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Clause and Effect: Prolog Programming for the Working Programmer Paperback – April 29, 2003
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Clause and Effect delivers well on promise to get professional programer up to speed on writing practical Prolog.
In the first chapter, the author gives some justification for programming in Prolog, such as its symbol manipulation capability, automatic backtracking, the view that data structures and programs are of the same form, and the relational form of clauses. The syntax of Prolog is then discussed, and examples given of the three kinds of terms in Prolog. Readers with some background in category theory will appreciate the discussion more, as the author does employ some of this in the discussion, for example the view of addition as being a functor of a term. Terms are drawn in tree form in this chapter and throughout the book. The author then characterizes a Prolog program as a set of procedures, with each defining a predicate, and consisting of one or more (Horn) clauses. Unification of terms is discussed as a basic operation that determines when two clauses can be made equivalent by a substitution of variables. The execution of a program is viewed as a querying of the clauses, and the goal or e nd of the program is a proof that the goal is true.
Data structures in Prolog are discussed in chapter 2 as generalizations of programs using compound terms instead of just constants and variables. Lists are defined and their syntax discussed, along with dot and bracket notation. The implementation of simple arithmetic in Prolog is discussed. Several effective examples are given to illustrate arithmetic and list manipulation in Prolog.
Mappings, which are relations between two data structures, are the topic of chapter 3, and the author gives many examples illustrating how it is used to compose Prolog programs and how they act an both lists and more general trees.
The built-in predicate "cut" is discussed in the next chapter as a predicate to allow backtracking control of the program. The author gives many examples illustrating the problems involved with the use of "cut".
Difference structures are discussed in chapter 5 as a tool to simplify and increase program efficiency. A generalization of the idea of an accumulator, they allow one to work with "holes" in data structures during actual program execution. A list for example, can be viewed as "open" with its elements known only up to a point. It can then be filled in with an empty or a proper list. A difference list, discussed in the chapter, is then a list represented as a pair of "front" and "back", with the back being variable.
Applications of term rewriting are given in chapter 6, with symbolic differentiation launching the discussion. This is the more popular example of what Prolog-type languages can do, and is usually the reason given for beginning the use of symbolic programming languages. The author also discussed matrix multiplication in this chapter.
The next two chapters discuss the representation and manipulation of logical circuits using Prolog, including shift registers and coding circuits. This is followed in chapter 9 by an interesting discussion on how to write a compiler in Prolog, with the author discussing compilation for a single-accumulator computer, a RISC machine, and a stack machine. This is followed in chapter 10 by an even more interesting discussion on how to write a Fast Fourier transform in Prolog.
The last chapter of the book discusses how to use higher-order functional programming techniques in Prolog. For individuals, like myself, who are convinced that functional and logic programming are the most effective programming paradigms, this chapter is very interesting reading. The author defines an evaluator written in Prolog for these higher-order functional programs. Functional programming views computation as a collection of function applications on an expression representing a particular problem, and these functions can then be viewed as arguments to other functions. The lambda calculus from mathematical logic serves as the foundation for functional programming, and the author reviews this quickly, along with the technique of currying, in order to obtain facilities for functional programming in Prolog. Although short, this chapter introduces the reader to a fascinating area, and helpful references are given at the end of the chapter.
This book is not perfect, however, and I can't in good faith give it five stars (though I really want to). There are a fair number of typos scattered through the text (the author has an errata sheet on his website). Most of these are obvious, but on occasion I had to spend a lot of time puzzling over whether some bit of code had a typo or not (most of the time, there was no typo; it was just my understanding that was deficient). When in doubt, entering the code into a Prolog interpreter works wonders; I really wish the author would make a zip file of all the code examples available for download. A more significant problem than the typos is that the author writes in a very condensed style and either doesn't explain the meaning of some critical terms at all (like tail recursion) or doesn't explain them well enough (like backtracking). Coming from a functional programming background, and having already read a book on Prolog, I could fill in the gaps without much trouble, but many readers will have a harder time than I did. For these readers, I recommend that they use this book as a complement to a book with a more extended discussion of the language; the author's book "Programming in Prolog" is a fine choice for this.
I think that all serious programmers owe it to themselves to learn about logic programming just to see how easy it makes some problems which are quite difficult to solve in more conventional languages (try writing a symbolic differentiation program in a single page of C++, for instance). I heartily recommend this book for this purpose; it will stretch your mind and make you a better programmer.