = assert : fold Natural (Some 2) Natural (λ(x : Natural) → x) 0 ≡ 2

let example1 =
      assert : fold Natural (None Natural) Natural (λ(x : Natural) → x) 0 ≡ 0

in  fold