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

From: David Abrahams (dave_at_[hidden])
Date: 2005-02-28 09:16:02

Larry Evans <cppljevans_at_[hidden]> writes:

> 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?

Not really.  As I said, "fold and reverse_fold don't even have to
_return_ sequences."  By that I mean that the result of invoking fold
might be, e.g. int.

>> 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.  

It's not a change.  Fusion is a compatible layer on top of MPL.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com