[Pyparsing] Visitor pattern in pyparsing

[Paul M. <pt...@au...>]

As I've read more about Visitors, I think this pattern may actually be more
applicable to the results that might be returned from the parsing process.
In the case of a parser that returns some structured tokens, a Visitor might
be created that walks this list, pretty-printing the output, or evaluating
some generated expression execution objects.

Unfortunately, pyparsing has little control over what kinds of objects will
be found.  The overall object returned is always a ParseResults object, but
the items within the ParseResults' list are likely to be just Python
strings.  They could also be ints, floats, or even user-defined objects, if
the user created parse actions that did some kind of type conversion.  So to
visit the results, each matched item or group would have to be wrapped in
some kind of "visitable" object, most likely created in a parse action.

I have implemented something *like* this, for example in the SimpleBool.py
example - here every matching expression returns an instance of a subclass
of BoolOperand, which implements its own version of __nonzero__.  After
parsing, the parseResults are evaluated by calling bool(item) for each item
in the results - bool(x) calls x.__nonzero__ in Python2.x.

So for visiting parsed results, I may have to just add another example to
the examples directory, showing how to use the Visitor pattern on the
matched tokens.

-- Paul