[boost] Re: best way to make {reverse_}fold a sequence

From: Larry Evans (cppljevans_at_[hidden])
Date: 2005-02-28 17:02:15

On 02/28/2005 10:01 AM, David Abrahams wrote:
[snip]
>> 
> That's misguided it seems to me.  Your tuple_type is not a
> specialization of fold.  Are you really interested in making
> specializations of fold into sequences?
> 
> You're folding some kind of cons cell, right?  Don't you want to
> implement begin and end on that?
> 
>   template <class A, class B>
>   struct begin<inherit<A,B> >
>   {
>      ...
>   };
> 
> or something?
Yes.  But how do I know what A or B is.  It has to be the result
of size<Sequence>-1 applications of ForwardOp.  Is there a way to
get this other than fold<Sequence,StartState,ForwardOp>::type ?
Hmmm.  This is what get_side_type does with Index == 0.  Maybe
I could use that.

BTW, get_side_type is also in vault.

[snip]
>>My other post mentioned visitors.hpp and its need for fold.
>>How could fusion's tuple be used to implement visitors where
>>the critical need is a StartState superclass for all instantiations
>>of ForwardOp?
> 
> 
> I'm not the guy to ask, sorry.  Joel is the fusion expert, and... I
> mean no offense by this, but I find your code hard to read and
> understand, so I'm trying to be helpful while not getting entangled in
> looking at its details because I think it will cost me too much time.

OK. Thanks for the help :)