expr.ml

(* A simple pattern matching example from http://www.c2.com/cgi/wiki?PatternMatching *)

(*
 * Define type `expr' to have two constructors: 
 * Constant(anInteger) represents a constant
 * Sum(anExpr, anotherExpr) represents the sum of two expressions.
 *)

datatype expr =
  Constant of int
| Sum of expr*expr

fun eval(Constant c) = c                  (* A constant evaluates to itself. *)
  | eval(Sum(x, y))  = eval(x) + eval(y)  (* eval(x+y)=eval(x)+eval(y). *)

(*
 * The compiler infers that eval takes an expr as its argument, and it
 * makes sure that both Constant and Sum are handled. If someone comes 
 * back later and adds, say, a Product constructor to expr, any 
 * pattern-match that dissects an expr without somehow handling the case 
 * of Product will raise a warning.
 *)