Re: [Pyparsing] operatorPrecedence: Problems with Power and Unary Minus

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

I worked on this problem a bit last night.  The opAssoc.RIGHT parameter is
there to address this problem, and in fact, this expression:

a**b**c

Does get correctly evaluated as (a**(b**c)).  But things get muddled when
unary signs are added, and I think this points up a bug in
operatorPrecedence.  As your pastebin code comments state:

#Power and unary operations are intertwined to get correct operator
precedence:
#   -a**-b == -(a ** (-b))

Which should be supported using this form of operatorPrecedence:

u_expr = Word(nums) | Word(alphas)
expression = operatorPrecedence( u_expr,
    [('**',           2, opAssoc.RIGHT),
     (oneOf('+ -'),   1, opAssoc.RIGHT),
     (oneOf('* /'),   2, opAssoc.LEFT),
     (oneOf('+ -'),   2, opAssoc.LEFT),
    ])

For the moment, this expression correctly parses "a**b**c" as "(a**(b**c))",
but can only parse "-a**-b" if it is given as "-a**(-b)".

Ideally, your request should be handled by the operatorPrecedence API as it
exists today - I don't think any more specialized interface should be
needed.  But I'll see if I make any progress with finding this problem in
the next day or so.

-- Paul