http://d.puremagic.com/issues/show_bug.cgi?id=4265

           Summary: It should be possible to query template parameters
                    with __traits
           Product: D
           Version: future
          Platform: Other
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: nfxjfg@gmail.com


--- Comment #0 from nfxjfg@gmail.com 2010-06-02 20:33:10 PDT ---
Consider you have

struct(T1, T2) Foo {
}

alias Foo!(int, float) X;

You should be able to extract (int, float) from X.
Often it is possible to get the parameters indirectly from members, but there
doesn't seem to be a way to do this in the general case.

I suggest:

__traits(getTemplateParameters, X)

which would yield  a (int, float) tuple.

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Trass3r <mrmocool@gmx.de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mrmocool@gmx.de


--- Comment #1 from Trass3r <mrmocool@gmx.de> 2010-06-03 16:53:07 PDT ---
What would you use that for?

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--- Comment #2 from nfxjfg@gmail.com 2010-06-03 20:05:20 PDT ---
Trass3r: meta programming. It often happened to me that I wanted to get the template params. Sometimes I had to add alias declarations just to get access to them. I think this feature would make meta programming code simpler in some cases.

PS: the example above should start with:

struct Foo(T1, T2) {
}

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--- Comment #3 from Trass3r <mrmocool@gmx.de> 2010-06-03 20:40:28 PDT ---
Well of course it's for metaprogramming.
But I can't imagine a usecase at the moment so I asked for some examples ;)

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nfxjfg@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID


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Brad Roberts <braddr@puremagic.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
                 CC|                            |braddr@puremagic.com
         Resolution|INVALID                     |


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Andrej Mitrovic <andrej.mitrovich@gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |andrej.mitrovich@gmail.com


--- Comment #4 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2012-12-20 14:55:16 PST ---
As a partial workaround if you know the original template a type was instantiated with you can use:

struct Foo(T1, T2)
{
}

alias Foo!(int, float) X;

template GetParams(R, alias X)
{
    static if (is(R x == X!T, T...))
    {
        alias T GetParams;
    }
}

void main()
{
    pragma(msg, GetParams!(X, Foo));
}

There's also an 'isTemplateInstance' now in std.traits.

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--- Comment #5 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2012-12-22 15:09:13 PST ---
I can have a pull ready soon for 2 traits:

1. Get the template out of the template instance (Foo!int => Foo)
2. Get the arguments used to instantiate the template (Foo!int => (int))

However I need some good names for these. I thought about using
'__traits(getTemplateArgs, T)' and '__traits(getTemplateParent, T)', any better
suggestions?

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--- Comment #6 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2012-12-22 15:24:32 PST ---
(In reply to comment #5)
> I can have a pull ready soon for 2 traits:
> 
> 1. Get the template out of the template instance (Foo!int => Foo)
> 2. Get the arguments used to instantiate the template (Foo!int => (int))
> 
> However I need some good names for these. I thought about using
> '__traits(getTemplateArgs, T)' and '__traits(getTemplateParent, T)', any better
> suggestions?

nazriel suggested:
'getTemplateName'
'getTemplateArgs'

I think it's better because they line up nicely.

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Andrei Alexandrescu <andrei@erdani.com> changed:

           What    |Removed                     |Added
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                 CC|                            |andrei@erdani.com


--- Comment #7 from Andrei Alexandrescu <andrei@erdani.com> 2013-03-24 05:30:25 PDT ---
C++ lacks this particular ability and makes up for it by requiring templates to define internal aliases for type parameters. However, this is more tenuous for value parameters and simply impossible for template template parameters (in our case alias parameters). This has led to certain contortions and limitations; based on that experience I think we should add this ability.

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