Re: [boost] [Fusion] Notice of breaking change

From: Joel de Guzman (joel_at_[hidden])
Date: 2007-06-17 15:52:16

dan marsden wrote:
> The definition of Fusion polymorphic function object will be changed shortly
> to be consistent with boost.resultof. Instead of requiring a nested result
> metafunction of the form:
> 
> template<typename T0, typename T1, ... typename TN> struct result { typedef
> xxx type; };
> 
> A polymorphic function object will instead simply be required to suppport
> 
> boost::result_of<F(A,B,...Z)>::type
> 
> as a valid expression.
> 
> This will break some Fusion client code, specifically, if you are using fold,
> accumulate, transform, or a lot of the components in the functional section 
> of the library, you may need to make changes to support this change.
> 
> The change should be entered into CVS head over the forthcoming weekend.
> 
> Apologies to anyone whos code this breaks, but this should make things more 
> consistent with the rest of boost going forwards.

Thanks Dan!

(CC'ing Doug)
Doug, I know the result_of interface is almost standard now, but I
still wonder about the redundancy of the function-object interface:

     F::result<F(T1, T2,..., TN)>::type

Is there a rationale? F is redundant here. The former fusion result API
does not require the F. Now, for example, instead of writing:

     struct square
     {
         template<typename T>
         struct result
         {
             typedef int type;
         };

         template <typename T>
         int operator()(T x) const
         {
             return x * x;
         }
     };

We have to write it as:

     struct square
     {
         template<typename T>
         struct result;

         template <typename T>
         struct result<square(T)>
         {
             typedef int type;
         };

         template <typename T>
         int operator()(T x) const
         {
             return x * x;
         }
     };

I'm not complaining, we already bit the conformance bullet :P
Just wondering why the latter is superior to the former.

Regards,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net