{-
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