Clay language

[Steven Schveighoffer]

On Thu, 30 Dec 2010 10:22:22 -0500, Michel Fortin  
<michel.fortin at michelf.com> wrote:

> On 2010-12-30 10:00:05 -0500, "Steven Schveighoffer"  
> <schveiguy at yahoo.com> said:
>
>> The thing I find ironic is that with the original operator overloading   
>> scheme, the issue was that for types that define multiple operator   
>> overloads in a similar fashion, forcing you to repeat boilerplate code.  
>>   The solution to it was a mixin similar to what you are suggesting.   
>> Except  now, even mundane and common operator overloads require verbose  
>> template  definitions (possibly with mixins), and it's the uncommon  
>> case that  benefits.  So really, we haven't made any progress (mixins  
>> are still  required, except now they will be more common).  I think  
>> this is one area  where D has gotten decidedly worse.  I mean, just  
>> look at the difference  above between defining the opcat operator in D1  
>> and your mixin solution!
>
> I'm with you, I preferred the old design.
>
>
>> As a compromise, can we work on a way to forward covariance, or to have  
>>  the compiler reevaluate the template in more derived types?
>
> I stubbled upon this yesterday:
>
> 	Template This Parameters
>
> 	TemplateThisParameters are used in member function templates to pick up  
> the type of the this reference.
> 	import std.stdio;
>
> 	struct S
> 	{
> 		const void foo(this T)(int i)
> 		{
> 			writeln(typeid(T));
> 		}
> 	}
>
> <http://www.digitalmars.com/d/2.0/template.html>
>
> Looks like you could return the type of this this way...
>

Damn!  That's very very close to what I wanted!  Thanks for stumbling on  
that ;)  I don't even need to repeat it at derived levels (just tried it  
out).

The one issue I see now is, I have to cast this to T, which involves a  
runtime cast.  From what I understand, covariance does not require a  
runtime cast penalty, because the adjustment lookup is done at compile  
time.

I wonder if the compiler can optimize out the runtime dynamic cast in this  
case, Walter?

Now, for a working (but not quite as effecient as D1) solution, all I need  
is the bug fix for allowing templates in interfaces.

-Steve