Prolog LanguageCoding guidelines


When programming in Prolog, we must pick two kinds of names:

  • names of predicates
  • names of variables.

A good predicate name makes clear what each argument means. By convention, underscores are used in names to separate the description of different arguments. This is because underscores_keep_even_longer_names_readable, whereas mixingTheCasesDoesNotDoThisToTheSameExtent.

Examples of good predicates names are:

  • parent_child/2
  • person_likes/2
  • route_to/2

Note that descriptive names are used. Imperatives are avoided. Using descriptive names is advisable because Prolog predicates can typically be used in multiple directions, and the name should be applicable also of all or none of the arguments are instantiated.

Mixed capitalization is more common when selecting names of variables. For example: BestSolutions, MinElement, GreatestDivisor. A common convention for naming variables that denote successive states is using S0, S1, S2, ..., S, where S represents the final state.


There are only a few language constructs in Prolog, and several ways for indenting them are common.

No matter which style is chosen, one principle that should always be adhered to is to never place (;)/2 at the end of a line. This is because ; and , look very similar, and , frequently occurs at the end of a line. Therefore, clauses that use a disjunction should for example be written as:

(  Goal1
;  Goal2 

Order of arguments

Ideally, Prolog predicates can be used in all directions. For many pure predicates, this is also actually the case. However, some predicates only work in particular modes, which means instantiation patterns of their arguments.

By convention, the most common argument order for such predicates is:

  • input arguments are placed first. These arguments must be instantiated before the predicate is called.
  • pairs of arguments that belong together are placed adjacently, such as p(..., State0, State, ...)
  • intended output arguments are placed last. These predicates are instantiated by the predicate.