Re: [boost] About all these metaprogramming libraries

Subject: Re: [boost] About all these metaprogramming libraries
From: Peter Dimov (lists_at_[hidden])
Date: 2017-03-18 22:08:25

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<T...> 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.