Head-Driven Phrase Structure Grammar (HPSG) is a formally well-defined approach to grammatical theory that seeks to provide a cognitively plausible model of human languages as systems of declarative constraints. Typed feature structures play a central role in this modeling. The HPSG community views explicit formalization as a pre-requisite for theory evaluation, and values efficient large-scale grammar implementations as well as psycholinguistic models of language processing. Computational HPSG implementations are useful not only in research (grammar comparison and hypothesis testing), but also in applied domains like machine translation, question answering, language tutoring, etc.

A comprehensive and detailed exposition of the state-of-the-art in HPSG theorizing is provided by the Head-Driven Phrase Structure Grammar: The Handbook volume, published (open-access) by Language Science Press. The proceedings of the annual International conference on HPSG are available here and for a bibliography of HPSG research go to this repository.

Some of the leading ideas of current work in this theory are the following.

  • Concrete, surface-oriented structures
      'Abstract' structures (e.g. empty categories and abstract functional projections like IP, NegP, SigmaP, etc.) are avoided wherever possible, in favor of minimal grammatical structures. Similarly, non-monotomic operations like movement are rejected, and instead modeled like all other phenomena, via features. Different languages are analyzed first and foremost on their own terms, and if any cross-linguist commonalities do exist, they emerge as empirically motivated linguistic generalizations. HPSG is not predicated on (neurogenetically implausible) linguistic innateness assumptions, and the construction-based nature of the theory is consistent with a wealth of empirical results in language acquisition, grammar induction, and probabilistic models of language. For an overview of the history of phrase structure grammar leading up to HPSG see Blevins and Sag (2013).
  • Geometric prediction
      Linguistic objects are modeled as feature structures organized via a system of types and constraint inheritance, drawing key insights from research in object-oriented paradigms. The organization of typed feature structures (i.e. the geometry of linguistic descriptions) plays a significant role in predicting the impossibility of certain kinds of linguistic phenomena. Although the formalism is very expressive (basically, a kind of first-order logic with sorted types), the feature geometry of the theory severely restricts - in a linguistically motivated fashion - what the grammar can express. This principled bottleneck not only makes linguistic predictions, but it also enables efficient computational implementations of linguistic grammars. For example, as Kay and Sag (2014) show, their account of idioms correctly predicts that some idiomatic dependencies can cross clausal boundaries, but only in a very limited way. A few other examples of geometric prediction are discussed below.
  • Declarativity
      In HPSG, the grammar is nothing but a static set of declarative constraints (stated as a certain sub-set of first-order logic statements), not an algorithm. The notion of derivation does not exist as part of the knowledge of language. Rather, derivations arise from the interaction between the language processing modules, the input, and the grammar constraints. In other words, it is up to the performance modules (i.e. comprehension and production) to consult the competence grammar module in order to determine which structures are licit. Consequently, the grammar is compatible with comprehension and production (see Sag and Wasow (2011, 2015)), and is by definition ‘crash-proof'. Consequently, there is no need for post-derivational filters or computationally demanding evaluation metrics that compare many alternative derivations in order to guarantee that only grammatical (convergent) outputs can be derived and interpreted at interface levels.
  • Locality
      All linguistic constraints are stated in terms of local mother-daughter configurations. For example, category selection, role assignment, case assignment, head agreement and semantic selection all obey a particular kind of locality determined by valence selection features. This is a kind of geometric prediction. For example, unbounded/long-distance dependencies are modeled not via movement, but rather in terms of certain feature specifications that are present throughout the 'path' from filler to gap. This feature-based theory correctly predicts the existence of grammatical phenomena sensitive to such specifications, as amply documented in languages such as Irish, Chamorro, Icelandic, Kikuyu and Thompson Salish (among many others). This is another instance of geometric prediction. For more on long-distance dependencies in HPSG see Hukari and Levine (2006), and for more on locality in HPSG see Sag (2014).
  • A distinction among types of agreement
      Agreement phenomena have been classified by Pollard and Sag (1994) as syntactic concord, anaphoric agreement, or pragmatic agreement. Their theory of indices predicts, inter alia, the absence of case agreement in anaphoric agreement. Work by Wechsler and Zlatic (2003) and various others has refined these proposals, extending the range of languages considerably and correcting mispredictions.
  • Obliqueness-based binding theory
      Generalizations about constraints on the binding of referentially dependent elements are stated in terms of relative obliqueness (o-command), at the lexical level, rather than configurational superiority (c-command). See for example Manning and Sag (1998).
  • Lexical cross-classification
      Within HPSG, words are rich in information. Lexical information is not simply listed, however; rather it is organized in terms of multiple inheritance hierarchies and lexical rules that allow complex properties of words to be derived from the logic of the lexicon. Current research is developing extensions of hierarchical lexicons that allow most kinds of lexical rule to be eliminated and linking patterns to be derived in a general fashion from semantic properties. See for example Koenig (1999) and Koenig and Davis (2003). Word structure and phrase structure are governed by partly independent principles. Words and phrases are two kinds (subtypes) of sign. For more recent work on morphology in HPSG see Crysmann and Bonami (2015) and Emerson and Copestake (2015).
  • Hierarchical cross-classification of grammatical patterns
      HPSG can model constructions, as well as signs, in terms of feature structures. This allows constructions to be analyzed via multiple inheritance hierarchies, which in turn provides a way of modeling the fact that constructions cluster into groups with a 'family resemblance' that corresponds to a constraint on a common supertype. This view is similar to the Berkeley-style conception of 'Construction Grammar' developed by Charles Fillmore, Paul Kay, and colleagues. More recently, a variant of HPSG called Sign-Based Construction Grammar (SBCG) has emerged from collaborative work between Paul Kay, Ivan Sag, and Laura Michaelis, which gives more prominence to constructions and is less reliant on the notion of syntactic 'head'. The latter allows for an elegant account of non-coordinate exocentric constructions such as free relatives, certain partitives, and comparative correlatives, among others (cf. with Sag (1999), Wright and Kathol (2003), Borsley (2011)).
  • Linearization theory
      HPSG has explored modes of serialization which are not based on the model of traditional phrase structure grammar (where sentences are word strings defined derivatively in terms of phrase structure). HPSG's linearization theory has sought to capture word order phenomena in various languages which, albeit correlated with notions of hierarchically defined constituency, are ultimately not derivable from the latter. This has implications for the treatment of discontinuous constituency, as the linear syntactic organization is to some extent dissociated from the combinatorial relationships among the items serialized. See for example Kathol (2004) and Wetta (2014), for construction-based accounts of word order phenomena in Germanic languages.