Re: [boost] proto::or_ arity

From: Larry Evans (cppljevans_at_[hidden])
Date: 2007-04-19 04:54:28

On 04/18/2007 01:06 PM, Eric Niebler wrote:
> Larry Evans wrote:
[snip]
>>I've just encounted a need for something like this.  For example,
>>if you have a list of types for which you want to apply the or_ to,
>>however, that list must by calculated, then the only way to apply
>>or to it is with a fold operation.  Since mpl::fold takes
>>an argument represening the initial state, this would naturally
>>be proto::or_<> in case you want to proto::or_ together a list
>>of types.
> 
> <snip>
> 
> Interesting. In order for this to work, proto::or_ would need to be an 
> extensible mpl sequence, right? 
[snip]

No. The input to fold has to be; however, the output produced doesn't.
Here's what I've got so far:

   struct
or_base
;
   template
   < class Head
   , class Tail
   >
   struct
or_op
{
         typedef
       proto::or_
       < Head
       , typename Tail::type
       >
     type
     ;
};
   template
   < class Head
   >
   struct
or_op
   < Head
   , or_base
   >
{
         typedef
       Head
     type
     ;
};
     typedef
   mpl::vector
   < inp<inp_0>::type
   , inp<inp_1>::type
   , out<out_0>::type
   , out<out_1>::type
   , out<out_2>::type
   >
grm_syms
;
   struct
grm_symbol
: mpl::fold
   < grm_syms
   , or_base
   , or_op
     < mpl::arg<2>
     , mpl::arg<1>
     >
   >::type
{
};

And it appears to work.