Template mixins - why only declarations

[Frustrated]

On Thursday, 6 March 2014 at 17:27:35 UTC, Steve Teale wrote:
> Pretty much what the subject says. Why can't template mixins 
> include statements ans so on?
>
> Is it just too hard, or is it just too much like C macros?
>
> Steve

template mixins mix in directly into the code as if you typed 
them. If they contained statements then you could mixin 
statements into classes, say, and it would then be illegal.

I guess there is no reason per se, but I guess that wasn't the 
desired behavior for template mixins. I imagine there could be a 
definite downside to having template mixins containing 
statements. Also, they can't be self contained.

e.g.,

mixin template C()
{
    i = i + 1;  // invalid
}

...

int i = 0;
mixin C();


The template itself can't be semantically checked in place 
because i is unknown inside the template. (it is not self 
contained so to speak)

In any case, just seems wrong for templates to do that. They are 
not grouping expressions but grouping definitions and 
declarations of things so you don't have to do them multiple 
times.

string mixins, OTOH, could do the above.

template C()
{
     string C()
     {
         return "i = i + 1;";
     }
}

...

int i = 0;
mixin(C);

and this will work. This is because the statement is contained 
within a string and the compiler simply inserts the string 
directly. The template can still be validated in place(since "i = 
i + 1" is a string and has no other meaning in the template).