Re: [boost] proto::or_ arity

From: Larry Evans (cppljevans_at_[hidden])
Date: 2007-04-19 16:15:32

On 04/19/2007 12:35 PM, Eric Niebler wrote:
> Larry Evans wrote:
> 
>>On 04/19/2007 11:46 AM, Eric Niebler wrote:
>>[snip]
>>
>>>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::_>.
>>
>>I couldn't get that to compile; however, proto::logical_not<proto::_>
>>does.  Thanks.
> 
> 
> 
> proto::not_<> is a fairly recent addition. Be sure you're sync'ed up. 
> logical_not<_> is certainly not what you want here. It will successfully 
> match proto expressions such as !as_expr('a'). You want something that 
> will never match anything. You could also use 
> proto::if_<mpl::always<mpl::false_> >.
> 
> 
Yeah, I thought about it some more and the name, logical_not, suggests
that if the argument, X, doesn't match, then logical_not<X>, would
match, which, as you've noted, is certainly not what I want.
However, I think proto::not_ also suffers the same name confusion.  Like
logical_not<X>, I'd think not_<X> would mean "if X fails then not_<X>
succeeds".  proto::if_<mpl::always<mpl::false_> > is much clearer 
although more verbose.  Maybe proto::always_false or maybe proto::no 
would be less confusing and about as verbose.