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Subject: Re: [boost] [metal] Feature Complete - Request for Feedback
From: paul (pfultz2_at_[hidden])
Date: 2017-02-22 23:11:54

On Wed, 2017-02-22 at 19:35 +0100, Bruno Dutra wrote:
> On Wed, Feb 22, 2017 at 2:33 AM, Paul Fultz II via Boost <boost_at_lists
>> wrote:
> >
> > > On Feb 21, 2017, at 1:16 PM, Bruno Dutra via Boost <boost_at_lists.b
> >> wrote:
> > >
> > >
> > > Your example is implemented in Metal like this
> > >
> > > using _ = metal::transform<metal::lambda<std::add_const_t>,
> > > metal::as_list<std::shared_ptr<X>>>; // metal::list<X const>
> > > metal::apply<metal::lambda<std::shared_ptr>, _> //
> > std::shared_ptr<X const>
> > >
> > > Admittedly a bit verbosier, but not too bad.
> > > .
> >
> > I like this. Its explicit and fast, which I think is good for
> > something that is core. From there you could build other frontends
> > that simplify it more by deducing the explicit types and then
> > passing them to the core components.
> Exactly. Thanks for your feedback!
> > Another thing I saw is that you no longer support gcc 4.8. Why is
> > that? As a core library, I would expect good portability, since the
> > portability limitations don’t just affect Metal but also any
> > libraries that decide to use it(or perhaps replace mpl with it).
> That is a very good point. I've strived to support the widest variety
> of compilers throughout the development of Metal and spent a _lot_ of
> time, working around nasty compiler bugs. It is kindda sad, but by
> now I'm sure I could ICE either GCC, Clang or MSVC at will. Simply
> put, at some point I just gave up on hammering Metal down on older
> GCC versions. 
> I just tried to running Metal test suite on GCC 4.9 and it fails the
> 3rd test of 200 already. According to the error log, it can't SFINAE
> away invalid instantiations of a template that expects N arguments
> but was given less than that.
> It most probably can be worked around by decltyping a an overloaded
> function call, but that kills performance and I'll have to #ifdef it,
> which sucks TBH. Nevertheless I'll give it a
> shot

I think picking a slower implementation on older compilers is
acceptable. This way I can develop with newer compilers which are fast,
and deploy with the older compilers. Also, having a faster compilation
with newer compilers is a good way to convince management to move to a
newer compiler.

> > Ideally, it would be nice if it supported gcc 4.6(even only
> > partially), as I would use it in my Tick library and get rid of my
> > homebrew metaprogamming. However, I know that might be a lot of
> > work since gcc 4.6 doesn’t have template aliases.
> Sadly without alias templates there's not much I could do :(

Well on gcc 4.6 inheritance can be used instead of an alias, but then
you would need an evaluation step, like:

METAL_EVAL(metal::drop<l3, metal::number<3>>)

And then the library would need to apply these evaluations internally
as well. I don't know know if you would accept a PR with changes to
support gcc 4.6 or not. Of course the first thing is to support for gcc
4.8 and then we could move backwards toward 4.7 and then 4.6.


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