Scope of Mixins

[Ali Çehreli]

On 8/26/21 10:06 AM, Adam D Ruppe wrote:
> On Thursday, 26 August 2021 at 17:01:06 UTC, DLearner wrote:
>> The object was to take a variable, and do alternative things with it 
>> depending on (say) whether it was an 'int' or an 'int*'.
> 
> That's *very* easy to do with the alias. You can just check `typeof(v)` 
> in there.

String mixins are appealing because they can inject code like C macros 
do. It's not trivially possible to do the same with template mixins.

import std.traits : isPointer;
import std.stdio : writeln;

mixin template valueFrom(alias var)
if (isPointer!(typeof(var))) {
   writeln("Dereferencing a pointer");  // ERROR
   x = *var;
}

mixin template valueFrom(alias var)
if (!isPointer!(typeof(var))) {
   writeln("Using a scalar");           // ERROR
   x = var;
}

void main() {
   int x;
   int i = 42;
   mixin valueFrom!i;

   int * p = &i;
   mixin valueFrom!p;
}

Yes, there are tricks one can play or change the design but when it 
comes to "injecting code", template mixins are not as convenient as 
string mixins.

Ali