dhall-1.3.0: Prelude/Optional/map

{-
Transform an `Optional` value with a function

Examples:

```
./map Natural Bool Natural/even ([+3] : Optional Natural)
= [False] : Optional Bool

./map Natural Bool Natural/even ([] : Optional Natural)
= [] : Optional Bool
```
-}
let map : ∀(a : Type) → ∀(b : Type) → (a → b) → Optional a → Optional b
    =   λ(a : Type)
    →   λ(b : Type)
    →   λ(f : a → b)
    →   λ(o : Optional a)
    →   Optional/fold
        a
        o
        (Optional b)
        (λ(x : a) → [f x] : Optional b)
        ([] : Optional b)

in  map