Tom Brinkman wrote:

Proto is the new backend for boost::spirit, boost::xpressive and the re-write of boost::phoenix, this is indeed an important library. It would seem that "Proto" already has the support of all of the top boost developers.

1) I wonder if you could give us a little bit of history, what motivated this library?

Good question. In Proto's docs, there is a History appendix: http://boost-sandbox.sourceforge.net/libs/proto/doc/html/boost_proto/appendi... There you'll find links to extensive discussions on the spirit developers list that informed much of Proto's design. In particular, the initial motivation for Proto was to simplify xpressive's metaprogramming, which after several year's work, had become brittle. Even with MPL, the job of implementing a domain-specific embedded language using expression templates required a large amount of very low-level code that is hard to maintain. With Proto, a DSEL is described and implemented at the level of grammars and semantic actions rather than low-level template hackery.

2) What were the defincies in previous design of boost::spirit. Why does spirit need a new backend?

The decision to reimplement Spirit was made before the decision to adopt Proto and was motivated both by efficiency and architectural concerns, IIUC, but Joel and Hartmut are better qualified to answer that. Very early on in the reimplementation effort, I convinced Joel and Hartmut to use Proto. The killer argument at the time was that it allows Qi and Karma share DSEL primitives (see http://osdir.com/ml/parsers.spirit.devel/2006-04/msg00035.html). Since then, many more of Proto's features have found use in Spirit-2. In particular, the job of transforming an expression template into a Qi parser or a Karma generator is handled by Proto transforms.

Learning to use boost:spirit is about a six month learing curve, but well worth it. For me, it was my introduction to functional programming. I'm excited to see what else is possible.

Spirit-2 is cool tech. I'll let Joel and Hartmut speak to that. -- Eric Niebler Boost Consulting www.boost-consulting.com