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

Subject: Re: [boost] [GSoC][MPL11] Post C++Now update
From: Larry Evans (cppljevans_at_[hidden])
Date: 2014-05-21 12:42:27

On 05/20/14 17:00, Roland Bock wrote:
> 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
>
Thanks very much Roland.  I had no idea there was this way
to do it.  However, I had a lot of trouble understanding
how it worked; hence, I added a bunch of comments, as shown
in the attached.  After adding the comments, I now understand
what you mean by "eat up" the first N-1 elements.

-regards,
Larry