5.3 DCGs for Long Distance Dependencies
Context free rules work locally. For example, the rule
tells us how an S can be decomposed into two parts, and NP and a VP.
But certain aspects of natural language seem to work in a non-local, long-distance way. Indeed, for a long time it was thought that such phenomena meant that grammar-based analyses had to be replaced by very powerful new mechanisms (such as the transformations used in transformational grammar).
In fact, a surprising amount can be done by using grammars enriched in a fairly restricted way: namely, by the addition of features. Now, we've already discussed the use of features to deal with simple facts of case, but it turns out that features can do a lot more work for us. In particular, they do make it possible to give grammar-based analyses of many long distance phenomena.
We're now going to discuss a central long distance phenomenon (namely, English relative clauses) and show that DCGs enable us to give a rather neat feature-based analysis of such phenomena. The technique we are going to use is called threading . We will then use the same technique for an analysis of English wh-questions.
