Re: [Pyparsing] Operator Precedence and Associativity.

[Paul M. <pa...@al...>]

> -----Original Message-----
> From: Ralph Corderoy [mailto:ra...@in...] 
> Sent: Wednesday, October 11, 2006 7:08 AM
> To: Paul McGuire
> Cc: pyp...@li...
> Subject: Re: [Pyparsing] Operator Precedence and Associativity. 
> 
> 
> Hi Paul,
> 
> > Well, why not?!  Check this out!
> > 
> > def operatorGrammar( baseExpr, opList ):
> >     ret = Forward()
> >     lastExpr = baseExpr | ( Suppress('(') + ret + 
> Suppress(')') ) ...
> 
> That's impressive and exactly what I was after.  You may want 
> to consider formalising it as part of pyparsing since 
> recursive descent parsers often use operator precedence 
> parsers for that part of things.
> 
> Thanks!
> 
> 
> Ralph.
> 

Ralph -

Note that this does not *quite* do the LR-style recursive descent you were
looking for.  That is to say, 
  9 + 2 + 3
parses as:
  [[9, '+', 2, '+', 3]]
and not as:
  [[[9, '+', 2], '+', 3]]
which is what I think you were looking for originally.

On the whole, though, it seems quite interesting.  It took me about 2
minutes to do the following boolean expression grammar:

boolExpr = operatorGrammar( Word(alphas),
    [
    ("not",1,"R"),
    ("or",2,"L"),
    ("and",2,"L"),
    ])
test = ["p and not q",
        "not not p",
        "not(p and q)",
        "q or not p and r",
        "q or not (p and r)"
        ]
for t in test:
    print t,'\n',boolExpr.parseString(t),'\n'

Giving:

p and not q 
[['p', 'and', ['not', 'q']]] 

not not p 
[['not', ['not', 'p']]] 

not(p and q) 
[['not', ['p', 'and', 'q']]] 

q or not p and r 
[[['q', 'or', ['not', 'p']], 'and', 'r']] 

q or not (p and r) 
[['q', 'or', ['not', ['p', 'and', 'r']]]] 


I need to give this a little more thought as far as including it into "core"
pyparsing (as opposed to say, just making it another example - the examples
directory is sort of becoming its own Pyparsing Cookbook).  I want to make
sure I get the API right.  For instance, I might want to support a 4'th
entry in each input tuple, representing a parse action to be attached to the
internally generated pyparsing expression.

-- Paul