dhall-1.8.0: Prelude/Optional/build

{-
`build` is the inverse of `fold`

Examples:

```
./build
Integer
(   λ(optional : Type)
→   λ(just : Integer → optional)
→   λ(nothing : optional)
→   just 1
)
= [ 1 ] : Optional Integer

./build
Integer
(   λ(optional : Type)
→   λ(just : Integer → optional)
→   λ(nothing : optional)
→   nothing
)
= [] : Optional Integer
```
-}
    let build
        :   ∀(a : Type)
          → (   ∀(optional : Type)
              → ∀(just : a → optional)
              → ∀(nothing : optional)
              → optional
            )
          → Optional a
        = Optional/build

in  build