Ply(LALR) and Yacc behaving differently

[Carsten Haese]

On Thu, 2005-04-07 at 14:51, Åsmund Grammeltvedt wrote:
> Hi.
> 
> I am trying to implement a small compiler in python and, trying to use 
> something a bit more pythonic than lex/yacc, ended up with ply 
> (http://systems.cs.uchicago.edu/ply/). The only problem is that whereas 
> yacc accepts the grammar and appears to parse it correctly, ply does not.
> 
> Perhaps this belongs on some compiler list, but I couldn't decide if it 
> was a compiler or a python problem, so bear with me.

Maybe this is a PLY bug? LALR(1) support appears to be a relatively
recent addition to PLY. Have you tried contacting PLY's author?

For what it's worth, it appears that you can make your example grammar
work in LALR-mode PLY by eliminating the empty production and making the
Block rule right recursive like this:

def p_Goal(p):
  """
  Goal : Block
  """

def p_Block(p):
  """
  Block : SEMI
        | T Block
        | S Block
  """

Of course, I don't know whether this rewrite is applicable to your
larger grammar.

Hope this helps,

-- 
  Carsten Haese - Software Engineer   |    Phone:  (419) 861-3331
        Unique Systems, Inc.          |      FAX:  (419) 861-3340
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