dhall-1.7.0: Prelude/List/concat

{-
Concatenate a `List` of `List`s into a single `List`

Examples:

```
./concat Integer
(   [   [0, 1, 2]    : List Integer
    ,   [3, 4]       : List Integer
    ,   [5, 6, 7, 8] : List Integer
    ]   : List (List Integer)
)
= [0, 1, 2, 3, 4, 5, 6, 7, 8] : List Integer

./concat Integer
(   [   [] : List Integer
    ,   [] : List Integer
    ,   [] : List Integer
    ]   : List (List Integer)
)
= [] : List Integer

./concat Integer ([] : List (List Integer)) = [] : List Integer
```
-}
    let concat
        : ∀(a : Type) → List (List a) → List a
        =   λ(a : Type)
          → λ(xss : List (List a))
          → List/build
            a
            (   λ(list : Type)
              → λ(cons : a → list → list)
              → λ(nil : list)
              → List/fold
                (List a)
                xss
                list
                (λ(xs : List a) → λ(ys : list) → List/fold a xs list cons ys)
                nil
            )

in  concat