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

Subject: Re: [boost] [GSoC][MPL11] Post C++Now update
From: Roland Bock (rbock_at_[hidden])
Date: 2014-05-21 17:30:28

On 2014-05-21 18:42, Larry Evans wrote:
> 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

It took me quite a while, too :-)

I am having a few difficulties with the documentation in the places that
use a '+1',  especially here:

"return (N+1)-th ts, where 0-th ts is first one."

With N==0, you get the argument at position 0. The above sentence seems
to be telling something different or am I just too tired?

Other than that: good job at documenting. I probably would have tried to
describe the concept instead of documenting the individual parts, but I
find it quite hard to describe this in natural language. I'll give it
some more thought, though :-)

Best,

Roland