fun zip ([], _) = []
 |  zip (_, []) = []
 |  zip (x::xs, y::ys) = (x,y)::(zip(xs,ys));

fun unzip [] = ([],[])
 |  unzip ((x,y)::pairs) = 
      let
        val (xs,ys) = unzip pairs
      in
        (x::xs, y::ys)
      end
 ;