MAS.632 Conversational Computer Systems - MIT OpenCourseWare

180 VOICE COMMUNICATION WITH COMPUTERS
SYNTAX
Syntactic Structure and Grammars
Syntax is structure in language imposed by the limited number of ways in which
words may be combined in various linguistic roles. Words have different characteristics
in a language. Nouns refer to persons, places, concepts, and sensations,
while verbs describe actions performed by or on nouns. Adjectives modify nouns,
qualifying or quantifying them, and adverbs similarly modify verbs by further
specifying the action.
Syntactic structure groups words into phrases and phrases into sentences and
describes the relationships between components at each level of structure. At a
low level, syntax constrains the gender and number of adjectives and pronouns to
agree with the noun to which they refer. The pronoun in "Jane ate his candy bar"
is ill-formed if the candy bar belongs to Jane. In many languages the gender of
nouns is more significant than in English, and adjectives exhibit different forms
for each. For example, in Spanish "el rey blanco" (the white king) and "la reina
blanca" (the white queen) illustrate the change in form of the adjective ("blanco")
as well as the determiner ("el"/"la") depending on whether they modify a masculine
or a feminine noun.
Conversational systems may utilize knowledge of syntax while trying to understand
a user's request using speech recognition; because syntax limits the possible
combinations of words, perplexity can be minimized. Syntax must also be
incorporated into language generation or else the system's replies may be incoherent
or less intelligible due to grammatical errors. For a conversational system
to make computational use of syntax, two components are required: a grammar
and a parser.' A grammar is a formal specification of language structure; it
defines how words can be combined into syntactic units and how these units can
be used to build sentences. A parser analyzes a sentence to determine its underlying
structure as defined by the grammar. The following sections describe
several grammatical representations and then present some basic parsing techniques.
Syntax provides structure in a language by constraining the manner in which
words can be combined according to a regular set of rules. In this section we consider
representations of syntactic structure and formalisms for expressing the
rules that comprise a grammar. We can represent the layers of syntactic structure
as a tree, starting by representing the complete sentence as the root node; this is
similar to the technique of diagramming sentences, which many of us learned in
elementary school. For example, the most simple sentences contain only a noun
ILanguage structure can also be represented probabilistically, e.g., using Hidden
Markov Models. Although such an approach is adequate for encoding constraints to aid
speech recognition, it is less well suited for representing the grouping of words into syntactic
components.