3 ms·
Pointing out ambiguities in a context-free grammar is an undecidable problem (http://www.cis.upenn.edu/~jean/gbooks/PCPh04.pdf http://www.cis.upenn.edu/~jean/gb
by panic 9y ago
Pointing out ambiguities in a context-free grammar is an undecidable problem (http://www.cis.upenn.edu/~jean/gbooks/PCPh04.pdf http://www.cis.upenn.edu/~jean/gbooks/PCPh04.pdf).
- zzzcpan 9y agoNo, that's not what your link proves and not what we are talking about.
- panic 9y agoTheorem 6.8.2 in the PDF: It is undecidable whether a context-free grammar is ambiguous. PEGs are unambiguous by construction, so I assumed we were talking about grammars which would be ambiguous as CFGs (replacing every ordered choice with a nondeterministic choice). What kind of ambiguity were you thinking about?
- zzzcpan 9y agoAny ambiguity that manifests itself as unreachable alternatives in PEGs can be detected for example. Doesn't matter if it's undecidable whether the whole grammar is ambiguous.