I need to process two sequences in parallel (select some elements of sequence A depending of the corresponding element of sequence B). How can I pass two sequences to a meta-function? I tried nesting AliasSeqs but I get Perl4 style flattening:

  AliasSeq!(AliasSeq!(int, float),
            AliasSeq!(char, double))
  -> (int,float,char,double)

I worked around the problem by passing a single AliasSeq to my meta-function and using indexation to access the two sub-lists but now I need to process three sequences in parallel and I am looking for a cleaner solution.

On 06/19/2017 12:54 PM, Jean-Louis Leroy wrote:
> I need to process two sequences in parallel (select some elements of
> sequence A depending of the corresponding element of sequence B). How
> can I pass two sequences to a meta-function? I tried nesting AliasSeqs
> but I get Perl4 style flattening:
>
>   AliasSeq!(AliasSeq!(int, float),
>             AliasSeq!(char, double))
>   -> (int,float,char,double)
>
> I worked around the problem by passing a single AliasSeq to my
> meta-function and using indexation to access the two sub-lists but now I
> need to process three sequences in parallel and I am looking for a
> cleaner solution.
>
>

Hi Jean-Louis! :)

One option is to nest templates:

template foo(Args1...) {
    void bar(Args2...)() {
    }
}

void main() {
    foo!(int, float).bar!(char, double)();
}

Ali

On Mon, Jun 19, 2017 at 01:59:33PM -0700, Ali Çehreli via Digitalmars-d-learn wrote:
> On 06/19/2017 12:54 PM, Jean-Louis Leroy wrote:
> > I need to process two sequences in parallel (select some elements of sequence A depending of the corresponding element of sequence B). How can I pass two sequences to a meta-function? I tried nesting AliasSeqs but I get Perl4 style flattening:
> > 
> >   AliasSeq!(AliasSeq!(int, float),
> >             AliasSeq!(char, double))
> >   -> (int,float,char,double)
> > 
> > I worked around the problem by passing a single AliasSeq to my meta-function and using indexation to access the two sub-lists but now I need to process three sequences in parallel and I am looking for a cleaner solution.
> > 
> > 
> 
> Hi Jean-Louis! :)
> 
> One option is to nest templates:
> 
> template foo(Args1...) {
>     void bar(Args2...)() {
>     }
> }
> 
> void main() {
>     foo!(int, float).bar!(char, double)();
> }
[...]

Another option is to encapsulate a sequence inside a non-eponymous template so that it doesn't auto-expand:

	template MySeq(T...)
	{
		alias elements = T;
	}

	template MyTemplate(T, U)
		if (is(T : MySeq!(V...), V) &&
		    is(U : MySeq!(V...), V))
	{
		// do stuff with T.elements and U.elements
	}

	MyTemplate!(MySeq!(int, float), MySeq!(char, double));

The sig constraint in MyTemplate is not strictly necessary; it's just additional insurance against passing the wrong thing to MyTemplate and causing inscrutable template errors.


T

-- 
Without outlines, life would be pointless.

On Monday, 19 June 2017 at 20:59:33 UTC, Ali Çehreli wrote:
> On 06/19/2017 12:54 PM, Jean-Louis Leroy wrote:
>> I need to process two sequences in parallel (select some elements of
>> sequence A depending of the corresponding element of sequence B). How
>> can I pass two sequences to a meta-function? I tried nesting AliasSeqs
>> but I get Perl4 style flattening:
>>
>>   AliasSeq!(AliasSeq!(int, float),
>>             AliasSeq!(char, double))
>>   -> (int,float,char,double)
>>
>> I worked around the problem by passing a single AliasSeq to my
>> meta-function and using indexation to access the two sub-lists but now I
>> need to process three sequences in parallel and I am looking for a
>> cleaner solution.
>>
>>
>
> Hi Jean-Louis! :)
>
> One option is to nest templates:
>
> template foo(Args1...) {
>     void bar(Args2...)() {
>     }
> }
>
> void main() {
>     foo!(int, float).bar!(char, double)();
> }
>
> Ali

Hi Ali :)

Thanks! It works perfectly for my purpose, which is to manipulate lists of types or values based on a list of types, e.g.:

  alias ParTypes = AliasSeq!(virtual!A, int, virtual!B);
  Signature!(ParTypes).UnqualArgs -> AliasSeq!(A, int, B)
  Signature!(ParTypes).VirtualArgs -> AliasSeq!(A, B)
  Signature!(ParTypes).Filter!(AliasSeq!(A, int, B)) -> AliasSeq!(A, B)

J-L