From: Eric Niebler (eric_at_[hidden])
Date: 2008-03-22 23:12:24
Larry Evans wrote:
> On 03/04/08 11:38, Eric Niebler wrote:
>> An expression is a grammar, but a grammar is not an expression. For
>> example, not_<X> is only a grammar and not an expression.
>>
>
> The following grammar I think summarizes the difference between
> grammar and expression:
>
> expr //describes an expression.
> = terminal
> | expr+
> //i.e. 1 or more expressions (e.g. expr<tag::plus,expr0,expr1>)
> ;
OK.
> gram //describes a grammar
> = expr
> | wildcard //struct wildcardns_::_ in matches.hpp
> | control //any struct or class template in namespace control in
> //matches.hpp
> ;
OK.
> control
> = not_ >> gram
> | or_ >> gram >> gram+
> | and_ >> gram >> gram+
> | if_ >> gram >> gram
> ;
I may have an issue with your notation here.
control
= not_<gram>
| or_<gram+>
| and_<gram+>
| if_<trans, gram, gram>
// or a vararg expr, or switch_, or ...
;
> The '= expr' as first alternate of gram reflects the statement
> 'an expression is a grammar'. The absence of gram on the
> rhs of the expr equation reflects the statement 'a grammar
> is not an expression'.
>
> Is that about right?
Yes, you have it.
-- Eric Niebler Boost Consulting www.boost-consulting.com