David Warren

Definite Clause Grammars for Language Analysis - A Survey of the Formalism and a Comparison with Augmented Transition Networks

Abstract A ciear and powerful formalism for describing languages, both natural and artificial, follows from a method for expressing grammars in logic due to Colmerauer and Kowalski. This formalism, which is a natural extension of context-free grammars, we call “definite clause grammars” (DCGs). A DCG provides not only a description of a language, but also an effective means for analysing strings of that language, since the DCG, as it stands, is an executable program of the programming language Prolog. Using a standard Prolog compiler, the DCG can be compiled into efficient code, making it feasible to implement practical language analysers directly as DCGs. This paper compares DCGs with the successful and widely used augmented transition network (ATN) formalism, and indicates how ATNs can be translated into DCGs. It is argued that DCGs can be at least as efficient as ATNs, whilst the DCG formalism is clearer, more concise and in practice more powerful.

PARSING AS DEDUCTION

By exploring the relationship between parsing and deduction, a new and more general view of chart parsing is obtained, which encompasses parsing for grammar formalisms based on unification, and is the basis of the Earley Deduction proof procedure for definite clauses. The efficiency of this approach for an interesting class of grammars is discussed.

Logic programming and compiler writing

The concept of ‘logic programming’, and its practical application in the programming language Prolog, are explained from first principles. The ideas are illustrated by describing in detail one sizable Prolog program which implements a simple compiler. The advantages and practicability of using Prolog for ‘real’ compiler implementation are discussed.

Prolog - the language and its implementation compared with Lisp

Prolog is a simple but powerful programming language founded on symbolic logic. The basic computational mechanism is a pattern matching process (“unification”) operating on general record structures (“terms” of logic). We briefly review the language and compare it especially with pure Lisp. The remainder of the paper discusses techniques for implementing Prolog efficiently; in particular we describe how to compile the patterns involved in the matching process. These techniques are as incorporated in our DECsystem-10 Prolog compiler (written in Prolog). The code it generates is comparable in speed with that produced by existing DEC10 Lisp compilers. We argue that pattern matching is a better method for expressing operations on structured data than conventional selectors and constructors - both for the user and for the implementor.

A view of the fifth generation and its impact

In October 1981, .Japan announced a national project to develop highly innovative computer systems for the 199Os, with the title “Fifth Generation Computer Systems ” This paper is a personal view of that project, its significance, and reactions to it.