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 (Unix version) - 6 Dec 2011