Polynomially parsable unification grammars

Unification grammars (UGs) are a grammatical formalism that underlies several contemporary linguistic theories, including lexical-functional grammar and head-driven phrase-structure grammar. UG is an especially attractive formalism because of its expressivity, which facilitates the expression of complex linguistic structures and relations. Formally, UG is Turing-complete, generating the entire class of recursively enumerable languages. This expressivity, however, comes at a price: the universal recognition problem is undecidable for arbitrary unification grammars. We define a constrained version of UG that is equivalent to range concatenation grammar, a formalism that generates exactly the class of languages recognizable in deterministic polynomial time. We thus obtain a constrained unification grammar formalism that guarantees efficient processing.