λ(nix : ../NixPrelude.dhall) →
  let identity = ../Function/identity.dhall

  let fold = ./fold.dhall nix

  let default
      : ∀(a : Type) → a → Optional a → a
      = λ(a : Type) →
        λ(default : a) →
        λ(o : Optional a) →
          fold a o a (identity a) default

  in  default