Re: [Pyparsing] Recursive grammar

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

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.