HPSG

Head-Driven Phrase Structure Grammar (HPSG) is a constraint-based approach to syntax and semantics developed by Carl Pollard and Ivan Sag in the late 20th century. It is a highly lexicalist framework that represents linguistic objects as richly structured, typed feature structures and relies on unification rather than transformation rules to combine elements.

Core formalism and ideas: In HPSG, linguistic knowledge is encoded in attribute-value matrices (AVMs). Each sign has features organized into a type hierarchy, with a prominent SYNSEM node that mirrors both syntactic and semantic information. The LOCAL feature captures immediate syntactic properties such as head, subcategorization, and valence, while the NON-LOCAL component handles long-distance dependencies (often via a SLASH feature). The lexicon contains detailed specifications for words and morphemes, including subcategorization frames, agreement, and in some analyses aspects of meaning. Grammar rules are constraints that describe how signs can combine, rather than transformational rules transforming structures.

Key characteristics: HPSG emphasizes a declarative, highly modular framework where most grammatical knowledge resides in the lexicon and in universal constraints. Movement is not part of the core mechanism; dependencies are encoded through feature structures and unification. The result is a theory that aims to describe cross-linguistic variation with a shared set of principles and a rich, language-specific lexicon.

Impact and use: HPSG has influenced theoretical syntax, typology, and computational linguistics. It has been implemented in various parsing systems and grammars for multiple languages, often in parallel with other constraint-based approaches such as GPSG and LFG.