Class STANDARD-OBJECT-TEMPLATE
Package:
COMMON-LISP.EXTENSIONS.DATA-AND-CONTROL-FLOW.UNIFICATION
Class Precedence List:
STANDARD-OBJECT-TEMPLATE, TYPE-TEMPLATE, TEMPLATE, STANDARD-OBJECT, T.
Known Subclasses:
None.
Slots:
None.
Description:
The STANDARD-OBJECT-TEMPLATE class denotes those object that are used to unify against any STANDARD-OBJECT.
Template Syntax:
#T(<CL class designator> <slot spec>*)
<slot spec> ::= <reader method> <object>
| (slot-value <slot name>) <object>
The STANDARD-OBJECT-TEMPLATE syntax denotes any CL object that is an instance of a STANDARD-OBJECT. The semantic of the template in a unification depends on which <slot spec>'s appear. In the first case the meaning of <slot spec> is such that the <reader method> gets called against the instance against which the STANDARD-OBJECT-TEMPLATE is being UNIFYed. In the second case, SLOT-VALUE is called with the instance being UNIFYed and <slot name>. The result value of the call to either <reader method>, or SLOT-VALUE is UNIFYed against <object>.
<object> can be any CL object, including a TEMPLATE (in which case the unification machinery proceeds recursively.)
Examples:
cl-prompt> (defclass foo () ((a :initform 42 :accessor foo-a)))
#<STANDARD-CLASS FOO XXXXX>
cl-prompt> (defclass bar (foo) ())
#<STANDARD-CLASS BAR XXXXX>
cl-prompt> (defclass baz ()
((a :reader the-a-in-a-baz)
(b :accessor bazb :initarg :b)))
#<STANDARD-CLASS BAZ XXXXX>
cl-prompt> (setf e (unify (make-instance 'foo) #T(foo foo-a 42)))
#<ENVIRONMENT xxx>
cl-prompt> (setf e (unify (make-instance 'bar) #T(foo foo-a 42)))
#<ENVIRONMENT xxx>
cl-prompt> (find-variable-value '?x e)
42
T
cl-prompt> (setf e (unify (make-instance 'bar) #T(foo foo-a 42)))
#<ENVIRONMENT xxx>
cl-prompt> (find-variable-value '?x e)
42
T
cl-prompt> (setf e (unify (make-instance 'baz) #T(baz the-a-in-baz 42)))
#<ENVIRONMENT xxx>
cl-prompt> (find-variable-value '?x e)
42
T
cl-prompt> (setf e (unify (make-instance 'baz) #T(baz (slot-value b) 42)))
#<ENVIRONMENT xxx>
cl-prompt> (find-variable-value '?x e)
42
T
cl-prompt> (setf e (unify (make-instance 'baz :b '?e) #T(baz bazb 42)))
#<ENVIRONMENT xxx>
cl-prompt> (find-variable-value '?x e)
42
T
cl-prompt> (setf e (unify (make-instance 'bar) #T(baz foo-a 42)))
--> Error: UNIFICATION-FAILURE
Affected By:
None.
Exceptional Situations:
Unifying a STANDARD-OBJECT-TEMPLATE against a non-STANDARD-OBJECT object results in an UNIFICATION-FAILURE error being signaled.
Unifying a STANDARD-OBJECT-TEMPLATE denoting an instance of class C1 against an instance of class C2, results in an UNIFICATION-FAILURE error being signaled, when C1 is not in the class precedence list of C2.
See Also:
UNIFY
Notes:
None.