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

Subject: Re: [boost] [fusion] pop_back doesn't work on fwd sequences
From: Joel de Guzman (joel_at_[hidden])
Date: 2011-08-10 02:17:59

On 8/10/2011 12:56 PM, Eric Niebler wrote:
> The documented requirement of pop_back are wrong. The docs say that
> pop_back works on Forward Sequences, however the following does not compile:
> 
>     // ERROR
>     fusion::list<int, int> l(1, 2);
>     fusion::pop_back(l);
> 
> The problem is that the implementation of pop_back uses prior, which
> only works on Bidirectional Sequences.
> 
> That's merely a doc problem, but this is a more serious problem:
> 
>     // ERROR
>     fusion::pop_back(fusion::pop_back(
>         fusion::push_back(fusion::push_back(fusion::nil(), 1), 2)));
> 
> 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).
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 think this is an implementable clean solution. I'll see if I can
work on it. Yours is a good test case.

Thanks for noticing this, Eric!

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://boost-spirit.com