Difference lists and Miniformat

difflist.ml

type ('ty,'v) t =
  | Nil : ('v, 'v) t
  | Cons : 'a * ('ty, 'v) t -> ('a -> 'ty, 'v) t

let cons x l = Cons (x,l)

let plus1 l = Cons ((),l)

let one x = Cons (x,Nil)

let l1 = Cons (1, Cons ("bla", Nil))
let l2 = Cons ((), Cons (2., Nil))
let l3 = Cons (1, Cons ("bla", Cons ((), Cons(2.,Nil))))

let rec append
  : type a b c.
    (a, b) t ->
       (b, c) t ->
    (a,    c) t
  = fun l1 l2 -> match l1 with
  | Nil -> l2
  | Cons (h, t) -> Cons (h, append t l2)

miniformat.ml

type ('ty,'v) t =
  | End : ('v,'v) t
  | Constant : string * ('ty,'v) t -> ('ty,'v) t
  | Hole : ('ty, 'v) t -> (string -> 'ty, 'v) t ;;

let rec kprintf
  : type ty res. (string -> res) -> (ty,res) t -> ty
  = fun k -> function
    | End -> k ""
    | Constant (const, fmt) ->
      kprintf (fun str -> k @@ const ^ str) fmt
    | Hole fmt ->
      let f s = kprintf (fun str -> k @@ s ^ str) fmt
      in f ;;

let printf fmt = kprintf (fun x -> x) fmt ;;

(* "%s | %s" *)
let ex = Hole (Constant (" | ", Hole End)) ;;

printf ex "foo" "zoo" ;;