Racklog: Prolog-Style Logic Programming

Dorai Sitaram

Adapted from Schelog by Dorai Sitaram for Racket by Dorai Sitaram, John Clements, and Jay McCarthy.

 (require racklog)

Racklog is an embedding of Prolog-style logic programming in Racket. “Embedding” means you don’t lose Racket: You can use Prolog-style and conventional Racket code fragments alongside each other. Racklog contains the full repertoire of Prolog features, including meta-logical and second-order (“set”) predicates, leaving out only those features that could more easily and more efficiently be done with Racket subexpressions.

The Racklog implementation uses the approach to logic programming for Scheme described in Felleisen [mf:prolog] and Haynes [logick]. In contrast to earlier Lisp simulations of Prolog [campbell], which used explicit continuation arguments to store failure (backtrack) information, the Felleisen and Haynes model uses the implicit reified continuations of Scheme. In Racket these are provided by the operator call-with-current-continuation (aka call/cc). This allows Racklog to be an embedding, ie, logic programming is not built as a new language on top of Racket, but is used alongside Racket’s other features. Both styles of programming may be mixed to any extent that a project needs.

The Racklog user does not need to know about the implementation mechanism or about call/cc and continuations to get on with the business of doing logic programming with Racklog.

This text is a gentle introduction to Racklog syntax and programming. It assumes a working knowledge of Racket and an awareness of, if not actual programming experience with, Prolog. If you need assistance for Prolog, you may consult [bratko, ok:prolog, aop] or many other excellent books and online documents available.

    1 Simple Goals and Queries

    2 Predicates

      2.1 Predicates Introducing Facts

      2.2 Predicates with Rules

      2.3 Solving Goals

      2.4 Asserting Extra Clauses

      2.5 Local Variables

    3 Using Conventional Racket Expressions in Racklog

      3.1 Constructors

      3.2 %is

      3.3 Lexical Scoping

      3.4 Type Predicates

    4 Backtracking

    5 Unification

      5.1 The Occurs Check

    6 Conjuctions and Disjunctions

    7 Manipulating Racklog Variables

      7.1 Checking for Variables

      7.2 Preserving Variables

    8 The Cut (!)

      8.1 Conditional Goals

      8.2 Negation as Failure

    9 Set Predicates

    10 Racklog Module Language

    11 Glossary of Racklog Primitives

      11.1 Racket Predicates

      11.2 User Interface

      11.3 Relations

      11.4 Racklog Variables

      11.5 Cut

      11.6 Racklog Operators

      11.7 Unification

      11.8 Numeric Predicates

      11.9 List Predicates

      11.10 Set Predicates

      11.11 Racklog Predicates

      11.12 Racklog Variable Manipulation