Suppose I have a function f that has a lazy parameter x, and I pass it into another function g that has a lazy parameter.  Will x be evaluated when it is passed to g?  For instance:

void g(lazy int x) {/* Do nothing */}

void f (lazy int x) {
    g(x);  //Is x evaluated here?
}

void main()
{
   int x = 0;
   f(++x);
    printf("%d\n", x); //Do we print 0 or 1?
}

Joe Gottman

Joe Gottman wrote:
> Suppose I have a function f that has a lazy parameter x, and I pass it into another function g that has a lazy parameter.  Will x be evaluated when it is passed to g?

No, it isn't evaluated. The evaluation of it is wrapped up into another delegate and passed to g.

"Walter Bright" <newshound@digitalmars.com> wrote in message news:edi8s6$olu$1@digitaldaemon.com...
> Joe Gottman wrote:
>> Suppose I have a function f that has a lazy parameter x, and I pass it into another function g that has a lazy parameter.  Will x be evaluated when it is passed to g?
>
> No, it isn't evaluated. The evaluation of it is wrapped up into another delegate and passed to g.

OK.  So what's the best way to force evaluation?  Would just mentioning it in a line by itself work?

Joe Gottman

Joe Gottman wrote:
> "Walter Bright" <newshound@digitalmars.com> wrote in message news:edi8s6$olu$1@digitaldaemon.com...
>> Joe Gottman wrote:
>>> Suppose I have a function f that has a lazy parameter x, and I pass it into another function g that has a lazy parameter.  Will x be evaluated when it is passed to g?
>> No, it isn't evaluated. The evaluation of it is wrapped up into another delegate and passed to g.
> 
> OK.  So what's the best way to force evaluation?  Would just mentioning it in a line by itself work?

Because g also uses lazy evaluation you have to evaluate x in f before giving it forward as an argument to g. Still, the evaluated value gets wrapped up into a delegate before it's finally usable in g as a simple value.

The best way might be not to use lazy evaluation if it's not necessary.

Joe Gottman wrote:
> OK.  So what's the best way to force evaluation?  Would just mentioning it in a line by itself work?

Yes.

Is it necessary to wrap it in another delegate?

Assuming it already has a delegate which returns int, it seems a possible optimization to simply pass this existing delegate on, without packaging it further...

Is that not the case, or simply a field for future optimization?

Thanks,
-[Unknown]


> Joe Gottman wrote:
>> Suppose I have a function f that has a lazy parameter x, and I pass it into another function g that has a lazy parameter.  Will x be evaluated when it is passed to g?
> 
> No, it isn't evaluated. The evaluation of it is wrapped up into another delegate and passed to g.