(* list-xprod.sml
*
* COPYRIGHT (c) 1993 by AT&T Bell Laboratories. See COPYRIGHT file for details.
*
* Functions for computing with the cross product of two lists.
*)
structure ListXProd : LIST_XPROD =
struct
(* apply a function to the cross product of two lists *)
fun appX f (l1, l2) = let
fun lp1 [] = ()
| lp1 (x::r) = let
fun lp2 [] = lp1 r
| lp2 (y::r) = (f(x, y); lp2 r)
in
lp2 l2
end
in
lp1 l1
end
(* map a function across the cross product of two lists *)
fun mapX f (l1, l2) = let
fun lp1 ([], resL) = rev resL
| lp1 (x::r, resL) = let
fun lp2 ([], resL) = lp1 (r, resL)
| lp2 (y::r, resL) = lp2 (r, f(x, y) :: resL)
in
lp2 (l2, resL)
end
in
lp1 (l1, [])
end
(* fold a function across the cross product of two lists *)
fun foldX f (l1, l2) = let
fun lp1 ([], accum) = accum
| lp1 (x::r, accum) = let
fun lp2 ([], accum) = lp1 (r, accum)
| lp2 (y::r, accum) = lp2 (r, f(x, y, accum))
in
lp2 (l2, accum)
end
in
fn init => lp1 (l1, init)
end
end; (* ListXProd *)