Coloring Maps and the Kowalski Doctrine

It is attractive to regard an algorithm as composed of the logic determining what the results are and the control determining how the result is obtained. Logic programmers like to regard programming as controlled deduction, and there have been several proposals for controlling the deduction expressed by a Prolog program and not always using Prolog’s normal backtracking algorithm. The present note discusses a map coloring program proposed by Pereira and Porto and two coloring algorithms that can be regarded as control applied to its logic. However, the control mechanisms required go far beyond those that have been contemplated in the Prolog literature. Robert Kowalski (1979) enunciated the doctrine expressed by the formula 1 ALGORITHM = LOGIC + CONTROL The formula isn’t precise, and it won’t be precise until someone proposes a precise and generally accepted notion of how control is to be added to an expression of the logic of a program. Nevertheless, the idea is attractive, and