Template type deduction

[Phil Lavoie]

On Sunday, 20 January 2013 at 16:11:08 UTC, Philippe Sigaud wrote:
> On Sun, Jan 20, 2013 at 5:01 PM, Phil Lavoie 
> <maidenphil at hotmail.com> wrote:
>
>> Is there also a way to extract per parameter information (ex: 
>> type) using
>> type deduction. Let me illustrate:
>>
>> struct Toto( T1, T2 ) {
>>  alias Key = T1;
>>  alias Value = T2;
>> }
>>
>> struct OuterToto( T ) if( isToto!T ) {
>>   T.Key key;
>>   T.Value value;
>> }
>>
>> Imagine I would want that but without using inner aliases, is 
>> this possible?
>
> Sure, the U... part is accessible inside the static if. Just 
> define a
> generic helper template:
>
>
> struct Toto(First, Second)
> {
>
> }
>
> template TemplateArguments(T)
> {
>     static if (is(T _ : a!(Args), alias a, Args...))
>         alias TemplateArguments = Args;
>     else
>         alias TemplateArguments = void;
> }
>
> template TemplateName(T)
> {
>     static if (is(T _ : a!(Args), alias a, Args...))
>         alias TemplateName = a;
>     else
>         alias TemplateName = void;
> }
>
> void main(string[] args)
> {
>     alias Type = Toto!(int, double);
>     writeln(TemplateArguments!(Type).stringof);
>     writeln(TemplateName!(Type).stringof);
>
>     alias T = TemplateName!(Type);
>     alias Swapped = T!(double, int); // And you can use T as a 
> template.
> }
>
> As you can see, you can even use the deduced template name.

YES, it works! I just didn't realized you could only use it 
inside the static ifs, thanks for pointing that out!

isInstanceOf( alias Template, T ) is nice but lacks support for 
expressions directly: but that ain't a big deal, example:

struct Pair( T1, T2 ) {...}

template isPair( alias T ) {
   static if( is( T ) ) {
     enum isPair = isInstanceOf!( Pair, T );
   } else {
     enum isPair = isInstanceOf!( Pair, typeof( T ) );  //Must add 
this to support expressions.
   }
}

//Returns the type of the first term of the pair.
template FirstType( alias T ) if( isPair!T ) {
   static if( is( T _: Pair!Args, Args... ) ) {
     alias FirstType = Args[ 0 ];
   } else {
     alias FirstType = FirstType!( typeof( T );
   }
}

The D programming language ROCKS :).