Semantic network

Revision as of 17:37, 26 March 2010

Semantics is the study of meaning, as inherent at the levels of words, phrases, sentences, and larger units of discourse. In the UNL approach, semantic values are expressed by three distinctive semantic entities: Universal Words, Attributes and Relations. Universal Words, or simply UWs, are expected to represent concepts, either simple or complex, that may figure as open class lexical items in natural languages. The set of UWs constitute the lexicon of UNL. Attributes and relations, on the other hand, are expected to represent semantic notions that are normally conveyed by closed class categories (such as prepositions and conjunctions) and other grammatical structures (such as affixes, prosody and word order).

Universal Words (UWs)

UWs are expected to be associated to lexical realisation units in the UNL-NL Dictionary, which is a bidirectional bilingual dictionary mapping lexical items between UNL and NL. A single UW may correspond to several different natural language entries (synonymy), and one single open-class natural language entry may correspond to several UWs (homography). Entries from closed classes are not associated to UWs, but to relations or attributes. Numerals (such as "six", "sixth", "6"), formulae (H₂0) and untranslatable expressions (such as "http://www.unlweb.net") are represented as temporary UWs, i.e., they are not expected to be included in the UNL-NL dictionaries. The same happens to most proper names. Temporary UWs are automatically assigned the feature TEMP, and may be addressed by named entity recognition modules in UNL-based applications.

Attributes

Attributes normally convey information that may be directly associated to grammar categories, such as aspect, degree, gender, number, tense, mood, register, voice and social deixis. This association is made through D-rules, such as the following:

@pl = PLR;
@past = PAS;
@passive = PSV;
@male = MCL;
@past,@progressive = PAS,PGS;

Some attributes, however, cannot be directly assigned to any value, and are rather treated as features to be addressed by other rules, such as the following:

(@ellipsis):=("",-@ellipsis); (L-rule)
(@square_bracket,%ref):= ("[")(%ref,-@square_bracket)("]"); (L-rule)
VC(@emphasis,%comp):=+IS(%comp)VC(%comp,TRACE); (S-rule)

Relations

Relations normally convey information that can be associated to S-rules:

agt(%source;%target):=VS(%source;%target);
tim(%source;%target):=VA(%source;PC([in];%target));