I've recently been doing a lot mpl::folding (in boost 1.47), and I was disappointed when I realized that a nested fold wouldn't work with a lambda expression passed to the inner call, as in this example which extracts elements of sequences: using namespace mpl; using namespace mpl::placeholders; typedef vector< vector<char>, vector<int> > vec_of_vecs; typedef vector< char, int > expected_result; typedef fold< vec_of_vecs, vector<>, fold< _2, _1, *push_back< _1, _2 >* // wrong - refers to outer arguments >
::type result;
BOOST_MPL_ASSERT(( equal<result, expected_result> )); After looking around for a solution I came across an old thread<http://lists.boost.org/boost-users/2004/12/9269.php> on the same subject, where it was suggested using mpl::protect might work. Indeed, the following modification does what I want, without any changes to the library: typedef fold< vec_of_vecs, vector<>, fold< _2, _1, *protect< lambda< push_back< _1, _2 > >::type
* // correct > ::type result;
Of course, this is a little on the verbose side. In the old thread, Daniel Wallin suggested suggested changing the definition of lambda to yield syntax more in line with the runtime version of boost::bind, but that would apparently break other code. What about instead adding a specialization to protect for placeholder expressions? Indeed, the source in protect.hpp seems to indicate such a thing might've been in the works at one point, but I didn't see any more references to it: template< typename BOOST_MPL_AUX_NA_PARAM(T) * , int not_le_ = 0 // not lambda expression?* > struct protect : T { ... typedef protect type; }; What about changing it to something like this? template< typename BOOST_MPL_AUX_NA_PARAM(T) * , typename not_le_ = typename is_lambda_expression< T >::type* > struct protect : T { ... typedef protect type; }; *template< typename T >* *struct protect< T, mpl::true_ > : lambda<T>* *{* *...* *typedef protect type;* *};* Doing so would remove the inner lambda<...>::type expression, yielding the expected syntax. Could that be done without messing up everything else? Thanks, *Nick Kitten* Software Engineer Center for Video Understanding Excellence ObjectVideo, Inc.