template isThing( alias symbol, A )
{
  enum isThing = false;
}

This template works in most contexts:

int x;
struct S {}

pragma(msg, isThing!x);
pragma(msg, isThing!S);

But this fails:
pragma(msg, isThing!int);

Why does it fail on a basic type (int), but not a user defined type (S)?
How can I fix the template declaration to not error in that case?
I tried:

template isThing( alias symbol, A )
{
  enum isThing = false;
}
template isThing( T, A )
{
  enum isThing = false;
}

Hoping the T version would catch the int, but this leads to different errors "matches more than one template declaration".

On Monday, 26 November 2012 at 16:37:08 UTC, Manu wrote:
> Why does it fail on a basic type (int), but not a user defined type (S)?

S is a D symbol name, int isn't...

> How can I fix the template declaration to not error in that case?

Try using only the (T,A) version, removing the alias version.

You might have to pass typeof(var) to it in some cases, but if you pass it typenames it should just work.

> How can I fix the template declaration to not error in that case?

If you want to have a template parameter that can be anything, including a a symbol or a built in type, you can use this ugly workaround:

template foo(bar...) if(bar.length == 1)
{
    enum foo = 1;
}

On 26 November 2012 18:40, Adam D. Ruppe <destructionator@gmail.com> wrote:

> On Monday, 26 November 2012 at 16:37:08 UTC, Manu wrote:
>
>> Why does it fail on a basic type (int), but not a user defined type (S)?
>>
>
> S is a D symbol name, int isn't...
>
>
>  How can I fix the template declaration to not error in that case?
>>
>
> Try using only the (T,A) version, removing the alias version.
>
> You might have to pass typeof(var) to it in some cases, but if you pass it typenames it should just work.
>

The template works on symbol aliases, so I can't do that. It needs to gracefully handle being given an int though somehow.

On 26 November 2012 18:49, jerro <a@a.com> wrote:

> How can I fix the template declaration to not error in that case?
>>
>
> If you want to have a template parameter that can be anything, including a a symbol or a built in type, you can use this ugly workaround:
>
> template foo(bar...) if(bar.length == 1)
> {
>     enum foo = 1;
> }
>

Ahhhhh, that explains why 90% of std.traits is written in that super-weird way! I always wondered what that was all about.

When you say 'anything' though, does that include alias parameters? Will bar[0] be the equivalent of template foo(alias bar) if called with a symbol?

On Monday, 26 November 2012 at 17:06:14 UTC, Manu wrote:
> When you say 'anything' though, does that include alias parameters? Will
> bar[0] be the equivalent of template foo(alias bar) if called with a symbol?

Yes.

D's inbuilt tuples can have both "expressions" and types. Tuples with only the former are called expression tuples. Tuples with only the latter are called type tuples (which is why people complain about the name of std.typecons.TypeTuple - it doesn't just take types).

On 26 November 2012 19:06, Manu <turkeyman@gmail.com> wrote:

> On 26 November 2012 18:49, jerro <a@a.com> wrote:
>
>>  How can I fix the template declaration to not error in that case?
>>>
>>
>> If you want to have a template parameter that can be anything, including a a symbol or a built in type, you can use this ugly workaround:
>>
>> template foo(bar...) if(bar.length == 1)
>> {
>>     enum foo = 1;
>> }
>>
>
> Ahhhhh, that explains why 90% of std.traits is written in that super-weird way! I always wondered what that was all about.
>
> When you say 'anything' though, does that include alias parameters? Will bar[0] be the equivalent of template foo(alias bar) if called with a symbol?
>

Ummm, immediate problem. That interferes with my argument list, inhibits consecutive parameters. You'll notice my template had 'A' on the end... it would make for a horrible API if the second arg were to come first in this case... is that the only option?

26.11.2012 20:36, Manu пишет:
> template isThing( alias symbol, A )
> {
>    enum isThing = false;
> }
>
> This template works in most contexts:
>
> int x;
> struct S {}
>
> pragma(msg, isThing!x);
> pragma(msg, isThing!S);
>
> But this fails:
> pragma(msg, isThing!int);
>
> Why does it fail on a basic type (int), but not a user defined type (S)?
> How can I fix the template declaration to not error in that case?
> I tried:
>
> template isThing( alias symbol, A )
> {
>    enum isThing = false;
> }
> template isThing( T, A )
> {
>    enum isThing = false;
> }
>
> Hoping the T version would catch the int, but this leads to different
> errors "matches more than one template declaration".

First, your `isThing` requires 2 parameters so all examples will fail to compile. Probably you meant `isThing(alias symbol)`.

Second, build-in type isn't a valid template alias parameter (probably as it isn't a "symbol"), see docs:
http://dlang.org/template.html#TemplateAliasParameter

Third, a workaround is to use template tuple parameter (as already suggested in this thread).

-- 
Денис В. Шеломовский
Denis V. Shelomovskij

26.11.2012 21:11, Manu пишет:
> Ummm, immediate problem. That interferes with my argument list, inhibits
> consecutive parameters. You'll notice my template had 'A' on the end...
> it would make for a horrible API if the second arg were to come first in
> this case... is that the only option?

Yes it is the only option. And it isn't really a big problem.


-- 
Денис В. Шеломовский
Denis V. Shelomovskij

On 26 November 2012 19:14, Denis Shelomovskij <verylonglogin.reg@gmail.com>wrote:

> 26.11.2012 21:11, Manu пишет:
>
>  Ummm, immediate problem. That interferes with my argument list, inhibits
>> consecutive parameters. You'll notice my template had 'A' on the end... it would make for a horrible API if the second arg were to come first in this case... is that the only option?
>>
>
> Yes it is the only option. And it isn't really a big problem.


It's a public user-facing API. It's not a small problem, but it is
workable...
However, going with that, the supplementary question is, how do I then
discover if bar[0] is a symbol or not? I can't see any traits to test for
builtin types...