Joel de Guzman wrote:
Tobias Schwinger wrote:
2. Fusion and MPL placeholder expressions
MPL placeholder expressions are very convenient for specifying the result because of their lazy nature (see lines 267, 312). Of course, this approach trades compile time for syntactic sugar. The syntactic sugar might not be entirely irrelevant, because it allows us to emphasize even complex problems with little code.
Sure.
So, I still wonder, whether it is a good idea to use MPL lambda for the result computation or not. Is it?
I think so, yes.
If so (even occasionanlly), it would be necessary to have at least accessors for pair members (they are needed to keep the evaluation lazy, see line 211) or, ideally, if there would be a normalization between the STL and the MPL pair concepts (then MPL's accessors, 'first' and 'second', could be borrowed).
If my suggestion to make mpl/fusion pair fully conforming mpl/fusion sequences is accepted, then you can use code like mpl::begin<_> or mpl::at<_, N>.
It's a very cool suggestion, IMO (just for the protocol: seems 'mpl::begin' was supposed to read 'mpl::front'). But it seems we're still talking about different pairs of shoes (shoes of pairs?): Given the (actually) unspecified type members of 'mpl::pair' get "_type"-suffixes, any pair would just work with MPL *as a pair*. That is, the 'first' and 'second' metafunctions are trivial and (unlike all sequence-stuff) not tag-dispatched, so their instantiation is very lightweight and especially nice for within iteration...
(It would also be cool if one could just typedef the lambda expression to nested result or inherit from a wrapper. I don't know whether it's possible and a good idea - this part is just loud thinking)...
Hmmm... can you elaborate. I lost you here.
OK. I believe it works best with some imaginary code: // rtc stands for runtime code // rmf stands for result metafunctions struct a_func : result< munch< _1, tweak<_2> > > { template<typename A, typename B> typename result<A,B>::type operator()(A const & a, B const & b) const { return rtc::munch(a,rtc::tweak(b)); } }; ------------- namespace rmf { typedef result< munch<_1,tweak<_2> > > a_func; } namespace rtc { struct a_func : rmf:: a_func { template<typename A, typename B> typename result<A,B>::type operator()(A const & a, B const & b) const { return munch(a,tweak(b)); } }; } Regards, Tobias