(************************************************************
   lib_sequence.ml	Created      : Fri Feb 22 00:42:54 2002
  			Last modified: Wed Feb 27 23:46:34 2002
  Compile: make #
************************************************************)
exception Empty
open Lazy
type 'a delayed = 'a Lazy.status ref
type 'a seq = Nil | Sequence of ('a * ('a seq) delayed)

let head = function
    Sequence (a,_) -> a
  | _ -> raise Empty
let tail = function
    Sequence (_,b) -> force b
  | _ -> raise Empty
let nil = Nil
let is_empty = function
    Nil -> true
  | _ -> false
let cons a b = Sequence(a,b)
let rec take stream n = 
  if n = 0 then []
  else match stream with
    Sequence (a,b) -> a::(take (force b) (n-1))
  | _ -> raise Empty
let rec nth n stream = 
  if n = 0 then head stream
  else 
    nth (n-1) (tail stream)