# File utils.ML

    (*  Title:      TFL/utils.ML
        ID:         $Id: utils.ML,v 1.2 2005/07/14 17:28:39 wenzelm Exp $
        Author:     Konrad Slind, Cambridge University Computer Laboratory
        Copyright   1997  University of Cambridge

    Basic utilities.
    *)

    signature UTILS =
    sig
      exception ERR of {module: string, func: string, mesg: string}
      val C: ('a -> 'b -> 'c) -> 'b -> 'a -> 'c
      val end_itlist: ('a -> 'a -> 'a) -> 'a list -> 'a
      val itlist2: ('a -> 'b -> 'c -> 'c) -> 'a list -> 'b list -> 'c -> 'c
      val pluck: ('a -> bool) -> 'a list -> 'a * 'a list
      val zip3: 'a list -> 'b list -> 'c list -> ('a*'b*'c) list
      val take: ('a -> 'b) -> int * 'a list -> 'b list
    end;

    structure Utils: UTILS =
    struct

    (*standard exception for TFL*)
    exception ERR of {module: string, func: string, mesg: string};

    fun UTILS_ERR func mesg = ERR {module = "Utils", func = func, mesg = mesg};

    fun C f x y = f y x

    fun end_itlist f [] = raise (UTILS_ERR "end_itlist" "list too short")
      | end_itlist f [x] = x
      | end_itlist f (x :: xs) = f x (end_itlist f xs);

    fun itlist2 f L1 L2 base_value =
     let fun it ([],[]) = base_value
           | it ((a::rst1),(b::rst2)) = f a b (it (rst1,rst2))
           | it _ = raise UTILS_ERR "itlist2" "different length lists"
     in  it (L1,L2)
     end;

    fun pluck p  =
      let fun remv ([],_) = raise UTILS_ERR "pluck" "item not found"
            | remv (h::t, A) = if p h then (h, rev A @ t) else remv (t,h::A)
      in fn L => remv(L,[])
      end;

    fun take f =
      let fun grab(0,L) = []
            | grab(n, x::rst) = f x::grab(n-1,rst)
      in grab
      end;

    fun zip3 [][][] = []
      | zip3 (x::l1) (y::l2) (z::l3) = (x,y,z)::zip3 l1 l2 l3
      | zip3 _ _ _ = raise UTILS_ERR "zip3" "different lengths";

    end;