Question

I didn’t understand the diffrence between Call-by-name and Call-by-need. As I understood, Call-by-need method restores the answer returned. But how It helps us, and Is there any fundamental difference between the results ?

For example,

begin integer n;
  procedure foo(e, n);
  integer e, n;
  begin
    for n := 1 step 1 until 10 do begin
      prints(`;;; the value of e is ');
      printnln(e)
    end
  end;
  foo(2 * n, n)
end

So in call-by-name, as I understood, We will get:

;;; the value of e is 2
;;; the value of e is 4
;;; the value of e is 8

and so on. This is because we pass 2*n to e, and e is evaluate with the new i everytime.
What would happen in call-by-need?

Answer 1

In call by need, we enter the loop, and evaluate the value only once. So In the code above, we will copy (2*n) inside the loop (macro-style) and we will evaluate the expression only one time (Not like call by name).
So, in the first iteration we will get e=2. this will be the value of e also in the next iteration, and the output will be:

;;; the value of e is 2
;;; the value of e is 2
;;; the value of e is 2