Template Question

[Mike Parker]

Bill Baxter wrote:
> Bill Baxter wrote:
>> Mike Parker wrote:
>>> Bill Baxter wrote:
>>>> Mike Parker wrote:
>>>>> Mike Parker wrote:
>>>>>> Bill Baxter wrote:
>>>>>>> Mike Parker wrote:
>>>>>>>> Bill Baxter wrote:
>>>>>>>>> Mike Parker wrote:
>>>>>
>>>>>>
>>>>>> This is exactly what I was looking for. I don't understand it, but 
>>>>>> it works.
>>>>>
>>>>> Actually, it doesn't work.
>>>>
>>>> Ok, so in that case...
>>>> I think you can put an alias for the type inside an interface, and 
>>>> use that to work around:
>>>>
>>>> interface Foo(T)
>>>> {
>>>>     alias T TheType;
>>>>     void init(T);
>>>> }
>>>>
>>>> ...
>>>>  static if ( is(U : Foo!(U.TheType)) ) {
>>>>    ...
>>>>   }
>>>>
>>>> That seems to work.
>>>>
>>>
>>> Yes, this works. But, the static if evaluation never completes if U 
>>> does not have TheType. It craps out with errors as soon as U.TheType 
>>> is encountered rather than printing out the more friendly static assert.
>>
>> I don't understand.  Can't you just make it
>>
>> static assert(is(U : Foo!(U.TheType))),
>>              "Use a Foo!() don't be a Foo! -- Mr. T");
> 
> Maybe you've found another bug?
> Expressions inside an "is" should never cause an error unless they're 
> syntactically mal-formed, so if you're saying you're getting an actual 
> error from the expression inside the static assert then that's a problem.

It errors out with the following:

Error: no property 'TheType' for type 'MyType'

However, I finally worked out a semi-solution using the traits module.

=======================================================================
private bool isFoo(T)()
{
     // make sure this T is a class so that the error
     // is reported from here and not from tango.core.Traits.
     static if(is(T == class))
     {		
         BaseTypeTupleOf!(T) tup;
	static if(tup.length > 0)
	{
	    static if(typeof(tup[0]).stringof ~ "(T)" == Foo.stringof)
	        return true;
         }
     }
	
     return false;
}

class FooManager(T)
{
     static if(!isFoo!(T)) static assert(0, "Mr. T ain't no Foo, Foo!");
}
=======================================================================

Each element of the tuple /should/ be iterated and compared with 
Foo.stringof. I was under the impression that foreach could be used at 
compile time, but using it here causes compilation to fail, saying that 
isFoo can't be run at compile time. Changing Foo to an abstract class 
and always checking the first element of the Tuple works, but it will 
still break of the class hierarchy goes deeper than one level.