Re: [boost] [fusion] pop_back doesn't work on fwd sequences

Subject: Re: [boost] [fusion] pop_back doesn't work on fwd sequences
From: Eric Niebler (eric_at_[hidden])
Date: 2011-08-10 02:51:29

On 8/9/2011 11:17 PM, Joel de Guzman wrote:
> On 8/10/2011 12:56 PM, Eric Niebler wrote:
>> A sequence created by push_back is a Forward Sequence, which means you
>> can't call pop_back on it. I think that's a serious shortcoming. Fusion
>> needs something like MPL's Back Extensible and Front Extensible concepts.
>>
>> http://www.boost.org/doc/libs/1_47_0/libs/mpl/doc/refmanual/back-extensible-sequence.html
>>
>> Unfortunately, this would likely require a redesign of the whole
>> (push|pop)_(front|back) mechanism.
> 
> Man, this is a major bummer, alright. One plausible clean way I can think of is:
> 
> 1) Make single_view random_access (if it's not already; IIRC No).

I implemented this not too long ago.

> 2) Make joint_view assume the least sequence category of its subsumed sequences.
>    There's already some work done related to this where joint_view inherits
>    the 'associative'-ness of its held sequences.

I don't think that's good enough. Take my example above. In,
push_back(nil(), 1), the traversal type of nil is Forward, and so too
would be the resulting view. It probably makes sense to see how MPL
manages it.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com