binary_tree.dhall

let Tree
    : Type → Type
    = λ(Elem : Type) →
        ∀(T : Type) → ∀(Branch : Elem → T → T → T) → ∀(Leaf : T) → T

let showTree =
      λ(elem : Type) →
      λ(show : elem → Text) →
      λ(tree : Tree elem) →
        tree
          Text
          ( λ(e : elem) →
            λ(l : Text) →
            λ(r : Text) →
              "(Branch '${show e}' ${l} ${r})"
          )
          "Leaf"

let tree
    : Tree Text
    = λ(t : Type) →
      λ(Branch : Text → t → t → t) →
      λ(Leaf : t) →
        Branch "foo" (Branch "bar" Leaf Leaf) Leaf

let identity = λ(A : Type) → λ(a : A) → a

in  showTree Text (identity Text) tree