9.1 Overview

This chapter gives a number of tips on how to organize your programs for increased efficiency. A lot of clarity and efficiency is gained by sticking to a few basic rules. This list is necessarily very incomplete. The reader is referred to textbooks such as [O’Keefe 90] for a thorough exposition of the elements of Prolog programming style and techniques.

• Do Not write code in the first place if there is a library predicate that will do the job.
• Write clauses representing base case before clauses representing recursive cases.
• Input arguments before output arguments in clause heads and goals.
• Use pure data structures instead of database changes.
• Use cuts sparingly, and only at proper places (see ref-sem-ctr-cut). A cut should be placed at the exact point that it is known that the current choice is the correct one: no sooner, no later.
• Make cuts as local in their effect as possible. If a predicate is intended to be determinate, then define it as such; do not rely on its callers to prevent unintended backtracking.
• Binding output arguments before a cut is a common source of programming errors. If a predicate is not steadfast, then it is usually for this reason.
• Replace cuts by if-then-else constructs if the test is simple enough (see Conditionals and Disjunction).
• Use disjunctions sparingly, always put parentheses around them, never put parentheses around the individual disjuncts, never put the ‘;’ at the end of a line.
• Write the clauses of a predicate so that they discriminate on the principal functor of the first argument (see below). For maximum efficiency, avoid “defaulty” programming (“catch-all” clauses).
• Do Not use lists ([…]), “round lists” ((…)), or braces ({…}) to represent compound terms, or “tuples”, of some fixed arity. The name of a compound term comes for free.
• Before trying to optimize your program for speed, use execution profiling to get an idea of where most of the time is being spent, and, more importantly, why.