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Subject: Re: [boost] [review][mp11] Formal review of Mp11
From: Peter Koch Larsen (peter.koch.larsen_at_[hidden])
Date: 2017-07-23 18:08:16

I would like to add my opinion about the mp_ prefix. Many do not like
it, but I actually believe it is very nice to have a name that stands
Apart for reducing the likelihood that there will be a name-clash if
the library should get accepted into the standard, there is also the
advantage that you can easily see that this is a mp_ function. Other
libraries have adopted a standard of simply appending an underscore
after the name so that the library char becomes char_.
If your eyesight is not perfect, this convention becomes a pain. I
have myself spent hours trying to decrypt weird error-messages (most
notably in spirit) to realise that an underscore was missing.
So hooray for mp_. ;-)


On Sun, Jul 23, 2017 at 7:44 PM, Glen Fernandes via Boost
<boost_at_[hidden]> wrote:
> What follows is my review of the Mp11 library:
> [Should the library be accepted into Boost?]
> Yes. I vote for an unconditional ACCEPT.
> [What is your evaluation of the design?]
> Excellent. The design where any template can serve as a list, and any template
> can serve as a meta-function is both elegant and powerful. The additional
> quoted meta-function support with utilities such as mp_bind, mp_bind_front,
> and mp_bind_back, addresses the other request I had when reviewing an earlier
> version of Mp11. While HOMF were never essential, as the article pointed out,
> users appreciate more ways to one-line things without having to write a helper
> type. Some additional primitives are desired, as mentioned later in this
> review (e.g. min and max functionality) but overall the toolkit is already
> vast and useful.
> [What is your evaluation of the implementation?
> Excellent. The focus on performance is definitely appreciated while we still
> have to depend on libraries for many of these meta-programming primitives.
> Ideally many of these would be provided in the standard library and have
> implementations that only a compiler vendor could provide with built in
> support, and offer performance beyond what a library could offer. In the
> meanwhile, the performance offered by Mp11 is great, more than sufficient for
> my needs, and in my opinion also for the majority of users. In comparison, the
> tiny C++11 meta-programming library that I have implemented, uses only the
> obvious C++11 implementations for most functionality, and does not offer the
> same fast compile times as the equivalents in Mp11. The techniques used by
> Mp11 via C++11 constexpr, C++14 constexpr, and even C++17 fold expressions,
> besides improving Mp11 performance, were a valuable source to learn from.
> Suggestion: Can all the workarounds for MSVC versions 1800 and older for the
> empty list cases be avoided by instead using a helper trait? i.e.
> template<template<class...> class L, class... T, template<class...> class P>
> struct mp_find_if_impl<L<T...>, P> {
> using type = typename mp_find_if_impl_2<P, T...>::type;
> };
> i.e. Introduce mp_find_if_impl_2 which would then have the obvious
> implementation as before, only without requiring the MSVC workaround? This is
> only worth considering if you find it to offer some benefit such as:
> Additional maintainability of the Mp11 code, with less workarounds, and no
> costs, e.g. no increase in compilation times as a consequence.
> [What is your evaluation of the documentation?
> Very good. The reference documentation is clear, and easily understood. More
> example and tutorial content before the reference section would always be
> welcome, but not necessary.
> Suggestion: The two-part article that Peter wrote (which is referenced in
> the documentation) is in my opinion so valuable for users to gain better
> understanding and appreciation of the library that it should be included in
> the library distribution as part of the documentation. Ideally in the same
> Asciidoc form as the current documentation, as a supplemental document (or
> Appendix entry).
> Suggestion: Does it make sense for the documentation to have a section
> describing the conventions the library uses, for potential contributors as
> much as users? e.g. If someone was designing an mp_min_element, should it
> yield the element type result, or the list index size constant result? Is
> there an established convention of when an error is acceptable for an empty
> list versus returning an index?
> Correction: Under heading mp_find_if<L, P>: "mp_find_f<L, P> is an" should be
> "mp_find_if<L, P> is an".
> [What is your evaluation of the potential usefulness of the library?
> Incredibly useful in its current form, and this is only going to increase as
> the library is extended based on user requests for additional functionality.
> Almost every meta-programming primitive that I have needed recently either
> already exists in Mp11, or I am able to implement using Mp11 with a little
> effort. During this review I replaced the use of my own meta-programming
> library in a project of mine without any issues. As part of this review I also
> tried several exercises where I implemented certain traits and meta-functions
> using Mp11. Of all the exercises that I attempted, the only one where the
> library lacked the necessary utility was an "mp_min_element" function:
> This was an implementation of the typical "type_with_alignment" trait, which
> can easily be implemented using a "min_element" functionality. Mp11 does not
> have mp_min, mp_max, mp_min_element, mp_max_element presently, and I believe
> they should be added because they are useful and users will probably find
> themselves implementing it. Sure, mp_sort exists, but I would not use a sort
> when a min_element can suffice. In this case, type_with_alignment<N> could use
> mp_min_element with a predicate that finds the type in an mp_list of several
> scalar types, that matches has an alignment of at least N and has the smallest
> size, and then check that the result element type of the mp_min_element is at
> least N.
> The litmus test for me for a meta-programming library is the question: Would I
> use Mp11 in library code? The answer for me is "yes". If it was part of Boost,
> I would use it in other Boost libraries (that already require C++11 or
> higher).
> [Did you try to use the library? With which compiler(s)?
> Did you have any problems?]
> Yes. I used the library as a replacement for an existing meta-programming
> library of mine in real (non-toy, non-exercise) project code that is compiled
> with g++ 4.8.5 in c++11 mode for x86 and x86_64 Linux, using Boost 1.64.0, on
> 64-bit RHEL 6 and measured compilation times. I also used the library in a few
> exercises that I compiled with g++ 7.1 and clang 4.0.0, for x86 and x86_64
> Linux, using the current Boost master branch, for c++11 and c++14, on 64-bit
> Fedora 26, and again measured compilation times. I also compiled and ran the
> Mp11 unit tests with the compilers, C++ standard modes, architectures, Boost
> distributions, and operating systems mentioned.
> [How much effort did you put into your evaluation? A glance? A quick reading?
> In-depth study?]
> More than ten hours over this weekend reading the documentation, reviewing the
> implementation and tests, using Mp11 in a project replacing my own C++11
> meta-programming library, using it in various exercises, and running the Mp11
> unit tests with the various compilers, C++ standard modes, targets, and Boost
> distributions listed above. Prior to the preparation for the review, I had
> spent some time with an earlier version of Mp11 and comparing it to Metal for
> implementing a few meta-functions, including the "instantiates_" example from
> my library:
> instantiates_<T, templates_<std::set, std::multiset, std::unordered_set> >
> Where instantiates_ should be implemented in terms of "is_instantiation_of_":
> is_instantiation_of_<T, std::set>
> Which has the obvious behavior indicated by the trait name. The Mp11 approach
> provided by Peter for this case, and the relative ease with which I was able
> to implement the other meta-functions using Mp11 were a convincing
> demonstration of the usefulness of the library.
> [Are you knowledgeable about the problem domain?]
> Like many of us, I had written a meta-programming library as C++11 language
> features have become available. Before that, if Boost.MPL solved a problem,
> more often than not it was convenient and useful enough to just use it, even
> if writing my own implementation would result in faster compilation times.
> Peter's article, however, opened my eyes to a more elegant design, and changed
> how I implemented my C++11 meta-programming library, and in general shaped
> what I think about meta-programming in modern C++.
> Thank you Peter for your work on Mp11, for submitting it for inclusion in
> Boost, and for the articles that motivated it and taught the rest of us.
> Glen
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