Structure Sharing

Figure 2.5: Structure sharing in a graph representation of a TFS.

Figure 2.6: Structure sharing in an AVM representation of a TFS.

From the long list of possible extensions to the basic formalism of typed feature structures, one of practical importance will be described before concluding this chapter. As seen in the example presented in Figure 2.3, both feature THROWER and feature THROWN have as value a feature structure of type index, which bear the feature NUMBER having as value the same type singular. Structure sharing avoids duplicating such information, by keeping a single instance of the node labeled with the type singular. Figure 2.5 shows this feature structure with the node singular shared, while in Figure 2.6, the AVM representation is shown (where the sharing is indicated by the tag [0]).

Structure sharing could lead to cyclic structures (cycles in the graph representing a feature structure), when a node has a substructure that incorporates that node. An example of such a cycle can be found in the feature structure representation for ``every book'' (adapted from [Pollard and Sag1994, p.50, p.361]):

$\left[\begin{array}{l} \textrm{SPR: } \left[\begin{array}{l} \textrm{PHON: \te... ...ht] \end{array}\right] \end{array}\right] \end{array}\right] \end{array}\right]$.