14.1 Feature-based Grammars

We motivated the introduction of feature structures in the previous section, by saying that we would like to be able to express things like

S\ \longrightarrow\ \textit{NP}\ \textit{VP}: number of \textit{NP} = number of \textit{VP},

i.e. NP and VP have to agree in number in order to form a sentence. The basic idea is that non-terminal symbols no longer are atomic, but are feature structures, which specify what properties the constituent in question has to have. So, instead of writing the (atomic) non-terminal symbols S, \textit{NP}, \textit{VP}, we use feature structures where the value of the \textsc{cat} attribute is \textit{s}, \textit{np}, \textit{vp}. The rule \textit{S}\ \rightarrow\ \textit{NP}\ \textit{VP} becomes

That doesn't look so exciting, yet. But what we can to now is to add further information to the feature structures representing the non-terminal symbols. We can, e.g., add the information that the np must have nominative case:

Further, we can add an attribute called \textsc{num} to the np and the vp and require that the values be shared. Note how we express this requirement by co-indexing the values.

Here is a feature based grammar for a (tiny) fragment of English.

Patrick Blackburn and Kristina Striegnitz
Version 1.2.4 (20020829)