mixin alias

[Derek Parnell]

On Mon, 17 Dec 2007 03:07:22 +0100, 0ffh wrote:

> Derek Parnell wrote:
>> On Sun, 16 Dec 2007 15:51:38 -0500, Jarrett Billingsley wrote:
>> 
>>> "Derek Parnell" <derek at psych.ward> wrote in message 
>>> news:jnak6l8ihhe1.k3tzm3in813z.dlg at 40tude.net...
>>>> Is there any reason why the alias template parameter cannot be a literal?
>>>>
>>>>
>>>> template Foo(alias b) {
>>>>   int X = b;
>>>> }
>>>>
>>>> void main() {
>>>>  int y = 4;
>>>>  mixin Foo!(y);  // This is okay
>>>>  mixin Foo!(4);  // This fails to compile
>>>>   // "mixin Foo!(4) does not match any template declaration"
>>>> }
>>>>
>>> This is correct.  You can only alias things that have names; the number 3 
>>> does not have a name.
>> 
>> I know that is 'correct', but I was really asking what is the rationale
>> behind such an apparently arbitrary design decision.
> 
> Well, for one we'd have to disallow assignment expressions using the
> aliased parameter (as well as pre- and post-increment)

Why? If you had such a template (one that used 'side effects') but supplied
a literal, it would/should fail at the mixin site. I can't see why that
necessarily prevents the same template being used with non-literals, as
your example below demonstrates.


> template Foo(alias b)
> {
>    int X=(b=4);
> }
> 
> void main()
> {
>    int a=3;
>    printf("%i\n",a); // 3
>    mixin Foo!(a);    // side effect here
>    printf("%i\n",a); // yeas, it's 4 now
>    printf("%i\n",X); // 4
     mixin Foo!(5) // Should fail because 'int X=(5=4);' is bad syntax.
> }

IMHO, such a template needs *heavy* justification as do all such 'hidden'
side effects code. Buts that's just the way I think.

-- 
Derek
(skype: derek.j.parnell)
Melbourne, Australia
17/12/2007 1:27:49 PM