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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, 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 ...
A definite clause grammar (DCG) is a way of expressing grammar, either for natural or formal languages, in a logic programming language such as Prolog. It is closely related to the concept of attribute grammars / affix grammars. DCGs are usually associated with Prolog, but similar languages such as Mercury also include DCGs.
Prolog is a logic programming language that has its origins in artificial intelligence, automated theorem proving and computational linguistics. [1] [2] [3]Prolog has its roots in first-order logic, a formal logic, and unlike many other programming languages, Prolog is intended primarily as a declarative programming language: the program is a set of facts and rules, which define relations.
In logic, a clause is a propositional formula formed from a finite collection of literals (atoms or their negations) and logical connectives.A clause is true either whenever at least one of the literals that form it is true (a disjunctive clause, the most common use of the term), or when all of the literals that form it are true (a conjunctive clause, a less common use of the term).
Prolog searches this space, top-down, depth-first, trying different clauses for solving the same (sub)goal in the order in which the clauses are written. This search strategy has the advantage that the current branch of the tree can be represented efficiently by a stack .
Given a goal clause, represented as the negation of a problem to be solved : with selected literal , and an input definite clause: . whose positive literal (atom) unifies with the atom of the selected literal , SLD resolution derives another goal clause, in which the selected literal is replaced by the negative literals of the input clause and the unifying substitution is applied:
Each such rule can be read as an implication: … meaning "If each is true, then is true". Logic programs compute the set of facts that are implied by their rules. Many implementations of Datalog, Prolog, and related languages add procedural features such as Prolog's cut operator or extra-logical features such as a foreign function interface.
Prolog has a syntax with a typical amount of punctuation, whereas Fril has an extremely simple syntax similar to that of Lisp. A (propositional) clause is a list consisting of a predicate followed by its arguments (if any). Among the types of top-level constructs are rules and direct-mode commands.