[boost] meaning of or_ and and_ (was Re: why or_, and_ names?

From: Larry Evans (cppljevans_at_[hidden])
Date: 2007-04-06 06:30:39

On 04/04/2007 02:49 PM, Eric Niebler wrote:
[snip]
> behaviors you want. For this, you write a domain-specific tree 
> transformation by writing out your DSEL's grammar and attaching 
> transforms to your grammar's rules.
> 
> This is where proto::or_ and proto::and_ come in. Imagine in your DSEL, 
> only terminals of type int and char* are allowed. Then you might define 
> a Terminal rule in your grammar as follows:
> 
> struct Terminal
>    : proto::or_<
>          proto::terminal<int>
>        , proto::terminal<char*>
>      >
> {};
> 
> When you write your tree transform, you might want to attach some 
> transformation to the Terminal rule to, for example, turn char* into 
> spirit::string_parser, or whatever.
>

I'm still unclear about and_.  I've looked at matches.hpp where
both or_ and and_ are defined and it looks like, as their names
suggest, that they are predicates of some sort.  Now I can
understand how or_ works as you described above for defining
a grammar's terminals; however, I'm unclear about and_.  Is an and_
instantiation a possible 2nd arg to matches and, in this case,
matches tests it's first argument to see that it satisfies both
arguments to the and_?

> Proto's meta-grammar facilities are good for more than just tree 
> transformations, by the way. You can use them with proto::matches<> to 
> make compile-time decisions based on the structure of an expression. 

So matches<Expr,Grammar> tests Expr to see if it matches the
meta-grammar, Grammar?  IOW, and_<X,Y> and or_<X,Y> are possible
values of Grammar in matches<Expr,Grammar>?