dhall-1.21.0: dhall-lang/Prelude/Optional/concat

{-
Flatten two `Optional` layers into a single `Optional` layer

Examples:

```
./concat Natural (Some (Some 1))
= Some 1

./concat Natural (Some (None Natural))
= None Natural

./concat Natural (None (Optional Natural))
= None Natural
```
-}
    let concat
        : ∀(a : Type) → Optional (Optional a) → Optional a
        =   λ(a : Type)
          → λ(x : Optional (Optional a))
          → Optional/fold
            (Optional a)
            x
            (Optional a)
            (λ(y : Optional a) → y)
            (None a)

in  concat