Mutiple AliasSeq as input to template

[David Sanders via Digitalmars-d-learn]

On Thursday, 25 May 2017 at 17:56:12 UTC, jmh530 wrote:
> On Thursday, 25 May 2017 at 16:36:45 UTC, jmh530 wrote:
>> [snip]
>
> I haven't played around with it fully, but it seems like the 
> following resolves my issue in a sort of manual way:
>
> template Process1(A, B)
> {
> 	static if (!isIndex!B)
> 		alias Process1 = A;
> 	else
> 		alias Process1 = B;
> }
>
> template Process(size_t n, A...)
> 	if (n > 0)
> {
> 	import std.meta : AliasSeq;
> 	
> 	alias B = A[0..n];
> 	alias C = A[n..$];
> 	
> 	static if (n == 1)
> 	{
> 		alias Process = AliasSeq!(Process1!(B[0], C[0]));
> 	}
> 	else static if (n > 1)
> 	{
> 		alias Process = AliasSeq!(Process1!(B[0], C[0]),
> 								  Process!(n - 1, B[1..$], C[1..$]));
> 	}
> }

You can use nested templates to process multiple AliasSeqs like 
so:

enum isIndex(I) = is(I : size_t);

template Process(A...)
{
     template With(B...)
     {
         import std.meta : AliasSeq;

         static if (A.length == 0 || B.length == 0)
             alias With = AliasSeq!();
         else
         {
             static if(!isIndex!(B[0]))
                 alias Process1 = A[0];
             else
                 alias Process1 = B[0];
             alias With = AliasSeq!(Process1, 
Process!(A[1..$]).With!(B[1..$]));
         }
     }
}

void main()
{
     import std.meta : AliasSeq;

     alias T = AliasSeq!(int[], int[]);
     alias U = AliasSeq!(int, string);
     static assert(is(Process!(T).With!(U) == AliasSeq!(int, 
int[])));
}