Larry Evans wrote:
On 03/03/08 19:29, Eric Niebler wrote:
Larry Evans wrote:
If T0,T1,...,Tn are all expression types, and Tag is an n-ary tag, then expr<Tag,T0,T,...,Tn> is an expression type.
Yes. It is also a grammar type that matches itself.
[snip] Hmm... So a grammar is an expression and an expression is a grammar. Sorta like lisp where a program is data and data is a program. I guess proto::matches is like lisp eval. Is that sorta right?
An expression is a grammar, but a grammar is not an expression. For example, not_<X> is only a grammar and not an expression. There's a similarity between lisp macros and proto transforms. They both operate on executable code in the form of expression trees. We haven't spoken about transforms yet, but just as there is a relation between expressions and grammars, there's also a relation between grammars and transforms. I certainly feel like there needs to be a section in Proto's docs that calls out these relationships explicitly. -- Eric Niebler Boost Consulting www.boost-consulting.com