Prolog guide warns against impure constructs and global state that silently drop solutions or create hidden dependencies

The article argues that Prolog programs can be defective not only by returning wrong answers, but also by failing to return intended solutions. It warns against impure, non-monotonic constructs like cuts, if-then, var checks, and global database updates, recommending declarative alternatives such as clean data structures, constraints, predicate arguments, and meta-predicates.

The piece reflects a broader lesson in logic programming: code that appears efficient or convenient can undermine correctness when it hides control flow or state. As interest in symbolic reasoning and constraint-based systems resurfaces alongside AI, declarative purity remains important for building programs that are easier to reason about, test, and compose.