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Tutorial Five (Supplementary): Syntax

For the most part, the syntax that the applet uses is the syntax used by CILog. However, it does not implement all of CILog's built-in features and functionality. There are three components in the language: terms, atoms, and clauses.

Terms: A term is a variable, a constant, or a compound term. A variable is a sequence of alphanumeric characters that starts with an uppercase letter. The only exception is "_", which is a variable that can unify with anything. A constant is either a sequence of alphanumeric characters starting with a lowercase letter or an integer. Finally, a compound term has the form f(t₁,...,t_n), where f is a function symbol (a sequence of alphanumeric characters starting with a lowercase letter) and the t_i are terms. However, arithmetic expressions are also considered to be compound terms. Some atoms require arithmetic expressions as arguments. The infix functions "+", "-", "*", and "mod" are implemented, with the usual interpretation.

Atoms: Atoms, also called goals, are almost identical in syntax to compound terms. They are of the form p(t₁,...,t_n), where p is a predicate symbol (a sequence of alphanumeric characters starting with a lowercase letter) and the t_i are terms. It is also possible to have a goal with no terms and only a predicate symbol. As with compound terms, there are also (infix) built-in predicates:

• X < Y, where X and Y must be ground arithmetic expressions when called, is true if the value of X is less than the value of Y.
• X > Y, where X and Y must be ground arithmetic expressions when called, is true if the value of X is greater than the value of Y.
• X =< Y, where X and Y must be ground arithmetic expressions when called, is true if the value of X is less than or equal to the value of Y.
• X => Y, where X and Y must be ground arithmetic expressions when called, is true if the value of X is greater than or equal to the value of Y.
• X =\= Y, where X and Y must be ground arithmetic expressions when called, is true if the value of X is different from the value of Y.
• V is E, where E must be a ground arithmetic expression when called, is true if expression E evaluates to V.
• X \= Y, where X and Y denote different objects under the unique names assumption, or are different compound terms.

Clauses: Clauses, which are also called rules, are made up of two parts. The first part, the head, is simply a goal. Every clause must have a head, and must end with a period. The body of a clause is a conjunction of atoms g₁ & ... & g_n. Thus, the clause is either of the form h. or h <- b where h is the head of the clause and b is the body.

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