Grammar - infinite recursion

Question

While trying to define the official grammar for the oz language, I hit an infinite recursion. I narrowed it down (I think) to these rules:

<declarationPart> ::= <variable> | <pattern> '=' <expression> | <statement>

<pattern> ::= ['!'] <variable>

in pyparser:

pattern             = Forward()
pattern     << (Optional(exclam_tkn) + variable)

declarationPart = ( variable \
            | (pattern + equal + expression) \
            | statement)

So, in declarationPart, variable can appear in variable and pattern. In the order as above, I don't have the recursion problem but the 'equal' is not detected and only the variable gets parsed. When I put 'variable' as second entry, I do have the infinite recursion.

The pattern rule is simplified - I do need it separately.

I can understand the failure mechanism, but I don't know if this is a language definition problem, or if I'm neglecting something in the grammar...

How do I make this work? Or a suggestion to debug the parsing process?

Answer 1

I'm not familiar with pyparsing in particular, but for general parsing purposes I would probably just combine the two rules into one, like so

<declarationPart> ::= ['!'] <variable> [ '=' <expression> ] | <statement>

and then deal with flagging eventual illegal constructs that are accidentally accepted by this grammar in the semantic analysis phase.