In chapter 1 we stressed that computer science deals with imperative (how to) knowledge, whereas mathematics deals with declarative (what is) knowledge. Indeed, programming languages require that the programmer express knowledge in a form that indicates the step-by-step methods for solving particular problems. On the other hand, high-level languages provide, as part of the language implementation, a substantial amount of methodological knowledge that frees the user from concern with numerous details of how a specified computation will progress.
Most programming languages, including Lisp, are organized around computing the values of mathematical functions. Expression-oriented languages (such as Lisp, Fortran, and Algol) capitalize on the ``pun'' that an expression that describes the value of a function may also be interpreted as a means of computing that value. Because of this, most programming languages are strongly biased toward unidirectional computations (computations with well-defined inputs and outputs). There are, however, radically different programming languages that relax this bias. We saw one such example in section , where the objects of computation were arithmetic constraints. In a constraint system the direction and the order of computation are not so well specified; in carrying out a computation the system must therefore provide more detailed ``how to'' knowledge than would be the case with an ordinary arithmetic computation. This does not mean, however, that the user is released altogether from the responsibility of providing imperative knowledge. There are many constraint networks that implement the same set of constraints, and the user must choose from the set of mathematically equivalent networks a suitable network to specify a particular computation.
The nondeterministic program evaluator of section also moves away from the view that programming is about constructing algorithms for computing unidirectional functions. In a nondeterministic language, expressions can have more than one value, and, as a result, the computation is dealing with relations rather than with single-valued functions. Logic programming extends this idea by combining a relational vision of programming with a powerful kind of symbolic pattern matching called unification.
This approach, when it works, can be a very powerful way to write programs. Part of the power comes from the fact that a single ``what is'' fact can be used to solve a number of different problems that would have different ``how to'' components. As an example, consider the append operation, which takes two lists as arguments and combines their elements to form a single list. In a procedural language such as Lisp, we could define append in terms of the basic list constructor cons, as we did in section :
(define (append x y) (if (null? x) y (cons (car x) (append (cdr x) y))))This procedure can be regarded as a translation into Lisp of the following two rules, the first of which covers the case where the first list is empty and the second of which handles the case of a nonempty list, which is a cons of two parts:
Using the append procedure, we can answer questions such as
Find the append of (a b) and (c d).
But the same two rules are also sufficient for answering the following sorts of questions, which the procedure can't answer:
Find a list y that appends with (a b) to produce (a b c d).
Find all x and y that append to form (a b c d).
In a logic programming language, the programmer writes an append ``procedure'' by stating the two rules about append given above. ``How to'' knowledge is provided automatically by the interpreter to allow this single pair of rules to be used to answer all three types of questions about append.
Contemporary logic programming languages (including the one we implement here) have substantial deficiencies, in that their general ``how to'' methods can lead them into spurious infinite loops or other undesirable behavior. Logic programming is an active field of research in computer science.
Earlier in this chapter we explored the technology of implementing interpreters and described the elements that are essential to an interpreter for a Lisp-like language (indeed, to an interpreter for any conventional language). Now we will apply these ideas to discuss an interpreter for a logic programming language. We call this language the query language, because it is very useful for retrieving information from data bases by formulating queries, or questions, expressed in the language. Even though the query language is very different from Lisp, we will find it convenient to describe the language in terms of the same general framework we have been using all along: as a collection of primitive elements, together with means of combination that enable us to combine simple elements to create more complex elements and means of abstraction that enable us to regard complex elements as single conceptual units. An interpreter for a logic programming language is considerably more complex than an interpreter for a language like Lisp. Nevertheless, we will see that our query-language interpreter contains many of the same elements found in the interpreter of section . In particular, there will be an ``eval'' part that classifies expressions according to type and an ``apply'' part that implements the language's abstraction mechanism (procedures in the case of Lisp, and rules in the case of logic programming). Also, a central role is played in the implementation by a frame data structure, which determines the correspondence between symbols and their associated values. One additional interesting aspect of our query-language implementation is that we make substantial use of streams, which were introduced in chapter 3.