1 Simple Goals and Queries

Racklog objects are the same as Racket objects. However, there are two subsets of these objects that are of special interest to Racklog: goals and predicates. We will first look at some simple goals. Predicates will introduce predicates and ways of making complex goals using predicates.

A goal is an object whose truth or falsity we can check. A goal that turns out to be true is said to succeed. A goal that turns out to be false is said to fail.

Two simple goals that are provided in Racklog are:

The goal %true succeeds. The goal %fail always fails.

(The names of all Racklog primitive objects start with %. This is to avoid clashes with the names of conventional Racket objects of related meaning. User-created objects in Racklog are not required to follow this convention.)

A Racklog user can query a goal by wrapping it in a %which-form.

(%which () %true)

evaluates to (), indicating success, whereas:

(%which () %fail)

evaluates to #f, indicating failure.

Note 1: The second subexpression of the %which-form is the empty list (). Later (Solving Goals), we will see %whiches with other lists as the second subform.

Henceforth, we will use the notation:

> E


to say that E evaluates to F. Thus,

> (%which () %true)