is this even possible? newbie + mixin template + foreach (allMembers)

[WebFreak001]

On Tuesday, 3 April 2018 at 18:49:00 UTC, Carlos Navarro wrote:
> QUESTION:
>
> Obviously I'm no geting mixins/templates nor traits and I'm 
> failing miserably to find/identify the right examples or 
> documentation to help me tackle this thing. What is wrong in 
> this code? is this pattern sintactically possible? what I'm 
> getting wrong?
>
> CONTEXT:
> I'm a newbie trying to extend a struct using a mixin template 
> like this one:
>
> struct World {
> 	float	 rotation;
> 	bool	 active;
> 	mixin BuildStuff;
> }
>
> mixin template BuildStuff() {
> 	foreach(elem; __traits(allMembers, typeof(this))) {
> 		//pragma(msg, elem);
> 	}
> } //full example here: https://run.dlang.io/is/OeXS4j
>
> COMPILER OUTPUT:
>
> 2.067.1 to 2.071.2: Failure with output:
> -----
> onlineapp.d(18): Error: declaration expected, not 'foreach'
> onlineapp.d(18): Error: declaration expected, not '__traits'
> onlineapp.d(22): Error: template member expected
> -----
>
> 2.072.2 to 2.075.1: Failure with output:
> -----
> onlineapp.d(18): Error: declaration expected, not 'foreach'
> onlineapp.d(18): Error: declaration expected, not '__traits'
> onlineapp.d(22): Error: matching '}' expected, not EOF
> -----
>
> Since      2.076.1: Segfault and no output

you need to use a static foreach for this. You can insert a 
static foreach basically where you can insert a function 
definition like void foo() {}

foreach is more like a function call like foo(), so you can't put 
it in the mixin template.

A mixin template is basically expected to be mixin-able in global 
scope or in a class/struct so it can't have any things like 
function calls because that would be the same as writing `foo();` 
at global level instead of in the main function.

static foreach on the other hand allows exactly that because it 
literally unrolls the contents and inserts it multiple times, so 
you can also put it in mixin templates if the content of the loop 
is valid in mixin templates.

If you need to support older compilers you can use normal foreach 
with ctfe by generating a code string or do recursive templates, 
but I would really encourage you to use static foreach instead, 
it is cleaner and faster.