Concatenating compile time sequences

[aliak]

On Saturday, 2 March 2019 at 02:38:09 UTC, H. S. Teoh wrote:
> On Sat, Mar 02, 2019 at 02:16:22AM +0000, Victor Porton via 
> Digitalmars-d-learn wrote:
>> [...]
>
> Keep in mind that sequences produced by AliasSeq are 
> auto-expanding, meaning the above construct will automatically 
> flatten into a flat AliasSeq!(int, "x", float, "y", double, 
> "z").  If that's not what you want, you need to wrap your 
> subsequences in a separate, non-eponymous template.
>
>
>> [...]
>
> I'm not sure what "alias enum" is supposed to mean; is that a 
> typo? Surely you mean just "alias"?
>
>
>>     [...]
>
> This line doesn't do what you think it does, because of 
> auto-expansion. It's essentially exactly the same thing as:
>
> 	private alias processFields(T, name, Fields...) =
> 		AliasSeq!(T, name, processFields!(Fields));
>
> i.e., the nested AliasSeq has no effect.
>
>
>> [...]
>
> If you want anything that retains a nested structure, you 
> cannot use AliasSeq because of auto-expansion.  You need to 
> define your own, non-eponymous template container, e.g.:
>
> 	template MySeq(T...) {
> 		alias data = T;
> 	}
>
> 	alias processFields(T, name, Fields...) =
> 		AliasSeq!(MySeq!(T, name), MySeq!(processFields!(Fields)));
>
> The MySeq!(...) "protect" their contents from flattening into 
> the outer list, while the outer AliasSeq causes individual 
> MySeq!(...)'s to be promoted to the top level sequence rather 
> than producing a tree-like structure.
>
> Note that to access the data inside a MySeq, you'll have to use 
> .data, for example:
>
> 	alias fields = processFields!(int, "x", float, "y");
>
> 	alias type0 = fields[0].data[0]; // int
> 	string name0 = fields[0].data[1]; // "x"
> 	alias type1 = fields[1].data[0]; // float
> 	string name1 = fields[1].data[1]; // "y"
>
> Hope this helps.
>
>
> T

There's a package called bolts that has an AliasPack defined that 
could allow you to do something like:

template split(seq...) if (seq.length % 2 == 0) {
     static if (seq.length >= 2) {
     	alias Pair = AliasPack!(seq[0], seq[1]);
     	alias split = AliasSeq!(Pair, .split!(seq[2..$]));	
     } else {
         alias split = AliasSeq!();
     }
}

Running code: https://run.dlang.io/is/lfTOBz

Cheers,
- Ali