Re: [Pyparsing] Operator Precedence and Associativity.

[Paolo L. <p....@hy...>]

Hi Ralph,

Ralph Corderoy wrote:

> whereas I'd like the left associativity explicit, i.e.
> 
>                          +
>                         / \
>     1 + 2 + 3    ==>   +   3
>                       / \
>                      1   2
> 
> I can achieve this with a right-associative operator, e.g. exponent, but
> how do I do this for left-associative ones whilst avoiding endless
> left-recursion?

I think you have no other choice but to rearrange ParseResults by
setting a specific parse action for the expression:

from pyparsing import *

plus = Literal('+')
num = Word(nums)

def left_associative_regroup(s,l,t):
     new_t = ParseResults( [ t[0], t[2] ] )
     for item in t[3:]:
         if item != "+":
             new_t = ParseResults( [new_t, item] )
     return new_t

expr = num + ZeroOrMore( plus + num )
expr.setParseAction(left_associative_regroup)

print expr.parseString("1 + 2 + 3 + 4 + 5")

> I wonder if PyParsing would benefit from being able to define operator
> tables, similar to K&R's, giving the tokens, precedence, and
> associativity, and being able to plumb that into the rest of the
> recursive descent parser at some point?

It's not trivial at all.

 From http://en.wikipedia.org/wiki/Left_recursion

"A formal grammar that contains left recursion cannot be parsed by a 
recursive descent parser"

PyParsing qualifies as recursive descent parser.

I think it would rather be possible to build Specilized Constructs
such as

LeftAssociativeOperator(term=Word(nums), operator=Literal('+') )

but those would only cope with immediate left recursion...

Hope this helps

	Ciao
	Paolo