(* Example of polymorphism and type checking
   in ML from pages 240-243 of
   Kenneth C. Louden, Programming Languages
   Principles and Practice 2nd Edition
   Copyright (C) Brooks-Cole/ITP, 2003
*)

fun max (x, y, gt) = if gt(x,y) then x else y;

fun gti (x:int,y) = x > y;
fun gtr (x:real,y) = x > y;
fun gtp ((x,y),(z,w)) = gti (x,z);

max(3,2,gti); (* returns 3 *)
max(2.1,3.2,gtr); (* returns 3.2 *)
max((2,"hello"),(1,"hi"),gtp); (* returns (2,"hello") *)

fun ident x = x;
fun makepair (x,y) = (ident x,ident y);
makepair(2,3.2); 
 (* ok -- ident has two different types, one for each call *)

fun makepair2 (x,y,f) = (f x, f y);
makepair2(2,3.2,ident); 
      (* type error -- ident cannot have general type *)