Re: [Boost-users] [mpl][fusion] Issues in the mpl/fusion interface

Subject: Re: [Boost-users] [mpl][fusion] Issues in the mpl/fusion interface
From: Joel de Guzman (joel_at_[hidden])
Date: 2011-08-09 23:32:34

On 8/9/2011 6:12 AM, Gabriel Redner wrote:
> Hi all,
> 
> I am writing some code which uses MPL and fusion together, and in
> general I am happy with their interoperability.  However, I have
> stumbled on a few areas in which functionality I expected was missing,
> or where expressions I expected to work did not.  All of these issues
> have workarounds, but I'd appreciate some input on whether the problem
> is with the libraries, or with my expectations.
> 
> 1. mpl::pair and fusion::pair
> 
> Fusion sequences are conforming MPL sequences, and MPL sequences can
> be transmuted into fusion sequences.  However, the same is not true of
> their pair classes.  I feel that fusion::pair ought to conform to some
> "MPL pair" concept such that:
> 
> - I can access its types by the same 'first' and 'second' members as
> in mpl::pair
> - mpl::first and mpl::second metafunctions should work against it

That sounds reasonable.

> Also, I feel there should be a canned way to translate mpl::pair into
> fusion::pair.  fusion::result_of::as_pair?

Ok. Sounds good.

> 2. mpl::map and fusion::map
> 
> This code works nicely:
> 
> typedef mpl::vector<int, char, bool> v1;
> typedef fusion::result_of::as_vector<v1>::type v2;
> v2 v(1, 'a', false);
> 
> However, this does not:
> 
> typedef mpl::map<mpl::pair<int, char>, mpl::pair<bool, short> > m1;
> typedef fusion::result_of::as_map<m1>::type m2;
> m2 m('a', 3);
> 
> The type 'm2' ends up as fusion::map<mpl::pair<int, char>,
> mpl::pair<bool, short> >, which clearly is no good.  I feel that
> as_map should automatically translate mpl::pair to fusion::pair so
> that the result is a usable container.
> 
> Please let me know if I am barking up the wrong tree, or whether these
> seem like useful features to anyone else.  If there is consensus that
> they are useful, I would be interested in developing patches for them.

All these sound reasonable. I'd definitely welcome a patch,
but I have to insist on docs too.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://boost-spirit.com