Re: [Pyparsing] Recursive grammar

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

Great, thanks for the help!

=Thomas

>
> Hi Thomas,
>
> > 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?
>
> I doubt it is.  If you look at a grammar for a language you tend to find
> arithmetical expressions defined:
>
>     expr := term (addop term)*
>     term := factor (mulop factor)*
>     factor := floatnum | '(' expr ')'
>     addop := '+' | '-'
>     mulop := '*' | '/'
>
> This makes the grammar, and the parse tree, represent the precedence of
> operators.  Note the parenthesis are handled with the recursive
> definition where `expr' was a Forward() initially.  Using ZeroOrMore()
> it's quite easy to turn the above into Python.  Maybe fourFn.py will
> make a little more sense now
>
> > > 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.
>
> Yes.  Or() tries all of them and then returns the longest.  MatchFirst()
> can give up trying after the first one that matches.
>
> Cheers,
>
>
> Ralph.
>
>