Why not move cast to the standard library?

[Adam D. Ruppe]

On Fri, Sep 25, 2009 at 09:29:21AM +0200, Don wrote:
> I demonstrated that the combination of CTFE + string mixins, 
> even in D1, was dramatically more powerful than the proposed macros. It 
> became clear that we didn't have a macro design that was anywhere near 
> powerful enough, and macros were postponed to D3. We don't know how they 
> should work.
> By all means make a proposal.

I was thinking of a simple one for D2 a while back, but canceled my post
since it is basically just sugar for what it does now. But, on the other
hand, that is a bit simpler to implement, so let me throw it out.

I propose that a macro (in D2; I don't know about a more elegant D3 design)
is merely a CTFE function that is mixed in, and takes symbols as strings.

Let me give an example:

====
macro max(int a, int b) {
	return a ~ " > " ~ b ~ " ? " ~ a ~ " : " ~ b;
}

void main() {
	auto num1 = 10;
	auto num2 = 20;

	auto result = max(num1, num2);
}
====

This would be the same as:

====
string max(string a, string b) {
	return a ~ " > " ~ b ~ " ? " ~ a ~ " : " ~ b;
}

void main() {
	auto num1 = 10;
	auto num2 = 20;

	auto result = mixin(max("num1", "num2"));
}

===============

So:
	a) A macro function always returns a string, which is mixed in at the
	   point where you used it

	b) Its arguments are actually strings. (I don't know if I'm happy with
	   what I did in the above example, using int when they are actually
	   string, but having some type checking seems more sane than actually
	   going all string. All string feels too C preprocessor like.)

	c) All the arguments are the name of the variable as a string. It
	   basically just tosses quotes around whatever you literally wrote
	   on the line when you used it. (So passing a string literal would
	   actually send "\"what you wrote\"" to the macro.)


This doesn't do anything we can't already do with the language, but I think
it looks a little prettier, which might make the functions a bit easier for
the user. Though it would still be the same for the lib writer, of course.



-- 
Adam D. Ruppe
http://arsdnet.net