Constraint logic programming: Difference between revisions

Revision as of 00:00, 11 November 2009

Revision as of 17:09, 12 November 2009

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Constraint logic programming with [[real number]]s uses real expressions as terms. When no functors are used, terms are expressions over reals, possibly including variables. In this case, each variable can only take a real number as a value.

PreciselyTo be precise, terms are expressions over variables and real constants. Equality between terms is a kind of constraint that is always present, as the interpreter generates equality of terms during execution. As an example, if the first literal of the current goal is <code>A(X+1)</code> and the interpreter has chosen a clause that is <code>A(Y-1):-Y=1</code> after rewriting is variables, the constraints added to the current goal are <code>X+1=Y-1</code> and <math>Y=1</math>. The rules of simplification used for functors are obviously not used: <code>X+1=Y-1</code> is not unsatisfiable just because the first expression is built using <code>+</code> and the second using <code>-</code>.

Reals and functors can be combined, leading to terms that are expressions over reals and functors applied to other terms. Formally, variables and real constants are expressions, as any arithmetic operator over other expressions. Variables, constants (zero-arity-functors), and expressions are terms, as any functor applied to terms. In other words, terms are built over expressions, while expressions are built over numbers and variables. In this case, variables ranges over real numbers ''and terms''. In other words, a variable can take a real number as a value, while another takes a term.