Re: [boost] proto::or_ arity

From: Eric Niebler (eric_at_[hidden])
Date: 2007-04-19 12:46:18

Larry Evans wrote:
> 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
>      ;

Oh! You're chaining or_'s together. Sure, that works. So for your 
initial state, all you need is a pattern that never matches anything. 
For that, you can use proto::not<proto::_>.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com