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

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

Eike -

Well, this came together a bit faster than I'd thought.  The solution begins
with an understanding of operatorPrecedence.  Here is the expression that is
published in the online example simpleArith.py:

integer = Word(nums).setParseAction(lambda t:int(t[0]))
variable = Word(alphas,exact=1)
operand = integer | variable

expop = Literal('**')
signop = oneOf('+ -')
multop = oneOf('* /')
plusop = oneOf('+ -')
factop = Literal('!')

expr = operatorPrecedence( operand,
    [(factop, 1, opAssoc.LEFT),
     (expop,  2, opAssoc.RIGHT),
     (signop, 1, opAssoc.RIGHT),
     (multop, 2, opAssoc.LEFT),
     (plusop, 2, opAssoc.LEFT),]
    )

As I mentioned earlier, this will only recognize "-a**-b" if given as
"-a**(-b)".  The reason for this is that operator precedence cannot look
"down" the precedence chain unless the next term is enclosed in parentheses.
My first attempt at fixing this was to change the definition of operand to
include an optional leading sign, and this parsed successfully.  But then it
dawned on me that this same thing can be accomplished by inserting another
signop operator *above* expop, as in:

expr = operatorPrecedence( operand,
    [(factop, 1, opAssoc.LEFT),
     (signop, 1, opAssoc.RIGHT),
     (expop,  2, opAssoc.RIGHT),
     (signop, 1, opAssoc.RIGHT),
     (multop, 2, opAssoc.LEFT),
     (plusop, 2, opAssoc.LEFT),]
    )

I think this is the correct solution, rather than mucking about with
operatorPrecedence, or requiring strange embellishments to one's operand
expression.  I don't think this is a general issue with unary operators or
right-associated operations, I think this is just a special case borne out
of the definition of exponentiation with respect to leading sign operators.

I'll fix simpleArith.py to correctly include the extra unary sign operator
precedence level, and also include some examples.  I'll also include
simpleArith2.py, which actually evaluates the parsed expression.

-- Paul