Re: [boost] About all these metaprogramming libraries

Vicente J. Botet Escriba wrote:

* do we want (can) to use as (HOMF) high-order meta-functions? if we nedd them at all). IIUC, Peter library doesn't works with HOMFs. * do we want a Hana style with "concepts" and with customizations? Do we want other data type than type lists? IIUC Peter's library works only with template aliases as data types and almost variadic class template with type parameters is a good candidate for a type list (even std::variant :( )

These two are the distinguishing features of mp11. All other libraries have chosen differently. To clarify, the mp11 approach does support higher-order metaprogramming, but it chooses to make the ordinary case easier at the expense of the HOMF case, which is made harder, whereas the other libraries choose to make HOMF easier, at the expense of taking quoted metafunctions as arguments, which requires aliases to be quoted. mp11, in contrast, requires quoted metafunctions to be de-quoted (Q::invoke) when passed to algorithms. To clarify further, std::variant is absolutely a supported typelist in mp11. If you have a std::variant V, you can remove the duplicate types from it with mp_unique<V>. This is again a deliberate design decision which keeps simple uses simple at the expense of making more complicated uses more complicated. With the other libraries, to remove the duplicates from a variant, you first have to convert it to the native typelist type, apply 'unique', then convert back to std::variant.