Practical Ambiguity Detection for Context-Free Grammars

The use of unconstrained context-free grammars for generalized parsing techniques has several advantages over traditional grammar classes, but comes with the danger of undiscovered ambiguities. The ambiguity problem for these grammars is undecidable in the general case, but this does not have to be a problem in practice. Our goal is to find ambiguity detection techniques that have sufficient precision and performance to make them suitable for practical use on realistic grammars. We give a short overview of related work, and propose new directions for improvement

Faster ambiguity detection by grammar filtering

Real programming languages are often defined using ambiguous context-free grammars. Some ambiguity is intentional while other ambiguity is accidental. A good grammar development environment should therefore contain a static ambiguity checker to help the grammar engineer. Ambiguity of context-free grammars is an undecidable property. Nevertheless, various imperfect ambiguity checkers exist. Exhaustive methods are accurate, but suffer from non-termination. Termination is guaranteed by approximative methods, at the expense of accuracy. In this paper we combine an approximative method with an exhaustive method. We present an extension to the Noncanonical Unambiguity Test that identifies production rules that do not contribute to the ambiguity of a grammar and show how this information can be used to significantly reduce the search space of exhaustive methods. Our experimental evaluation on a number of real world grammars shows orders of magnitude gains in efficiency in some cases and negligible losses of efficiency in others

Ambiguity Detection Methods for Context-Free Grammars

The Meta-Environment enables the creation of grammars using the SDF formalism. From these grammars an SGLR parser can be generated. One of the advantages of these parsers is that they can handle the entire class of context-free grammars (CFGs). The grammar developer does not have to squeeze his grammar into a specific subclass of CFGs that is deterministically parsable. Instead, he can now design his grammar to best describe the structure of his language. The downside of allowing the entire class of CFGs is the danger of ambiguities. An ambiguous grammar prevents some sentences from having a unique meaning, depending on the semantics of the used language. It is best to remove all ambiguities from a grammar before it is used. Unfortunately, the detection of ambiguities in a grammar is an undecidable problem. For a recursive grammar the number of possibilities that have to be checked might be infinite. Various ambiguity detection methods (ADMs) exist, but none can always correctly identify the (un)ambiguity of a grammar. They all try to attack the problem from different angles, which results in different characteristics like termination, accuracy and performance. The goal of this project was to find out which method has the best practical usability. In particu