Formally, HPSG is organized as a set of grammar principles, rules, and lexical descriptions [Cooper1996]. Each constituent is described by a typed feature structure, as in the examples presented in Chapter 2. HPSG is a refinement of the GPSG formalism, and from a linguistic perspective it shares similar principles [Meurers2002]: lexical entries, lexical rules, Immediate Dominance (ID) rules (specifying a dominance relation between categories, without imposing an order of categories), Linear Precedence (LP) rules (defining and order relation between some daughters in a rule; these, together with ID rules, are the grammar rules), and grammatical principles. To an extent, a HPSG can be viewed as a feature structure that must satisfy a description of the form:
are grammatical
principles,
are grammar rules, and
are
lexical entries.
One of the major differences between HPSG and formalisms such as CFG is the underlying mathematical structure [Pollard1997]: the CFG formalism is based on sequences of phrase-markers (trees where nodes are labeled with categories), while HPSG is based on feature structures (which represent linguistic entities in graphs where arcs are labeled with feature names and nodes with types of linguistic objects). The relation between entities in HPSG is formulated as well-formedness constraints on typed feature structures. Another major difference between HPSG and CFG-like formalisms is that HPSGs are considered to be nonderivational, meaning there are no transformations or destructive operations to derive one structure from another[Pollard1997].
Figure 3.1 presents a simple example [Matheson1997] of a HPSG. The type hierarchy contains three basic types (sign, cat, and head), where head has two subtypes (noun and verb). Appropriateness is specified through declarations like CAT:cat together with the type sign. It is said that sign introduces the feature CAT, meaning sign is the least type for which the feature CAT is appropriate. This declaration also specifies that possible values for CAT have to be of type cat3.1 or one of its subtypes.
Lexical entries are specified using the descriptions presented in
Section 2.3.3. For example, the lexical head of
``snored'' is verb, and the subject of ``snored'' is a
noun. Although the description for ``Mary'' might suggest
that it only specifies a noun, total well-typedness
guarantees the presence of all appropriate features:
![$ mary \longrightarrow
\left[\begin{array}{l}
\textrm{\textbf{sign}}\\
\textrm{...
...bf{noun}}\\
\textrm{SUBJ: \textbf{head}}
\end{array}\right]
\end{array}\right]$](img189.png)
.
This simple grammar also contains a rule implementing the subject-head schema. Similar to the lexical entry for ``Mary'', the descriptions in rules are specified only at features having values different than ones inferable from appropriateness. The rule states that the mother has a verb as its head (meaning the head of the phrase is a verb), while the first daughter (the subject) has the same value for its feature head as the second daughter (the head) has for its subj feature. This is achieved through the use of the variable HeadVar, which creates structure sharing between the most general satisfiers of the two daughters.
HPSG is a complex formalism, under constant improvement through various extensions. It is outside the scope of this work to present an in-depth overview of HPSG. More formal details can be found in [Pollard and Sag1987,Pollard and Sag1994] or [Cooper1996]. A concise introduction can be found in [Pollard1997]. Information about current research trends and HPSG developments is maintained on several HPSG-related web servers: http://www.sfs.uni-tuebingen.de/hpsg/, http://hpsg.stanford.edu/, http://www.ling.ohio-state.edu/research/hpsg/, etc.
![\includegraphics[width=8cm]{example_hpsg.eps}](img184.png)
