Term Description: A Simple Powerful Extension to Prolog Data Structures

Term description is a simple, powerful extension of terras. For example, functional notation and lazy execution of a program is introduced in a very natural manner without changing the basic mechanism of the computation, such as unification and backtracking. Especially , the readability of functional languages is introduced without actually introducing functional concepts. A term description is an extension to Prolog data structure. A term description is a term with some description (constraints) on it: : It means that must satisfy which is a predicate. In other words, whenever the term is unified with another term, the substitution must satisfy the description. B. Motivation Although unification in Prolog is a useful tool for manipulating structures, it lacks the ability to express complex patterns and operations over them. In Prolog, dividing a list into its first element and the rest, and constructing a list from its first element and the rest are very easy. There is no need to call a procedure to perform the operation. A simple list notation , [Carl Odr] does both of them. However, it is not so easy to divide a list into two lists or construct a single list from two lists. This operation is usually called "append" and requires a special program. Some operations are expressed as patterns and others are expressed as procedures. This destroys the readability and coherence of program notations. The distinction is not the essential part of the programming. The notation is simply a syntax sugar for a term cons(X,Y). We could similarly give a syntax sugar for a term description, : is unified with another term T, is first unified with T and then is checked. A constraint is described as a Prolog program, and executed as if it were written at the top-level. If the execution of the constraint fails, the unification also fails. A constraint is executed only when it is necessary, ie., only when the term description is unified with non-variable terms. When two term descriptions are unified, only one of them is executed first. For example, when two term descriptions: X:p(X,Y) and Z:g(Z) are unified, X is unified with Z:q(Z) first*, and p(Z:q(Z),Y) is executed. Then Z:q(Z) is in turn unified with the first argument of 'p'. A term description may be used to produce a value. The term description is similar to the macro In ESP [Chikayama 83] …