6 Definite Clause Grammars

This lecture has three main goals:

1. To briefly review last semester's discussion of DCGs.

2. To show that the inbuilt feature-passing mechanism of DCGs enables them to handle long distance dependencies. We first give a rather naive DCG for simple English relative clauses, and then show how the gap-threading technique can be used to improve it.

3. To discuss the good and bad points of DCGs, and set the stage for our later work with grammars and features.

• 6.1 Review of DCGs
  • 6.1.1 DCGs are a natural notation for context free grammars
  • 6.1.2 DCGs are really syntactic sugar for difference lists
  • 6.1.3 DCGs give us a natural notation for features

• 6.2 DCGs for Long Distance Dependencies
  • 6.2.1 Relative Clauses
  • 6.2.2 A First DCG
  • 6.2.3 A Second DCG
  • 6.2.4 Gap Threading
  • 6.2.5 Questions

• 6.3 Pros and Cons of DCGs

• 6.4 Exercises