On 3/12/2015 5:05 PM, Louis Dionne wrote:
Eric Niebler <eniebler <at> boost.org> writes:
[...]
using factorial_ = lambda_rec<_a, lazy::if_<lazy::greater<_a, meta::size_t<0>>, lazy::multiplies<_a, lazy::apply<_self, lazy::dec<_a>>>, meta::size_t<1>>>; template<std::size_t N> using factorial = apply<factorial_, meta::size_t<N>>;
Metafunctions? We don't need no stinkin' metafunctions!
FWIW, here's how you can implement a factorial with lazy metafunctions, here done with MPL11:
template <typename N> struct fact : if_c<N::value == 0, ullong<1>, mult<N, fact<pred<N>>> > { };
Lambda expressions are a nice hammer, but a factorial metafunction is seemingly not a nail.
Ha! That works in Meta too: using namespace meta; template<typename N> struct factorial : eval< if_c<N::value == 0, meta::size_t<1>, lazy::multiplies<N, factorial<dec<N>>>>> {}; But now that I think about it, there *is* a difference between MPL-style lazy metafunctions and meta::lazy that could complicate these uses of Meta. The metafunctions in meta::lazy don't assume their arguments are themselves lazy metafunctions, which would make lazy metafunction composition difficult. It's really just not the Meta way. -- Eric Niebler Boost.org http://www.boost.org