Fluid construction grammar

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Fluid Construction Grammar (FCG) is a construction grammar formalism designed by Luc Steels implementing the notion of emergent grammar and operates from a multi-agent perspective (none of the agents will speak the exact same language), useful for studies in evolutionary linguistics.

FCG is a fully operational formalism for construction grammars and proposes a uniform mechanism for parsing and production. It integrates many notions from contemporary computational linguistics such as feature structure and unification-based language processing. Rules are considered bi-directional and hence usable both for parsing and production. Processing is flexible in the sense that it can even cope with partially ungrammatical or incomplete sentences. FCG is called 'fluid' because it acknowledges the premise that language users constantly change and update their grammars. The research on FCG is conducted at Sony CSL in Paris and the AI Lab at the Vrije Universiteit Brussel


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[edit] Coupled feature structure

FCG provides two transient feature structure objects that are coupled during a parsing or production operation. One feature structure, designated the 'Left Pole', contains the semantic units. Each named unit is a set of features and their values. The other feature structure, the 'Right Pole', contains the syntactic units. Words are represented by units. Groupings of sub-units into grammatical constructions are also represented by units. The 'subunits' feature links subordinate units into a tree rooted at the 'top' unit. The majority of units in the FCG Left Pole have corresponding same-named units in the Right Pole, thus forming similar unit trees.

During the FCG parse of an utterance, the initial state of the Coupled Feature Structure is for a set of word strings and their ordering to be present on the sole unit of the Right (syntactic) Pole named 'top', and for the Left (semantic) Pole to consist of an empty top unit. As production rules apply consecutively to the Coupled Feature Structure, units and lexical features are linked into the Right Pole tree for each word, and if the word is significant, same-named units with semantic features are added to the Left Pole tree. At the end of a successful parse, the meaning (logical form (linguistics)) is extracted from the 'meaning' features of Left Pole units.

Conversely, because the FCG grammar is bi-directional, the production of an utterance from an initial set of meanings begins with the meanings as feature values of the sole Left Pole unit named 'top'. As successive grammar rules are applied, same-named units are added to both pole trees. At the end of a successful production, the word string is extracted from the 'form' features of Right Pole units.


[edit] Grammar production rules

FCG rules are a persistent specialization of Coupled Feature Structures, having the additional attributes of type, name, and weight. FCG applies rules by type, and distinguishes among them by name. Weights can be used to choose between competing rules. Some types of FCG rule match the rule Left Pole with the corresponding Coupled Feature Structure Left Pole. The remaining types of FCG rule match Right Poles with the Coupled Feature Structure. A match generally follows the process of unification. With rule variables bound, the rule pole not used in the match is merged with the corresponding pole of the Coupled Feature Structure. One type of rule, the Construction Rule, is an exception and is allowed to merge both Left and Right Poles with the Coupled Feature Structure after the matching process binds its free variables.


[edit] Flexibility

FCG provides a framework for bi-directional parsing and production. But aside from reserving the 'top' unit name and a few feature names, FCG makes no commitments to any grammatical theory of word categories, and makes no commitment to any particular theory of grammatical constructions. Users of FCG make these choices by writing FGC grammar rules accordingly.

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