The system creates a choice-point and tries the first alternative, whose body is father_child(Z, sally). The next goal to be proved is the leftmost one of this conjunction, i.e., parent_child(Z, sally). This is obtained as follows: Initially, the only matching clause-head for the query sibling(sally, erica) is the first one, so proving the query is equivalent to proving the body of that clause with the appropriate variable bindings in place, i.e., the conjunction (parent_child(Z,sally), parent_child(Z,erica)). This execution strategy is called chronological backtracking. If any goal fails in the course of executing the program, all variable bindings that were made since the most recent choice-point was created are undone, and execution continues with the next alternative of that choice-point. In that case, the system creates a choice-point, unifies the goal with the clause head of the first alternative, and continues with the goals of that first alternative. Operationally, Prolog's execution strategy can be thought of as a generalization of function calls in other languages, one difference being that multiple clause heads can match a given call. In that case, all generated variable bindings are reported to the user, and the query is said to have succeeded. If the negated query can be refuted, it follows that the query, with the appropriate variable bindings in place, is a logical consequence of the program. The resolution method used by Prolog is called SLD resolution. Logically, the Prolog engine tries to find a resolution refutation of the negated query. The built-in predicate true/0 is always true.Įxecution of a Prolog program is initiated by the user's posting of a single goal, called the query. There are two types of clauses: Facts and rules. Pure Prolog is restricted to Horn clauses, a Turing-complete subset of first-order predicate logic. Prolog programs describe relations, defined by means of clauses. Strings: A sequence of characters surrounded by quotes is equivalent to a list of (numeric) character codes, generally in the local character encoding or Unicode if the system supports Unicode.(3, ))) can also be written as ]]], or even more compactly as. There exists special syntax for denoting lists. (dot) and arity 2, whose second argument is a list, is itself a list. Lists are defined inductively: The atom is a list.The notation f/n is commonly used to denote a term with functor f and arity n. Users can declare arbitrary functors as operators with different precedences to allow for domain-specific notations. For example, the terms -(z), +(a,b) and =(X,Y) can also be written as -z, a+b and X=Y, respectively. Compound terms with functors that are declared as operators can be written in prefix or infix notation. An atom can be regarded as a compound term with arity zero.Įxamples of compound terms are truck_year('Mazda', 1986) and 'Person_Friends'(zelda,). The number of arguments is called the term's arity. Compound terms are ordinarily written as a functor followed by a comma-separated list of argument terms, which is contained in parentheses. Unlike other variables, the underscore does not represent the same value everywhere it occurs within a predicate definition.Ī compound term is composed of an atom called a "functor" and a number of "arguments", which are again terms. A single underscore ( _) denotes an anonymous variable and means "any term". A variable can become instantiated (bound to equal a specific term) via unification. Variables closely resemble variables in logic in that they are placeholders for arbitrary terms. Variables are denoted by a string consisting of letters, numbers and underscore characters, and beginning with an upper-case letter or underscore. Many Prolog implementations also provide unbounded integers and rational numbers. Other examples of atoms include x, blue, 'Taco', and 'some atom'.
The empty list, written, is also an atom. Atoms beginning with a capital letter must also be quoted, to distinguish them from variables. However, atoms containing spaces or certain other special characters must be surrounded by single quotes. Atoms are usually bare words in Prolog code, written with no special syntax. It is composed of a sequence of characters that is parsed by the Prolog reader as a single unit. It has a single data type, the term, which has several subtypes: atoms, numbers, variables and compound terms.Īn atom is a general-purpose name with no inherent meaning.
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