dhall-1.4.2: Prelude/Optional/filter

{-
Only keep an `Optional` element if the supplied function returns `True`

Examples:

```
./filter Natural Natural/even ([+2] : Optional Natural)
= [+2] : Optional Natural

./filter Natural Natural/odd ([+2] : Optional Natural)
= [] : Optional Natural
```
-}
let filter : ∀(a : Type) → (a → Bool) → Optional a -> Optional a
    =   λ(a : Type)
    →   λ(f : a → Bool)
    →   λ(xs : Optional a)
    →   Optional/build
        a
        (   λ(optional : Type)
        →   λ(just : a → optional)
        →   λ(nil : optional)
        →   Optional/fold
            a
            xs
            optional
            (λ(x : a) → if f x then just x else nil)
            nil
        )

in  filter