gistfile1.txt

(*
  let s = Stream.of_string "foo\nbar\nbaz" in
  let lines = Stream.refoldl (fun s c -> if c = '\n' then Done s else Acc (s ^. c)) s in
  Stream.iter print_string lines;
  print_newline ();;
  
  foobarbaz
*)

type 'a accum = Acc of 'a | Done of 'a

let some x = Some x
let (@.) f g x = f (g x)
let maybe v f x = match x with None -> v | Some y -> f y
let optMap f x = match x with None -> None | Some y -> Some (f y)

module Stream =
struct
  include Stream

  let nextOpt st = try Some (next st) with Failure -> None

  let rec foldlB f init st =
    match nextOpt st with
      | None -> init
      | Some x ->
        begin match f init x with
          | Acc a -> foldlB f a st
          | Done b -> b
        end

  let rec foldl f init st = foldlB (fun s i -> Acc (f s i)) init st

  let map f st = from (fun _ -> optMap f (nextOpt st))

  let refoldl f init st =
    from (fun _ ->
      match peek st with
        | None -> None
        | Some x -> Some (foldlB f init st))
end