Re: [boost] [GSoC][MPL11] Post C++Now update

Subject: Re: [boost] [GSoC][MPL11] Post C++Now update
From: Roland Bock (rbock_at_[hidden])
Date: 2014-05-20 18:00:03

On 2014-05-20 20:59, Larry Evans wrote:
> On 05/18/14 15:41, Louis Dionne wrote:
>> Dear Boost,
>>
>> A lot happened during the past week in Aspen; the goal of this message
>> is to summarize insights, changes and feedback received wrt MPL11.
>>
>> The slides of the presentation are available at [1]. The second part of
>> the presentation is especially relevant to get an overview of the
>> designs
>> considered so far.
>>
>>
>> Merging the MPL and Fusion
>> --------------------------
>> After discussing the issue several times during the week, I (and others)
>> think it might be possible to merge Fusion and the MPL into a single
>> library. I am currently trying to write a library that does that. Since
>> this constitutes a large reorientation, I created a new repository which
>> is available at [2]. Those with interest should consider subscribing to
>> the repository to be updated as I make progress.
>>
>>
>> Iterator-free design
>> --------------------
>> I was a bit anxious that people would complain about this, but the
>> contrary
>> happened. Nobody complained and some people expressed support. This
>> design
>> aspect will stay as-is.
>>
>>
>> Request for guinea pigs
>> -----------------------
>> I am looking for people with hardcore C++1y metaprogramming needs who
>> will
>> be willing to test the library I come up with (MPL11 or MPL + Fusion
>> merge).
>> I also need to see use cases for the library, so please reply even if
>> you
>> are not willing to test.
>>
>
> As mentioned in this post:
>
> https://groups.google.com/forum/?hl=en#!msg/comp.std.c++/_-6X_xZlKlA/-HhvKh5ccEAJ
>
>
> One use case is:
>
> > 3) A guaranteed non-recursive way to access elements of parameter
> > packs
>
> and Doug Gregor (quoting from above post) says:
>
>   This is probably the most-requested feature for variadic templates,
>   and it never it made it because we never found a good, unambiguous
>   syntax.
>
> The attached seems to do that.  What about adding something
> like this to the library?
>
> -regards,
> Larry
>
Attached is a solution that uses C++14 index sequence (which is
recursive, afaik), but is otherwise recursion free. Maybe there is a
non-recursive alternative to index_sequence to "eat up" the first N-1
elements?

The code based on the third attachment of this clang bug report
http://llvm.org/bugs/show_bug.cgi?id=13263

Regards,

Roland