Re: [boost] Formal Review Request: mp11

Subject: Re: [boost] Formal Review Request: mp11
From: Peter Dimov (lists_at_[hidden])
Date: 2017-03-19 11:06:43

Vicente J. Botet Escriba wrote:

> About your mp_count example. Compare your definition with this the HOF one
>
>      template <class TL, class V>
>          using mp_count_if<TL, P> = mp_plus<transform<TL, P>;

First, you need to keep in mind that the code in the article compiled and 
worked at the time on the compilers that I had available, which included 
Visual Studio 2013. Using mp_transform was a good way to crash MSVC and 
remains so to this day, which is why the implementations often inline it by 
hand.

Second, the above doesn't work because P<T>::value is allowed to be 42.

>      template <class TL, class V>
>          using mp_count<TL, V> = mp_cont_if<TL, is_same_t<_1,V>>;

This - assuming that is_same_t is a template alias - cannot be made to work. 
Template aliases are evaluated eagerly and is_same_t<_1, V> gives you 
mp_true or mp_false depending on whether V is _1 (the literal _1) or not.

You either have to use std::is_same<_1, V> and make the algorithms take and 
evaluate such lambdas by replacing placeholders and looking at ::type, or 
you need to bind or curry is_same_t. mp_bind<is_same_t, _1, V>::invoke, if 
it existed (it's a few lines, I'll probably add it) or mp_quote<is_same_t, 
V>::invoke.

(I'm considering renaming ::invoke to something shorter such as ::fn.)

> BTW, your mp_plus doesn't take a type list as parameter.

It doesn't. There's an annoying dilemma here as most functions are useful in 
either form. mp_second<L> for instance takes a list, but mp_second<T...> 
would also have been useful. Not sure what to do here.

> About mp_contains. I've not yet finished the second article yet. IMO, we 
> need different concepts as we can have different implementations. I 
> believe mp_contains is an example where we can have different 
> implementations for type list and for type set.

It does, see mp_contains and mp_set_contains in the actual mp11 source. The 
latter requires the elements to be unique, although the data structure is 
still a list. I like Lisp.

> About C++11, C++14, C++17.  I believe that meta-programming can profit a 
> lot of C++17 fold expressions.

That belief is not necessarily true, but there are places where fold 
expressions are obviously useful, such as mp_plus. mp_fold can also be 
rewritten in their terms, but the performance gains are disappointingly 
small, although it does considerably increase the list size limit that can 
be handled without the compiler crashing.

We'll add BOOST_NO_CXX17_FOLD_EXPRESSIONS at some point, presumably. Support 
is currently a bit hard to detect as Clang doesn't define an __is_feature 
for it. (I already had to make creative use of fold expressions to work 
around a Clang 3.9+ bug with mp_find.)