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 , $\textit{NP}$ , $\textit{VP}$ , 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.