IsExpression question

[Sean Kelly]

Stewart Gordon wrote:
> Sean Kelly wrote:
>> Kirk McDonald wrote:
>>> Even after writing most of a library founded on the thing, I still 
>>> don't get something. What exactly is the expression
>>>
>>>     is(T == function)
>>>
>>> testing? I would expect it to test whether T is a function pointer 
>>> type (like how "is(T == delegate)" tests whether T is a delegate 
>>> type), but it doesn't.
>>
>> It's testing whether T is a function type, not a function pointer 
>> type.  For template code, it's mostly useful in this sort of situation:
>>
>> template isFunctionType( alias ref ) {
>>     const bool isFunctionType = is( ref == function );
>> }
>>
>> Though it can be used to detect a function pointer type like so:
>>
>> template isFunctionPointerType( T ) {
>>     const bool isFunctionPointerType = is( typeof(*T) == function );
>> }
> <snip>
> 
> The spec states:
> 
> "If TypeSpecialization is one of  typedef  struct  union  class 
> interface enum  function  delegate  then the condition is satisifed if 
> Type is one of those."
> 
> However, that doesn't excuse the behaviour of function being 
> inconsistent with the behaviour of delegate.  This would make more sense:
> 
>     template isFunctionPointerType(T) {
>         const bool isFunctionPointerType = is(T == function);
>     }
> 
>     template isFunction(alias ref) {
>         const bool isFunction = is(typeof(&ref) == function);
>     }
> 
> But I do wonder what you can really do with a function in a template 
> without knowing at least how many parameters it has.
> 
> Indeed, the function as a kind of data type ought not to exist in D.  At 
> the moment, it appears to exist only in the form of IsExpression that 
> you've used.  We have function pointers, indicated by the function 
> keyword, and function delegates, indicated by the delegate keyword.  So 
> whatever is(... == delegate) does, is(... == function) logically ought 
> to act correspondingly.

I believe this is also true:

     class C { void fn() {} int val; }

     static if( is( typeof( C.fn ) == function ) )
         pragma( msg, "fn is function" );
     static if( is( typeof( C.val ) == function ) )
         pragma( msg, "val is function" );

Taking the address of C.blah here to test for a function is a bit 
counter-intuitive, and I suspect could lead to some odd corner-cases.


Sean