dhall-1.3.0: Prelude/Optional/fold

{-
`fold` is the primitive function for consuming `Optional` values

Examples:

```
./fold Integer ([2] : Optional Integer) Integer (λ(x : Integer) → x) 0 = 2

./fold Integer ([]  : Optional Integer) Integer (λ(x : Integer) → x) 0 = 0
```
-}
let fold
    :   ∀(a : Type)
    →   Optional a
    →   ∀(optional : Type)
    →   ∀(just : a → optional)
    →   ∀(nothing : optional)
    →   optional
    =   Optional/fold

in  fold