On Sun, Nov 22, 2009 at 4:50 AM, Diez B. Roggisch <de...@nospam.web.de> wrote: > Steve Howell schrieb: >> >> On Nov 21, 4:07 pm, MRAB <pyt...@mrabarnett.plus.com> wrote: >>> >>> I don't see the point of EvalNode and PrettyPrintNode. Why don't you >>> just give Integer, Sum and Product 'eval' and 'pprint' methods? >> >> That's a good question, and it's the crux of my design dilemma. If >> ALL I ever wanted to to with Integer/Sum/Product was to eval() and >> pprint(), then I would just add those methods to Integer, Sum, and >> Product, and be done with it, as you suggest. But what happens when >> somebody wants to extend capability? Should every future software >> developer that wants to use Integer/Sum/Product extend those classes >> to get work done? What if they want to store additional state for >> nodes? >> > > What's usually done is to create visitors/matchers. Those traverse the AST, > and either only visit, or return transformed versions of it (think e.g. > algebraic optimization) > > A visitor will roughly look like this: > > > class ASTVisitor(object): > > > > def visit(self, node): > name = node.__class__.__name__.lower() > if hasattr(self, "visit_%s" % name): > getattr(self, "visit_%s" % name)(node) > for child in node: > self.visit(child) > > > > You can of course chose another type of dispatch, using e.g. a generic > method. > > Then you create Visitors for specific tasks - pretty-printing, evaluation, > rewriting. Those don't have the overhead of your current design with all > those factory-mapping stuff, and yet you aren't forced to put logic into AST > you don't want there. >
FWIW I often use John Aycock's SPARK (Scanning, Parsing, and Rewriting Kit) for this sort of thing. It has a GenericASTTraversal which "is a Visitor pattern according to Design Patterns." http://pages.cpsc.ucalgary.ca/~aycock/spark/ It's apparently distributed with the python source, but it's not in the standard library, more's the pity IMO. There's a bit of a learning curve but it's well worth it. ~Simon -- http://mail.python.org/mailman/listinfo/python-list
