Is pyparsing really a recursive descent parser?

[Neil Cerutti]

On 2007-11-07, Just Another Victim of the Ambient Morality
<ihatespam at hotmail.com> wrote:
> "Neil Cerutti" <horpner at yahoo.com> wrote in message 
> news:slrnfj13e4.1hc.horpner at FIAD06.norwich.edu...
>> You might be interested in the Early parsing algorithm. It is
>> more efficient than the naive approach used in your prototype,
>> and still handles ambiguous grammars.
>

I'll take this opportunity to correct my misspelling. It's
"Earley".

> I think I might be interested in this algorithm, thank you!
>
>> http://www.cpsc.ucalgary.ca/~aycock/spark/
>
>     You know, I tried this thing but, for the life of me, I
> can't figure out how to use it and the few tutorials out there
> are less than illuminating...

I'll send you the code I composed. 

The tricky part of Spark is the Token and AST classes you have to
use. A type used as a token class is required to provide a
__cmp__ function that behaves in the following confounding
manner:

  class Token(object):
    def __cmp__(self, o):
      return cmp(self.type, o)

If you attempt to use a list, string or a tuple as token, it just
barfs. AST's are required to provide an even weirder interface.

In effect, you have to write badly designed wrappers around
tuples and lists, respectively to take advantage of the generic
classes.

Go to the examples directory of the distribution to find working
versions of these stupid classes.

Once you get over that hurdle it becomes easier. Be sure to
provide your Parser and Scanner classes with an error method to
prevent the library from raising SystemExit(!) on errors. Scanner
classes are also required to override the t_default method to
prevent this mishap.

In short, it hasn't really evovled into a user-friendly package
yet.

-- 
Neil Cerutti