Re: [Pyparsing] Recursive grammar

[thomas_h <th...@go...>]

Thanks for the fast answers!

> and your recursion wont recurse properly.  Instead, you must use the '<<'
> operator.  In your example, you just need to replace:

Ah, silly me. I had the '<<' operator in my original code, but forgot
it when I tried to stip it down to a minimal example ... Yes, now it
works :-).

The reason I tried to build a minimal example in the first place was
that I had double recursion in my initial version, something like

aexp << Or([...,
                 aexp + '+' + aexp,
                 ...])

which results in infinite recursion. I felt this might be the right
way to do it, but maybe I'm wrong. Is this generally possible in
pyparsing?


> While this all works, I'm curious why you are not using the operators
> defined for creating compound constructs such as Or.  The form you have
> feels very tedious to me, compared to:
>
> aexp   << ( number ^ number + '+' + aexp )

I tried the operators superficially and got error messages from
Python, but that was probably my fault (e.g. not importing enough). I
will improve on that :-).

> And I would also suggest that you try to use the MatchFirst construct
> instead of Or, especially with recursive expressions:
>
> aexp   << ( number + '+' + aexp | number )

I presume this will gain me some run-time efficiency.

> Note that I had to reorder the terms in order to try the more restrictive
> test first.  But MatchFirst will stop at the first matching alternative,
> while Or will evaluate all alternatives and select the longest.  In
> recursive expressions, Or can descend down a neverending sequence of
> self-referencing alternatives.

I will keep this in mind, although I might stick with Or for the
beginning and use MatchFirst as an optimization later on (my target is
parsing JavaScript).

>
> Welcome to pyparsing!

Thanks :-). Apart from my beginner's problems I've got the impression
that it is really good stuff. Thanks for making the effort!

=Thomas