LOGLISP: an alternative to PROLOG
John A. Robinson, Ernest E. Sibert · 2013
Seven years or so after it was first proposed (Kowalski 1974), the technique of 'logic programming' today has an enthusiastic band of users and an increasingly impressive record of applications. For most of these people, logic progamming means PROLOG, the system defined and originally implemented by the Marseille group (Roussel 1975). PROLOG has since been implemented in several other places, most notably at Edinburgh (Warren et al. 1977). Much of the rapid success of logic progamming is due to these implementations of PROLOG (as well as to the inspired missionary work of Kowalski, van Emden, Clark and others). The Edinburgh PROLOG system is in particular a superb piece of software engineering which allows the logic progammer to compile assertions into DEC-10 machine code and thus run logic programs with an efficiency which compares favourably with that of compiled LISP. All other implementations of logic programming (including our own, which we describe in this paper) are based on interpreters. However, PROLOG is not, and does not claim to be, the definitive logic programming formalism. It has features which (however useful in practical applications) are somewhat foreign to Kowalski's original simple and clear conception of a deductive, assertional programming language based entirely on the LUSH resolution of Horn clauses. In this conception there are no imperative constructs and indeed there is no notion of control or of anything 'happening'. The spirit is very like that of Strachey's static Platonic vision — elaborated by Landin and later by Scott — of programs as purely denotative expressions in which entities are represented as the results of applicative combinations of functions with their arguments. Running a program is, on this view, nothing more than finding out what it denotes. The program merely does the denoting, and should not be taken for a prescription of any sort of activity, such as 'computing'.