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

Examples:

```
./filter Natural Natural/even (Some 2)
= Some 2

./filter Natural Natural/odd (Some 2)
= None 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(some : a → optional)
          → λ(none : optional)
          → Optional/fold
            a
            xs
            optional
            (λ(x : a) → if f x then some x else none)
            none
        )

in  filter