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Subject: Re: [boost] [metal] Feature Complete - Request for Feedback
From: Hans Dembinski (hans.dembinski_at_[hidden])
Date: 2017-02-21 09:53:31

Dear Bruno,

>> What I do however wonder is, where does it fit in with MPL, Fusion and
>> especially Hana? If you could give an overview of where your library is
>> compared to those, you probably increase the support for your library.
> That is a very valid concern and I wondered myself whether there was space
> for Metal in Boost for quite a while, before finally deciding to put this
> proposal forward.
> It basically boils down to whether template metaprogramming (TMP) is still
> useful to modern C++, given decltype and the paradigm shift it enables,
> which is very well explored by Hana. I believe that yes, TMP has its unique
> use cases, mainly because:
> * it scales much better in terms of both compile time and memory usage on
> all major compilers. To convince yourself, just check and be
> sure to play with the 'subtract baseline' switch;
> * It integrates nicely with standard type_traits and utilities;
> * It draws a clear line between type-level and value programming, which
> improves readability when the goal is to trigger SFINAE and drive overload
> resolution.

I am on the skeptical side. I just learned boost::mpl and boost::fusion to implement compile-time computations for my proposed boost.histogram library, so I am by no means an expert on meta-programming. Nevertheless, I was positively surprised how fast I made progress with these libraries once I overcame my general attitude of "TMP is weiiiiird". The structural similarity of boost::mpl with the STL eased the learning curve a lot.

At first glance (I am looking at the code in the section "In a Glimpse" of your documentation), I don't see much syntactic difference to boost::mpl, could you elaborate on the differences? It seems you mostly you drop the trailing ::type, because you use alias templates. I like your metal::lambda, I am not sure if boost::mpl has that. If not, one could probably add it to boost::mpl.

I suppose my big question is this: can't you just merge your ideas/improvements into boost::mpl? With ifdefs you could enable C++14 features like templated aliases when they are available. Even more so, it should be possible to merge your speed improvements into boost::mpl.

I also looked at your benchmarks and they are a bit confusing. I am mostly interested in how your lib compares to the mpl, but not all benchmarks have mpl data, and those that have (see at or count_if) only use up to 50 elements. Is that a hard limitation of the current mpl implementation?

I am looking forward to the comments from the authors of boost::hana and boost::mpl.

Best regards,

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