I recently purchased David Abrahams' and Aleksey Gurtovoy's new book on the MPL, "C++ Template Metaprogramming" (obviously recently since it's only been out a couple of weeks), and am working through some of the examples. I'm having trouble with the factorial example in section 8.3.1, page 160: #include <boost/mpl/int.hpp> #include <boost/mpl/multiplies.hpp> #include <boost/mpl/equal.hpp> #include <boost/mpl/eval_if.hpp> #include <boost/mpl/prior.hpp> #include <boost/static_assert.hpp> namespace mpl = boost::mpl; template <class N> struct factorial : mpl::eval_if< mpl::equal_to<N,mpl::int_<0> > // check N == 0 , mpl::int_<1> // 0! == 1 , mpl::multiplies< // N! == N * (N-1)! N , factorial<typename mpl::prior<N>::type> > > { BOOST_STATIC_ASSERT(N::value >= 0); // for nonnegative N }; There's a minor issue in that equal.hpp is (apparently) mistakenly included instead of equal_to.hpp, but even after fixing that, I can't get this to compile in the form in which it's written. Should it, or is it broken? I'm using gcc 3.3-23 on Linux. The first meaningful line in the error novel I get is: error: no type named `tag' in `struct factorial<mpl_::int_<0> >' I was able to get a modified version to work, but only by explicitly invoking the eval_if metafunction and wrapping the result in a constant wrapper, like so: template <class N> struct factorial : mpl::int_< mpl::eval_if< mpl::equal_to<N,mpl::int_<0> > , mpl::int_<1> , mpl::multiplies< N , factorial<typename mpl::prior<N>::type> > >::type::value > { }; But as this is a less-than-trivial departure from the book version, I don't feel comfortable moving on without first gaining a better understanding of what's going wrong. Thanks in advance to anyone who can offer some insight.