3 Syntax

3 Syntax

The syntax of logical terms is based on Prolog's syntax in its convention for variables, but uses a different syntax for operators (because Prolog's are so confusing and to emphasize it is not Prolog).

A variable is a sequence of alphanumeric characters (possibly including "_") that starts with an upper case letter or "_". For example, X, Letter, Any_cat, A_big_dog are all variables. The variable "_" in an anonymous variable which means that all occurrences are assumed to be different variables. If a variable occurs only once in a clause, it should probably be written as "_".

A constant is either:

a sequence of alphanumeric characters (possibly including "_") starting with a lower case letter, such as: david, comp_intell, ciLOG, and a45_23
an integer or float, such as 123, -5, 1.0, -3.14159, 4.5E7, -0.12e+8, and 12.0e-9. There must be a decimal point in floats written with an exponent and at least one digit before and after a decimal point.
any sequence of characters delimited by single quotes, such as 'X', '2b~2b', '../ch2/foo.pl', 'A Tall Person'

A term is either a variable, a constant, of of the form f(t₁,...,t_n), where f, a function symbol, is a sequence of alphanumeric characters (possibly including "_") starting with a lower case letter and the t_i are terms.

An atom is either of the form p or p(t₁,...,t_n), where p, a predicate symbol, is a sequence of alphanumeric characters (including _) starting with a lower case letter and the t_i are terms.

An body is either an atom, of the form alpha&beta, where alpha and beta are bodies, or of the form ~alpha, where alpha is a body.

A clause is either an atom or is a rule of the form h<-b where h is an atom (the head of the clause) and b is a body.

Some predicate and function symbols can be written using infix notation. For example "X is 4+3*Y" means the same as "is(X,+(4,*(3,Y)))", where "is" is a predicate symbol and "+" and "*" are function symbols. The operator precedence follows Prolog's conventions.

The symbol "<=" is defined to be infix, but there are no clauses defining it. This is designed to be used for meta-programming where "<=" can be used as a meta-level predicate symbol defining the object-level implication. (See Computational Intelligence [1], Chapter 6).

3 Syntax