(* 
Call by Value
-------------
ml uses "call by value". What does this mean?
Well, when we make a call to some function 
f(E), ml will evaluate all the arguments before it actually
calls f. Consider the following function: 
*)

fun cond (p,e1,e2) = if p then e1 else e2;

(*
We might think of a call to cond(p,e1,e2) as being
equivalent to "if p then e1 else e2". But is it?
Lets define a factorial function f using cond *)

fun f n = cond(n=0,1,n*f(n-1));

(* f 2 => cond(false,1,2*f(1))
       => cond(false,1,2*cond(false,1,1*f(0)))
       => cond(false,1,2*cond(false,1,1*cond(true,1,0*(f-1))))
       :
       :
Recursion does not stop!

NOTE: conditional expressions, and boolean operators 
      CANNOT be functions within ml. What we would require
      is something other than "call by value".

Call by Name
------------
To compute the value of cond(p,e1,e2), substitute p, e1, and
e2 into the body of the function cond. Then compute the
value of the resulting expression.

    f 2 => cond(2=0,1,2*f(2-1))
        => cond(2=0,1,2*cond(1=0,1,1*f(1-1)))
        => cond(2=0,1,2*cond(1=0,1,1*cond(0=0,1,f(0-1))))
        => cond(2=0,1,2*cond(1=0,1,1*1))
        => cond(2=0,1,2*1)
        => 2*1
        => 2

Call by Need
------------
Lazy evaluation (aka "call by need") 

*)