Re: [Pyparsing] Recursive grammar

[Ralph C. <ra...@in...>]

Hi Thomas,

> from pyparsing import *
> number = Word(nums)
> aexp   = Forward()
> aexp   = Or ([
>           number,
>           number + '+' + aexp ,
>           ])
> 
> Running the code yields:
> 
> aexp.parseString("3+4") => (['3'], {})

Examine aexp shows

    >>> aexp
    {W:(0123...) ^ {{W:(0123...) "+"} Forward:None}}

which indicates to me that the Forward hasn't been set to anything.
That's because Forwards have to be assigned with `<<' rather than `='.
You're forgetting about the Forward created with the initial assignment
to aexp by altering aexp to be an Or.

I can't find the documentation online, but have a look for something
like /usr/share/doc/python2.4-pyparsing/htmldoc/index.html on your
machine and see Forward:

    Forward declaration of an expression to be defined later - used for
    recursive grammars, such as algebraic infix notation. When the
    expression is known, it is assigned to the Forward variable using
    the '<<' operator.

When I try:

    >>> aexp   = Forward()
    >>> aexp << Or ([
    ...           number,
    ...           number + '+' + aexp ,
    ...           ])
    Forward:{W:(0123...) ^ {{W:(0123...) "+"} ...}}

Which looks a bit better.

    >>> aexp.parseString("3+4")
    (['3', '+', '4'], {})

Cheers,


Ralph.