HPSG: a theory of signs

The Head- Driven Phrase Structure Grammar (HPSG) is a grammar that attaches importance on information encoded in lexical heads. It is based on developments arising from research in Generalized Phrase Structure Grammar (GPSG) (Gazdar et al 1985) which sought to provide a non transformational syntactic framework (as

opposed to the Principle and Parameters framework) by employing meta-rules which applied to lexically headed phrase structure rules and which constrained context free grammars. Parallel developments in LFG by Bresnan (1976 and 1982) provided a lexical based explanation to phenomena such as passivization.

Three phases in the development of the theory are described in the literature. Phase one refers to Pollard and Sag (1987), Phase two refers to chapters 1 to 8 of Pollard and Sag (1994) and Phase three refers to chapter 9 of Pollard and Sag (1994), Sag and Wasow(1999), Ginzburg and Sag(2001) as well as researches to date.

Not all details of the discussion in the following are relevant for my discussion in the later chapters. The discussion is nevertheless relevant since a comprehensive account

of the matrix framework is not found else where in the literature except in Hellan (2003).

The fundamental concept of HPSG is that of the sign. It is a grammar theory whereby emphasis is based on representation of the lexicon as a system of structured linguistic objects represented as types and constraints on the types. Lexical entries⁹ correspond to lexical types that are related to each other in type hierarchies.

Following Ferdinand de Saussure’s idea of a linguistic sign, a sign in HPSG is a collection of different kinds of properties that include phonological, syntactic, semantic and contextual constraints that are represented as a typed attribute-value matrix (AVM). Example (21) below illustrates this:

(21)

AVMs are descriptions of feature structures. A feature structure is a way of

representing grammatical information. It is a specification of a set of features called attributes (written in capital letters) each of which is paired with a particular value (written in italics). The value must belong to a type. Types are classes of linguistic entities (words, phrases, categories, sounds, meaning, theoretical entities such as grammar rules etc) that form the grammar of a language. Entities are assigned to classes due to certain properties they share. Feature structures allow generalizations to be captured.

The linguist uses feature descriptions containing a certain part of the information that is present in the feature structure that models the linguistic object. A feature structure description can be partial (satisfied by many distinct feature structures) or total (satisfied by one).

In (21) above, the type sign has a specification of the following five attributes that constrains it: STEM, SYNSEM, ARGS, INFLECTED and ROOT.

The attribute STEM has a list as value. The constraint on the object contained in this list must include a list of phonemes.

The attribute SYNSEM has a type synsem as value. The attribute LOCAL constrains synsem and has the type local as value. Local information encompasses syntactic cat (category) and semantic cont (content) information. Cat contains category and

valence information while mrs contains information on instances of linguistic objects.

This will be discussed further in 1.4.3.3 below. The feature NON-LOCAL has a value non-local. Non-local information constrains relationships between an entity realized non-canonically and the lexical head that subcategorizes for it, as in unbounded dependencies.

ARGS has a list as value. Lists have avm as supertype with immediate subtypes: cons (non-empty), null (empty) and olist (optional). ARGS specify the daughters of a type.

INFLECTED allows for information on inflectional patterns of lexemes, words and phrases to be captured and has the type boolean as value. Boolean has two subtypes + and -. This allows for the distinction between lexemes and words to be captured.

Lexemes are neutral to inflectional variants for which they are defined and words realize inflectional variants.

Lastly, the feature ROOT captures what a grammar licenses as a “stand alone”

utterance or a start symbol. This is captured by the constraint [IC boolean] in

Ginzburg and Sag (2001), IC meaning independent clause. Bender (2002) represents words as [ROOT-] and phrases as [ROOT boolean]. Thus in this grammar words cannot function as start symbol.

In addition to being a sign based grammar, HPSG is constraint based. A constraint-based grammar consists of feature declarations that are also called appropriateness conditions on types. It declares which attributes and attribute values are appropriate for which type of objects. The constraint on a type must be consistent and compatible with inherited information from a parent type. The type sign in example (21) above is a parent for the type word-or-lexrule which has subtypes which includes lex-item and word types and phrase-or-lexrule which has subtypes which include phrase. This

means that these types must satisfy the constraint for the parent type sign. Examples (22) and (23) illustrate this:

(22) _ARG-ST word or lexrule list

− −

⎡ ⎤

⎢ ⎥

⎣ ⎦

Word-or-lexrule inherits all constraints of the parent type sign with the additional constraint that it must have an ARG-ST (Argument Structure). ARG-ST consists of all subcategorized constituents a lexical head combines with. It is a feature only found on lexical heads and the ordering of elements in its value imposes a ranking on the phrases in the phrase structures corresponding to these values.

The type lex-item is a parent for the type lexeme. Lexeme inherits ARG-ST

information from word-or-lexrule with the additional constraint that it has a – value for the feature INFLECTED. Lexemes are abstract proto-words that give rise to words:

(23) ^⎡⎢_INFLECTED-^{lexeme ⎤⎥}

⎣ ⎦

The type lex-rule and sub-types that include lexeme-to-word-rule introduce

inflectional variants. Inflection is not declared on the type word but is declared on the type lexeme-to-word-rule that induces inflection on a lexeme, deriving a word. Words realize inflectional variants. This is discussed in chapter 2 below.

The type word is also a subtype of word-or-lexrule and also inherits the constraint that lexical items have an ARG-ST. In addition, words cannot function as start symbols.

This is captured by the following constraint:

(24) ROOT

-⎡word ⎤

⎢ ⎥

⎣ ⎦

In addition to inheriting constraints on the super types sign and word-or-lexrule, the value for the feature ROOT is declared on word as having the value -. This

differentiates the type word from the type phrase.

The type phrase inherits information from a super type phrase-or-lexrule, which has the type sign as parent. Phrase-or-lexrule contains constraint on semantic information of a rule in a construction and has the constraints in (25) below:

(25)

Phrase is different from the type word-or-lexrule in that it has an empty ARG-STR list. This captures the generalization that ARG-STR is a feature relevant only for lexical heads.

(26) SYNSEM.LOCAL.ARG-STR ROOT

The constraint on a type must be consistent and monotonic.¹⁰ As seen in (21) through (26) above, this means that constraints on super types affect all instances of subtypes without exceptions.

Every type must be defined or declared. This involves specification of types position in a hierarchy, as well as, specification of what attributes are appropriate to it and specification of possible values for each attribute. The types described so far are declared in the hierarchy in (30) below. This is discussed immediately below in section 1.4.3.

10 A non-monotonic system would allow for default inheritance.