Arguments bug: is input rewind wrong?

[Enrico]

Hello, maybe this is a bug or there's something I don't understand. After some debug I found a simple example that shows the problem:

rule_x = """
    first ::= <letter>:a <letter>:b => [a,b]
    let :n ::= <first>:xs (<letter>:x ?(len(xs)<n+2) !(xs.append(x)))* => xs
    two_or_three ::= (<let 2>:x <end>)|(<let 3>:x <end>)
"""
g = OMeta.makeGrammar(rule_x, globals())
print g('ab').apply('first')
print g('abcc').apply('two_or_three')
print g('abccc').apply('two_or_three')

.....ParseError.

The problem is in "xs.append". If I omit this everything is fine. It seems that it is not possible to append a result to another one.
Inspecting the code with a debugger I can say that:
if the match with (<let 2>:x <end>) fails the input is not rewind properly, as in the last test

[Enrico (ricercar)]

I think the problem is in runtime.py, line 194:

            memoRec = self.input.setMemo(ruleName,
                                         [rule(), self.input])

If the result from rule() is a list or a mutable object, then it can be changed affecting the rest of the process in case a failure in the parser and a following rewind. At line 214 and 215 we have:

        self.input = memoRec[1]
        return memoRec[0]

i.e. we restore the input but we return a result, memoRec[1], that could have been changed.

A dirty solution is to rewrite my previous example as:

rule_x = """
    first ::= <letter>:a <letter>:b => [a,b]
    let :n :xx ::= <first>:xs (<letter>:x ?(xx or len(xx)<n+2) !(xx.append(x)))* => xs+xx
    two_or_three ::= (<let 2 []>:x <end>)|(<let 3 []>:x <end>)
"""

[Enrico (ricercar)]

Sorry, it is:

i.e. we restore the input but we return a result, memoRec[0], that could have been changed.