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