めも: Narrowingとか

3.13.4 Programming †
Often, programming with variables leads to conceptually simple, terse and elegant code at the cost of an acceptable loss of efficiency. The logic variables of a program and/or a query are not much different from the variables that are typically used to solve algebra or geometry problems. In both cases, some unknown entities of the problem are related to each other by expressions involving functions. Narrowing allows us to evaluate these expressions―and in the process to find values for the variables. The simplest application, and the most familiar for those used to solve algebra and geometry problems with variables, is when the expression to evaluate is an equation.

The evaluation strategy of the PAKCS compiler/interpreter, which is used for all our examples, is lazy, but incomplete. The strategy evaluates non-deterministic choices sequentially instead of concurrently.