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

From: Larry Evans (cppljevans_at_[hidden])
Date: 2005-02-28 01:19:54

On 02/28/2005 01:56 AM, David Abrahams wrote:
> Larry Evans <cppljevans_at_[hidden]> writes:
> 
> 
>>After creating:
>>
>>      typedef range_c<int,0,3> types;
>>      typedef fold
>>      < types
>>      , empty_base
>>      , inherit< arg<2>, arg<1> >
>>      >::type tuple_type;
>>
>>I'd like to use for_each on tuple_type.  For example:
>>
>>struct print_integral_c
>>{
>>    template<int i, typename Right>
>>    void operator()(inherit<integral_c<int,i>, Right>)const
>>    {
>>        std::cout<<i<<"\n";
>>    }
>>};
>>
>>for_each<tuple_type>(print_integral_c);
>>
>>However, tuple_type is not a sequence; hence, for_each won't work.
>>I could define the necessary specializations for mpl::begin, etc.,
>>to make it a sequence, but I was wondering if there's any way
>>or any plans to somehow automagically make fold or reverse_fold
>>into a sequence so I could use for_each on it.
> 
> 
> No.  Since fold and reverse_fold are metafunctions (and therefore
> class templates), they can't possibly be sequences (which are always
> types).  fold and reverse_fold don't even have to _return_ sequences.

 From my OP, the sentence:

   tuple_type is not a sequence;

I thought would make it obvious that I didn't mean the metafunction, but
the result of invokation of that metafunction.

OTOH, I really should have said "make the result of a fold metafunction
invokation into a sequence".  Would that have been sufficient to make
my meaning clear?

> 
> I can't understand why you're not using Fusion, or at least trying to
> fit into the Fusion framework, but I'm going to stop flogging that
> horse now.
> 

IIUC, Fusion is in flux, based on the post:

http://article.gmane.org/gmane.comp.lib.boost.devel/118011

and mpl has an easy to use reference manual:

   http://www.boost.org/libs/mpl/doc/refmanual/refmanual_toc.html

OTOH, I haven't really looked at fusion, so maybe I should.  It's
just I've found mpl very good and I've gotten used to it; hence,
I'm reluctant to change.  I'll take a closer look at fusion and
see if I can get it to work, but It's got to do the range_all

   http://article.gmane.org/gmane.comp.lib.boost.devel/117764

"thing" also, and I had some difficulty getting that to work with
mpl; hence, I don't want to go thru that again with fusion, so,
no promises.

BTW, the range_all.zip has moved in the vault to cppljevans/mpl.