Let me put this another way: do any of the other metaprogramming libraries add *in pure API or capability terms* anything over Hana?
I don't mean to derail the discussion again, but since you asked, Metal provides SFINAE-friendliness guarantees that Hana does not. Think std::enable_if on steroids.
Thanks for the info. However to continue to play devil's advocate, is that a highly motivating reason for an additional metaprogramming library over Hana in Boost when Concepts will ship in every major compiler this year or next? If someone were proposing a metaprogramming library which allowed me to write code which uses Concepts on a supporting compiler, but fell back to a C++ 14 emulation on older compilers, that I'd find that a compelling reason for an additional Boost metaprogramming library. Niall -- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/