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

Subject: [Boost-users] [mpl][fusion] Issues in the mpl/fusion interface
From: Gabriel Redner (gredner_at_[hidden])
Date: 2011-08-08 18:12:43

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

For instance, I'd like to be able to extract a sequence of key types from a map:

// OK!
typedef mpl::map<mpl::pair<int, char>, mpl::pair<bool, short> > m1;
  typedef mpl::transform<
    m1,
    mpl::first<mpl::placeholders::_1>,
    mpl::back_inserter<mpl::vector<> > >::type keys1;

// Not OK!
typedef fusion::map<fusion::pair<int, char>, fusion::pair<bool, short> > m2;
typedef mpl::transform<
  m2,
  mpl::first<mpl::placeholders::_1>,
  mpl::back_inserter<mpl::vector<> >  >::type keys2;

// OK, but requires different code for mpl and fusion maps
typedef mpl::transform<
  m2,
  fusion::result_of::first<mpl::placeholders::_1>,
  mpl::back_inserter<mpl::vector<> >  >::type keys3;

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

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.

Thanks,
-Gabe