Re: [boost] [review][mp11] Formal review of Mp11

Subject: Re: [boost] [review][mp11] Formal review of Mp11
From: Peter Dimov (lists_at_[hidden])
Date: 2017-07-17 01:00:37

Brook Milligan wrote:
> Before I write a review, I have a couple of questions that are the 
> outgrowth of having been using mp11 since some time after the first two 
> articles of Peter’s.  That should suggest I generally like the library, 
> which I do.
>
> Nevertheless, in my code, I have found need for the following additional 
> metafunctions:
>
> - mp_map_keys: creates a list of keys from a map

With metaprogramming libraries, it's always quite difficult to decide what 
to include and what to leave out, as there are so many useful things that 
are also one-liners. As everyone has probably surmised by now, I prefer 
minimalism, that is, omit by default, only include when a clear need arises.

mp_map_keys, for example, is mp_transform<mp_first, M>.

> - mp_is_set: type trait to determine whether a type is a set, i.e., has 
> unique elements

This at the moment is std::is_same<S, mp_unique<S>>, although as I 
mentioned, perhaps either mp_is_set or mp_distinct should indeed be 
provided.

> - mp_is_map: type trait to determine whether a type is a map, i.e., has a 
> unique set of keys

This is mp_is_set<mp_map_keys<M>>, although not quite if M has an element 
that is not a list.

> I have also found need for a metafunction that takes a map and a set of 
> keys and returns a map with only elements with the selected keys.  This is 
> perhaps more specialized, but I can also see a general use case.

template<class M, class L> using mp_map_subset =
    mp_copy_if<M, mp_bind<mp_set_contains, L, mp_bind<mp_first, 
_1>>::template fn>;

mp_copy_if_q, when I add it, will eliminate the need for the ::template fn 
here.