Re: [boost] [metal] Feature Complete - Request for Feedback

Subject: Re: [boost] [metal] Feature Complete - Request for Feedback
From: Deniz Bahadir (dbahadir_at_[hidden])
Date: 2017-02-21 18:21:38

Am 21.02.2017 um 18:07 schrieb Bruno Dutra via Boost:
> On Tue, Feb 21, 2017 at 2:38 PM, Deniz Bahadir via Boost <
> boost_at_[hidden]> wrote:
>
>> Am 21.02.2017 um 14:03 schrieb Bruno Dutra via Boost:
>>
>>> On Tue, Feb 21, 2017 at 10:53 AM, Hans Dembinski <
>>> hans.dembinski_at_[hidden]>
>>> wrote:
>>>
>>>>
>>>> [...]
>>>
>>> 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.
>>>>
>>>
>>> It does sound like a good idea from the user's point of view since Metal
>>> looks so similar to MPL, but because they are so drastically distinct
>>> internally, merging them into a single library doesn't really make any
>>> sense. Fortunately, if you are coming from some legacy metaprogram, it is
>>> easy to incrementally migrate to Metal using the helper metal::from_mpl,
>>> but there is really no reason to ever use MPL anymore on a new project.
>>>
>>
>> Could you elaborate a little more on this metal::from_mpl?
>> How is it used?
>> Can I just replace all occurrences of "boost::mpl" in any code by
>> "metal::from_mpl" and it works?
>
>
> The idea is to be able to connect some preexisting MPL code to Metal, so
> that new features may be implemented using Metal already, before eventually
> porting everything in a gradual fashion and only if necessary. Thus
> metal::from_mpl converts MPL Sequences, Integral Constants and Metafunction
> Classes to their equivalents in Metal. It is recursive, so if you have an
> mpl::list of mpl::int_, it will produce a metal::list of metal::number.
> Check out the examples here: http://brunocodutra.github.io/metal/group__
> external.html (click the panel to expand).
>

Ahh, clicking the panel makes sense. :-)
I did not try this before and thought that doc-generation just failed 
there (or that none was available).
Maybe, you should change the default view to be expanded (if possible).

>
>> At our company we had to increase the number of elements to 100 and it
>> works fine except for the obvious reasons Bruno already talked about. It is
>> so damn slow and wastes so much RAM that we can only compile
>> single-threaded which slows down compile-performance even more.
>
>
>> That is why I am so interested in a drop-in replacement for Boost.MPL and
>> asked about metal::from_mpl above.
>>
>
> While Metal is not able to replace MPL like this, it can complement it with
> the help of metal::from_mpl, which makes it possible to rewrite critical
> sections that take the longest to compile for example. Give it a try, gains
> with respect to performance and memory consumption are huge!

Our problem is, that we never really used Boost.MPL directly. We just 
got it for "free" as a (costly) dependency through other Boost libraries 
(e.g. Boost.MSM). So, I am not really looking forward to changing 
anything there.
Using "find/replace", however, to just change the namespaces would have 
been a great (= easy) solution.

But if find time I will see if Metal can help here.

>
> Regards,
> Bruno
>

Thanks,
Deniz