Eckhard Bick - VISL

3.8 Mapping: From word class to syntax
Rules at the mapping level of a Constraint Grammar exploit (more or less)
disambiguated morphological/PoS information for assigning a context dependent
syntactic function potential to each target word in the text. Rules can address words
individually, but are usually bundled for certain types of word class targets.
Contextually safe mapping rules are able to map a more precise syntactic tag list
(ideally, one tag only) than more broad rules. Therefore, unlike disambiguation rules,
which remove information rather than add it, mapping rules are inherently sequential
and mutually exclusive: Safe, specific, context rich rules have to be applied before more
general, poor context rules, and once targeted, a word has to be “closed” for further
mapping. Otherwise, every word will receive the full combined syntactic tag potential
of all mapping rules targeting it, which would “erase” the visibility of any individual,
more specific rule. In the rule compiler formalism used here, ordinary sequential
mapping rules are marked by the MAP operator, and they are applied in the order given
in the rules file of the grammar. Rules with the alternative ADD operator are cumulative
and, in principle, non-sequential. Basically, ADD rules provide a way of splitting a
complex MAP rule into smaller, more manageable parts.
There is no clear border line between mapping rules and (syntactic)
disambiguation rules. In theory, all mapping rules could be crafted with a perfect and
complete list of context conditions such that no mapping would need to be ambiguous -
with no need for ordinary disambiguation rules. However, in none of the presently
available rule compilers can mapping rules “see” the output of other (earlier) mapping
rules, making it difficult if not impossible to address syntactic context (@tags) other
than that provided by lexicon entries. Also, a perfect (i.e. unambiguous) MAP rule is
like a SELECT rule in the way it works - a risky kind of rule, stating a grammatical
“fact” all in one go. REMOVE rules, operating on broadly mapped - and therefore
ambiguous - @tag strings, are much more cautious and robust, working together step by
step, relying on each other’s context condition safety nets.
The basic skeleton of syntactic mapping is the target word class condition. Even
without further context conditions, word class mapping can provide a working mapping
module for a syntactic disambiguation CG to work on. The Portuguese mapping rule set
is structured in word class “chapters”, with each chapter concluded by a “pure” word
class mapping rule, preceded by more specific rules for that word class, and headed by a
section with word or base form mapping rules. Though there are some prototypical
relations, most form-function pairs (PoS-@tag pairs) are not very closely knitted. Thus,
nouns are typical of subject (@SUBJ) and direct object (@ACC) function, but still,
subjects do come as infinitive clauses (#ICL), too, and objects can be finite subclauses
(#FS, “acho que não faz nada”). Adjectives and participles often occur with adnominal
and predicative function, but they can head noun phrases, too, and thus usurp typical
- 182 -