[Boost-users] mpl lambda facilities

From: Matt Calabrese (mcalabre_at_[hidden])
Date: 2004-03-09 05:47:45

I'm trying to create a metafunction which takes in a boost::mpl::vector
containing spirit parser types, and yields a vector of the original vector's
length plus one with each element representing the parser type made of the
recursive boost::spirit::alternative of all of the original parser types
from the corresponding element in the original vector and an epsilon_parser
at the end.

in other words, the metafunction should be able to take in:

(using boost::mpl and boost::spirit)

vector< chlit<>, strlit<>, range<> >

and yield a result:

vector< alternative< chlit<>, alternative< strlit<>, alternative< range<>,
epsilon_parser > > >,
             alternative< strlit<>, alternative< range<>, epsilon_parser >
>,
             alternative< range<>, epsilon_parser >,
             epsilon_parser >

where empty is my own parser type, similar to epsilon_parser

I'm still new to metaprogramming and the mpl lambda facilities and I'm
having some trouble creating a metafunction which produces my desired type.
This is what I've come up with, though it doesn't work - I'm assuming
because of my second use of mpl::_1 in the nested template parameter of
mpl::front in mpl::push_front (which I'd like to have represent the current
state of the mpl::fold).

template< typename vector_t >
struct get_alternatives
{
  typename mpl::fold_backward< vector_t,
                               mpl::vector< epsilon_parser >,
                               typename mpl::push_front< mpl::_1,

spirit::alternative< mpl::_2,

typename mpl::front< mpl::_1 >::type > >::type >::type type;
};

Any help is greatly appreciated. Thanks in advance.