| Title: Anonymous Variable
Description: None
Copyright (c) 1999 Steven J. Metsker.
Copyright (c) 2001 The Open For Business Project - www.ofbiz.org
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An anonymous variable unifies successfully with any other
term, without binding to the term.
Anonymous variables are useful for screening out unwanted
terms. For example, if a program describes a marriage in
terms of an id, the husband, wife, and the beginning and
end dates of the marriage, its facts might look something
like:
marriage(001, balthasar, grimelda, 14560512, 14880711);
// ...
marriage(257, kevin, karla, 19790623, present);
A rule that extracts just the husband from
marriage facts is:
husband(Id, Hub) :- marriage(Id, Hub, _, _, _);
The underscores in this rule represent anonymous variables.
When the rule executes, it will unify its
marriage structure with marriage
facts, without regard to the last three terms of those
facts.
Without anonymous variables, the husband rule
would need three unused variables. Note that the following
approach would fail:
husband(Id, Hub) :-
marriage(Id, Hub, Anon, Anon, Anon); // wrong
This approach, while tempting, will not work because the
variable Anon will bind to the structures it
encounters. Issued against the example program,
Anon will first bind to grimelda.
Next, Anon will attempt to bind to
14560512. This will fail, since
Anon will already be bound to
grimelda.
The essential behavior anonymous variables provide is that
they unify successfully without binding.
author:
Steven J. Metsker
version:
1.0 |
