dhall-1.42.3: dhall-lang/Prelude/Natural/fold.dhall

{-|
`fold` is the primitive function for consuming `Natural` numbers

If you treat the number `3` as `succ (succ (succ zero))` then a `fold` just
replaces each `succ` and `zero` with something else
-}
let fold
    : Natural →
      ∀(natural : Type) →
      ∀(succ : natural → natural) →
      ∀(zero : natural) →
        natural
    = Natural/fold

let example0 = assert : fold 3 Text (λ(x : Text) → "A" ++ x) "B" ≡ "AAAB"

let example1 =
      λ(zero : Text) →
          assert
        :   fold 3 Text (λ(x : Text) → "A" ++ x) zero
          ≡ "A" ++ ("A" ++ ("A" ++ zero))

let example2 =
      λ(succ : Text → Text) →
      λ(zero : Text) →
        assert : fold 3 Text succ zero ≡ succ (succ (succ zero))

in  fold