4.2.2.1 Hook Predicates

Hook predicates are called by the system. They enable you to modify SICStus Prolog’s behavior. Most of them are undefined by default. The idea of a hook predicate is that its clauses are independent of each other, and it makes sense to spread their definitions over several files (which may be written by different people). In other words, a hook predicate is typically declared to be multifile (see Multifile Declarations). Often, an application needs to combine the functionality of several software modules, among which some define clauses for such hook predicates. By simply declaring every hook predicate as multifile, the functionality of the clauses for the hook predicate is automatically combined. If this is not done, then the last software module to define clauses for a particular hook predicate will effectively supersede any clauses defined for the same hook predicate in a previous module. Most hook predicates must be defined in the user module, and usually only their first solution is relevant.