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.
Read more about this topic: Fluid Construction Grammar
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