{-
Only keep elements of the list where the supplied function returns `True`

Examples:

```
./filter Natural Natural/even ([+2, +3, +5] : List Natural)
= [+2] : List Natural

./filter Natural Natural/odd ([+2, +3, +5] : List Natural)
= [+3, +5] : List Natural
```
-}
let filter : ∀(a : Type) → (a → Bool) → List a → List a
    =   λ(a : Type)
    →   λ(f : a → Bool)
    →   λ(xs : List a)
    →   List/build
        a
        (   λ(list : Type)
        →   λ(cons : a → list → list)
        →   List/fold
            a
            xs
            list
            (λ(x : a) → λ(xs : list) → if f x then cons x xs else xs)
        )

in  filter