import std.typecons;

auto foo2(R)(R foopara){
    return tuple(foopara, is(R==int));
}

void main(){
    auto tuple(a,b) = foo2(1);
}


I'm expecting some error such as can not act as left value but when I compiled this, no error occured. DMD version is DMD64 v2.065.(ldc2 exited with error function declaration without return type)

Why does this work? Or it is a bug?

On Monday, 23 June 2014 at 08:30:44 UTC, h_zet wrote:
> import std.typecons;
>
> auto foo2(R)(R foopara){
>     return tuple(foopara, is(R==int));
> }
>
> void main(){
>     auto tuple(a,b) = foo2(1);
> }
>
>
> I'm expecting some error such as can not act as left value but when I compiled this, no error occured. DMD version is DMD64 v2.065.(ldc2 exited with error function declaration without return type)
>
> Why does this work? Or it is a bug?

Looks grammaticallyu correct: R type is guessed/infered from the parameter

On Monday, 23 June 2014 at 08:30:44 UTC, h_zet wrote:
> import std.typecons;
>
> auto foo2(R)(R foopara){
>     return tuple(foopara, is(R==int));
> }
>
> void main(){
>     auto tuple(a,b) = foo2(1);
> }
>
>
> I'm expecting some error such as can not act as left value but when I compiled this, no error occured. DMD version is DMD64 v2.065.(ldc2 exited with error function declaration without return type)
>
> Why does this work? Or it is a bug?

You declared a variable template named "tuple" (with unused type parameters a, b) on that line.
http://dlang.org/template.html#variable-template

I think this is very confusable syntax...

void main()
{
    auto tuple(a, b) = foo2(1);

    writeln(tuple!(int, int)); // writes "Tuple!(int, bool)(1, true)"

    tuple!(int, int) = foo2(20);
    writeln(tuple!(int, int)); // writes "Tuple!(int, bool)(20, true)"

}

On Monday, 23 June 2014 at 08:30:44 UTC, h_zet wrote:
> import std.typecons;
>
> auto foo2(R)(R foopara){
>     return tuple(foopara, is(R==int));
> }
>
> void main(){
>     auto tuple(a,b) = foo2(1);
> }
>
>
> I'm expecting some error such as can not act as left value but when I compiled this, no error occured. DMD version is DMD64 v2.065.(ldc2 exited with error function declaration without return type)
>
> Why does this work? Or it is a bug?

Strange behavior, indeed. It took me a minute, but I think I know what's going on, and I'm pretty sure it's a bug. D recently introduced a short syntax for function-like templates:

  enum a(b) = "some_value";

It looks like this also (sort of) works with other qualifiers, which I believe it shouldn't. Here's a minimal example that might be good to put in a bug report:

  void main() {
      enum a(x) = "some_value"; // Good.
      auto b(x) = "some_value"; // Huh?
      // This also works for `const`, `static`, etc.
  }

On Monday, 23 June 2014 at 09:29:15 UTC, Mason McGill wrote:
> Strange behavior, indeed. It took me a minute, but I think I know what's going on, and I'm pretty sure it's a bug. D recently introduced a short syntax for function-like templates:
>
>   enum a(b) = "some_value";
>
> It looks like this also (sort of) works with other qualifiers, which I believe it shouldn't. Here's a minimal example that might be good to put in a bug report:
>
>   void main() {
>       enum a(x) = "some_value"; // Good.
>       auto b(x) = "some_value"; // Huh?
>       // This also works for `const`, `static`, etc.
>   }

It looks like I'm mistaken. Variable templates are supposed to exist. Please ignore the previous post.

h_zet:

> Why does this work? Or it is a bug?

When you play a little with this code it's easy to see _error_ that should not appear. So there's surely something worth reporting as bug, but I don't yet know what.

Bye,
bearophile

On Monday, 23 June 2014 at 09:09:56 UTC, hane wrote:
> On Monday, 23 June 2014 at 08:30:44 UTC, h_zet wrote:
>> import std.typecons;
>>
>> auto foo2(R)(R foopara){
>>    return tuple(foopara, is(R==int));
>> }
>>
>> void main(){
>>    auto tuple(a,b) = foo2(1);
>> }
>>
>>
>> I'm expecting some error such as can not act as left value but when I compiled this, no error occured. DMD version is DMD64 v2.065.(ldc2 exited with error function declaration without return type)
>>
>> Why does this work? Or it is a bug?
>
> You declared a variable template named "tuple" (with unused type parameters a, b) on that line.
> http://dlang.org/template.html#variable-template
>
> I think this is very confusable syntax...
>
> void main()
> {
>     auto tuple(a, b) = foo2(1);
>
>     writeln(tuple!(int, int)); // writes "Tuple!(int, bool)(1, true)"
>
>     tuple!(int, int) = foo2(20);
>     writeln(tuple!(int, int)); // writes "Tuple!(int, bool)(20, true)"
>
> }

Problem solved, Thank you so much!

h_zet:

> Problem solved, Thank you so much!

I don't think it's solved. There are probably bugs worth reporting here.

Bye,
bearophile

> I don't think it's solved. There are probably bugs worth reporting here.

I have not found them, sorry for the noise.

Bye,
bearophile

On Tuesday, 24 June 2014 at 10:11:05 UTC, bearophile wrote:
>> I don't think it's solved. There are probably bugs worth reporting here.
>
> I have not found them, sorry for the noise.
>
> Bye,
> bearophile

This looks really bad. I thought we weren't going to allow variable templates, just enums and aliases?