Unification grammars are widely accepted as an expressive means for describing the structure of natural languages. In general, the recognition problem is undecidable for unification grammars. Even with restricted variants of the formalism, off-line parsable grammars, the problem is computationally hard. We present two natural constraints on unification grammars which limit their expressivity and allow for efficient processing. We first show that non-reentrant unification grammars generate exactly the class of context-free languages. We then relax the constraint and show that one-reentrant unification grammars generate exactly the class of mildly context-sensitive languages. We thus relate the commonly used and linguistically motivated formalism of unification grammars to more restricted, computationally tractable classes of languages.
Bibliographical noteFunding Information:
Acknowledgements This research was supported by The Israel Science Foundation (grant no. 136/01). We are grateful to Yael Cohen-Sygal, Nissim Francez and James Rogers for their comments and help. We greatly benefited from comments made by an anonymous JoLLI reviewer. This paper is an extended and revised version of Feinstein and Wintner (2006).
- Generative capacity
- Linear indexed grammars
- Mildly context- sensitive languages
- Unification grammars
ASJC Scopus subject areas
- Computer Science (miscellaneous)
- Linguistics and Language