This thesis is concerned with the effect of surface stylistic constraints (SSC) on syntactic
and lexical choice within a unified generation architecture. Despite the fact that these
issues have been investigated by researchers in the field, little work has been done with
regard to system architectures that allow surface form constraints to influence earlier
linguistic or even semantic decisions made throughout the NLG process. By SSC we
mean those stylistic requirements that are known beforehand but cannot be tested
until after the utterance or — in some lucky cases — until a proper linearised part
of it has been generated. These include collocational constraints, text size limits, and
poetic aspects such as rhyme and metre to name a few.
This thesis introduces a new NLG architecture that can be sensitive to surface stylistic
requirements. It brings together a well-founded linguistic theory that has been used
in many successful NLG systems (Systemic Functional Linguistics, SFL) and an exist¬
ing AI search mechanism (the Assumption-based Truth Maintenance System, ATMS)
which caches important search information and avoids work duplication.
To this end, the thesis explores the logical relation between the grammar formalism and
the search technique. It designs, based on that logical connection, an algorithm for the
automatic translation of systemic grammar networks to ATMS dependency networks.
The generator then uses the translated networks to generate natural language texts
with a high paraphrasing power as a direct result of its ability to pursue multiple paths
simultaneously. The thesis approaches the crucial notion of choice differently to previ¬
ous systems using SFL. It relaxes the choice process in that choosers are not obliged to
deterministically choose a single alternative allowing SSC to influence the final lexical
and syntactic decisions. The thesis also develops a situation-action framework for the
specification of stylistic requirements independently of the micro-semantic input. The
user or application can state what surface requirements they wish to impose and the
ATMS-based generator then attempts to satisfy these constraints.
Finally, a prototype ATMS-based generation system embodying the ideas presented in
this thesis is implemented and evaluated. We examine the system's stylistic sensitivity
by testing it on three different sets of stylistic requirements, namely: collocational,
size, and poetic constraints.