/Prelude-v21.1.0/Natural/fold.dhall

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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

Examples

fold 3 Text (λ(x : Text) → "A" ++ x) "B" ≡ "AAAB"

Source

{-|
`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 zerosucc (succ (succ zero))

in fold