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The linguistics encyclopedia 42
‘noun’ and ‘determiner’ in order to segment the text and extract the relations between the
concepts represented by the words in the text—exactly what we always regarded as the
task of syntactic analysis. We could weaken their claim to say that by building the
semantic representation by direct analysis of the relations of individual words in the input
text they avoid constructing an unnecessary intermediate set of structures. This, however,
fails to provide any serious contrast with theories like Montague grammar (Dowty et al.,
1981), generalized phrase-structure grammar (GPSG) (Gazdar et al., 1985) and
unification categorical grammar (UCG) (Klein, 1987). These theories contain extremely
complex and specific rules about permissible configurations of structures, of the sort that
the Yale School seems to avow. They also, however, contain very straightforward
mappings between syntactic rules and rules for semantic interpretation, so that any
structure built up using them can equally easily be seen as semantic and syntactic.
Phrase-structure grammar and programs
For most of the 1970s the main example of this third approach was Woods’ (1970, 1973)
augmented transition network (ATN) formalism, which incorporated virtually
unchanged the basic operations of the programming language LISP. Many very
successful systems were developed using this formalism, but it had comparatively little
effect on linguistics as a whole because the choice of the operations from LISP seemed to
have very little explanatory power. ATNs work quite well, but they do not seem to
capture any significant properties of language.
More recent work which uses notions from the logic programming language PROLOG
seems to have had a wider effect. This is presumably because of PROLOG’S status as an
attempt to mechanize the rules of logic, which are themselves attempts to capture the
universal rules of valid inference. These grammars use the PROLOG operation of
unification, a complex pattern-matching operation, to capture phenomena such as
agreement, subcategorization, and long-distance dependency, rather than using the more
standard programming operations of variable assignment and access.
The first such unification grammar was Pereira and Warren’s (1980) definite clause
grammar (DCG). This was simply an attempt to capitalize on the facilities which came
for free with PROLOG, without any very strongly held views on whether language was
really like this or not. Since then, however, variants of unification seem to have taken
over grammatical theory. Generalized phrase-structure grammar (Gazdar et al., 1985),
lexical-functional grammar (Bresnan and Kaplan, 1982), functional-unification grammar
(Kay, 1985), restricted-logic grammar (Stabler, 1987)—the list seems to be growing
daily. Unlike DCG, these later formalisms are generally defended in wider terms than
their suitability for computer implementation, though at the same time they all respect the
need to consider processing issues. This seemed, in the late 1980s, one of the most
significant contributions of AI/NLP to general linguistic theory—a growing consensus on
the general form of syntactic rules, which emerged initially from the AI literature but
later came to be taken seriously within non-computational linguistics.
SYNTACTIC PROCESSING
As well as choosing an appropriate syntactic theory, it was necessary to construct
programs which could apply the theory, either to analyse the structure of input texts or to