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A.3 Defining Relations

The normal method of defining relations in Common Prolog is to use the defrel macro:

(defrel <relation name>
  [(declare declaration*)]
    <clause1>
     .
     .
    <clauseN>)

where each <clause> is of the form:

(<clause-head>
 <subgoal1>
    .
    .
 <subgoalN>)

and declarations may include: (mode arg-mode*) and any of the normal Lisp optimization declarations. Mode declarations determine how much clause indexing will be done on the predicate and can also streamline generated code for a predicate that will only be used in certain ways. A mode declaration consists of the word "MODE" followed by a mode spec for each argument position of the predicate. The possible argument mode specs are:

?
Generate completely general code for this arg and don't index on it.
?*
Generate completely general code and index.
+
Generate code assuming this argument will be bound on entry and index.
-
Generate code assuming this argument will be unbound on entry and don't index.

The default mode specs are ?* for the first argument and ? for all the rest.

KnowledgeWorks and Prolog User Guide (Macintosh version) - 01 Dec 2021 19:35:39

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