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<code>mpl::remove</code> works correctly. The problem is with your template for permutations of singleton lists: it catches only types that are <code>mpl::list</code>s, while the result of remove has another sequence type. With other words, the result of <code>mpl::remove</code> is <code>mpl::equal</code> to the singleton list, but not <code>std::is_same</code>: <pre><code>#include <boost/mpl/list.hpp> #include <boost/mpl/remove.hpp> #include <boost/mpl/equal.hpp> namespace mpl = boost::mpl; struct test_type1 {}; struct test_type2 {}; typedef mpl::list2<test_type1, test_type2> typelist; typedef mpl::remove<typelist, test_type1>::type t; typedef mpl::list1<test_type2> t_corr; static_assert(mpl::equal<t,t_corr>::value, "t equals t_corr"); // the following will fail: // static_assert(std::is_same<t,t_corr>::value, "t same type as t_corr"); int main() { return 0; } </code></pre> You can fix this problem by specializing your template for the singleton list not based on the exact type <code>mpl::list</code>, but on the property to have length 1: <pre><code>#include <boost/mpl/list.hpp> #include <boost/mpl/transform.hpp> #include <boost/mpl/fold.hpp> #include <boost/mpl/push_front.hpp> #include <boost/mpl/joint_view.hpp> #include <boost/mpl/remove.hpp> #include <boost/mpl/assert.hpp> #include <boost/mpl/equal.hpp> #include <boost/mpl/size.hpp> #include <boost/mpl/front.hpp> #include <boost/mpl/begin.hpp> #include <boost/mpl/next.hpp> namespace mpl = boost::mpl; struct test_type1 {}; struct test_type2 {}; struct test_type3 {}; template< typename T, typename _ENABLE=void > struct permutations; template <typename T> struct permutations<T, typename std::enable_if<mpl::size<T>::value==1>::type> { typedef typename mpl::list1<T> type; }; template< typename value, typename T> struct permutation_helper: mpl::transform< typename permutations< mpl::list1<test_type3> >::type, mpl::push_front< mpl::_1, value> > { }; template< typename value, typename T> struct broken_helper: mpl::transform< typename permutations< typename mpl::remove<T, value>::type >::type, mpl::push_front< mpl::_1, value> > { }; template< typename T > struct permutations<T, typename std::enable_if<(mpl::size<T>::value>1)>::type>: mpl::fold< T, mpl::list0<>, mpl::joint_view< mpl::_1, broken_helper<mpl::_2, T > > > { }; typedef mpl::list2<test_type1, test_type2> typelist; typedef permutations<typelist>::type perms; typedef mpl::list<mpl::list<test_type1, test_type2>, mpl::list<test_type2, test_type1> > perms_corr; int main() { static_assert(mpl::size<perms>::value == 2, "perms has correct size"); static_assert(mpl::equal<mpl::front<perms>::type, mpl::front<perms_corr>::type>::value, "perms has correct front"); typedef mpl::next<mpl::begin<perms>::type>::type perms_2nd; typedef mpl::next<mpl::begin<perms_corr>::type>::type perms_corr_2nd; static_assert(mpl::equal<perms_2nd, perms_corr_2nd>::value, "perms has correct 2nd element"); return 0; } </code></pre> By the way, <pre><code>static_assert(mpl::equal<perms, perms_corr>::value, "perms correct"); </code></pre> will fail for exactly the same reasons. <ul> <li>lars</li> </ul>
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