On Wednesday 20 October 2010 05:19:17 Joel de Guzman wrote: > On 10/20/2010 12:08 AM, Eric Niebler wrote: > > On 10/19/2010 1:33 AM, Thomas Heller wrote: > >> On Tue, Oct 19, 2010 at 6:21 AM, Joel de Guzman wrote: > >>> Can we also focus on one very specific use-case that demonstrates > >>> the motivation behind the need of such a refactoring and why the > >>> old(er) design is not sufficient? I'd really want to sync up with > >>> you guys. > >> > >> With the old design (the one which is currently in the gsoc svn > >> sandbox) I had problems with defining what phoenix expressions really > >> are. We had at least two types of expressions. First were the ones we > >> reused from proto (plus, multiplies, function and so on), Second were > >> these proto::function constructs which had a funcwrap<T> struct and > >> an env placeholder. This env placeholder just wastes a valuable slot > >> for potential arguments. The second point why this design is not > >> good, is that data and behaviour is not separated. The T in funcwrap > >> defines how the phoenix expression will get evaluated. > >> > >> This design solves this two problems: Data and behaviour are cleanly > >> separated. Additionally we end up with only one type of expressions: > >> A expression is a structure which has a tag, and a variable list of > >> children. You define what what a valid expression is by extending the > >> phoenix_algorithm template through specialisation for your tag. The > >> Actions parameter is responsible for evaluating the expression. By > >> template parametrisation of this parameter we allow users to easily > >> define their own evaluation schemes without worrying about the > >> validity of the phoenix expression. This is fixed by the meta grammar > >> class. > > > > What Thomas said. We realized that for Phoenix to be extensible at the > > lowest level, we'd need to document its intermediate form: the Proto > > tree. That way folks have the option to use Proto transforms on it. > > (There are higher-level customization points that don't expose Proto, > > but I'm talking about real gear-heads here.) > > > > There were ugly things about the intermediate form we wanted to clean > > up before we document it. That started the discussion. Then the > > discussion turned to, "Can a user just change a semantic actions here > > and there without having to redefine the whole Phoenix grammar in > > Proto, which is totally non-trivial?" I forget offhand what the use > > case was, but it seemed a reasonable thing to want to do in general. > > So as Thomas says, the goal is two-fold: (a) a clean-up of the > > intermediate form ahead of its documentation, and (b) a way to easily > > plug in user-defined semantic actions without changing the grammar. > > > > I think these changes effect the way to define new Phoenix syntactic > > constructs, so it's worth doing a before-and-after comparison of the > > extensibility mechanisms. Thomas, can you send around such a > > comparison? How hard is it to add a new statement, for instance? > > Yes, exactly, that's what I want. Anyway, while I'd still want to see > this, I looked at the code and I like it, except for some nits here > and there (especially naming). More on that later.
Yep, naming was one of the major problems i had ;) I don't have much time now. But here is a side by side comparision of the old and new design: http://www.scribd.com/doc/39713335/Design-ion When looking at it, please keep in mind, that with the new design, we can reuse for_expr in a proto grammar. Additionally, we can reuse it in any extendable Actions class. While in the old design, everything is wrapped behind a proto::function<funcwrap<for_eval>, ....>. Reusing it, for example for generating debug output, is not directly supported by the old design. While in the new design, we have a unified, documented way on how to interact with out phoenix expression AST. Additionally, keep in mind, that in the old design, everything basically was a phoenix expression. The grammar wasn't really definining what can be used and what not. While in the new design, we explicitly say: here is the expression, this is how we want to use it. Sorry, I am in kind of a hurry right now. Hope that helped this far. Thomas _______________________________________________ proto mailing list email@example.com http://lists.boost.org/mailman/listinfo.cgi/proto