Toccata

(* Sorting a list of integers using insertion sort *)

module InsertionSort

  type elt
  val predicate le elt elt

  clone relations.TotalPreOrder with
    type t = elt, predicate rel = le, axiom .
  clone export list.Sorted with
    type t = elt, predicate le  = le, goal Transitive.Trans

  use list.List
  use list.Permut

  let rec insert (x: elt) (l: list elt) : list elt
    requires { sorted l }
    ensures  { sorted result }
    ensures  { permut (Cons x l) result }
    variant  { l }
  = match l with
    | Nil -> Cons x Nil
    | Cons y r -> if le x y then Cons x l else Cons y (insert x r)
    end

  let rec insertion_sort (l: list elt) : list elt
    ensures { sorted result }
    ensures { permut l result }
    variant { l }
  = match l with
    | Nil -> Nil
    | Cons x r -> insert x (insertion_sort r)
    end

end