On Nov 22, 7:55 am, Simon Forman <sajmik...@gmail.com> wrote: > 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. >
Thanks, Simon, I think something like GenericASTTraversal should work for me in most cases. A couple of folks have suggested an idea that is in his paper, which is to use a single class for something PrettyPrinter, and then use reflection to find methods on PrettyPrinter to pretty-print sums, products, integers, etc. -- http://mail.python.org/mailman/listinfo/python-list
