This example determines a class precedence list for the
class pie. The following classes are defined:
(defclass pie (apple cinnamon) ())
(defclass apple (fruit) ())
(defclass cinnamon (spice) ())
(defclass fruit (food) ())
(defclass spice (food) ())
(defclass food () ())
The set S_{pie}~= {pie, apple, cinnamon, fruit, spice, food, standard-object, t}. The set R~= {(pie, apple), (apple, cinnamon), (apple, fruit), (cinnamon, spice), \break (fruit, food), (spice, food), (food, standard-object), (standard-object, t)}.
The class pie is not preceded by anything, so it comes first;
the result so far is (pie). Remove pie from S and pairs
mentioning pie from R to get S~= {apple, cinnamon,
fruit, spice, food, standard-object, t} and R~=~{(apple, cinnamon), (apple, fruit), (cinnamon, spice),\break (fruit,
food), (spice, food), (food, standard-object),
(standard-object, t)}.
The class apple is not preceded by anything, so it is next; the
result is (pie apple). Removing apple and the relevant
pairs results in S~= {cinnamon, fruit, spice, food,
standard-object, t} and R~= {(cinnamon, spice),
(fruit, food), (spice, food), (food, standard-object),\break
(standard-object, t)}.
The classes cinnamon and fruit are not preceded by
anything, so the one with a direct subclass rightmost in the
class precedence list computed so far goes next. The class apple is a
direct subclass of fruit, and the class pie is a direct
subclass of cinnamon. Because apple appears to the right
of pie in the class precedence list,
fruit goes next, and the
result so far is (pie apple fruit). S~= {cinnamon,
spice, food, standard-object, t}; R~= {(cinnamon,
spice), (spice, food),\break (food, standard-object),
(standard-object, t)}.
The class cinnamon is next, giving the result so far as (pie apple fruit cinnamon). At this point S~= {spice,
food, standard-object, t}; R~= {(spice, food), (food,
standard-object), (standard-object, t)}.
The classes spice, food, standard-object, and
t are added in that order, and the class precedence list
is (pie apple fruit cinnamon spice food standard-object t).
It is possible to write a set of class definitions that cannot be ordered. For example:
(defclass new-class (fruit apple) ())
(defclass apple (fruit) ())
The class fruit must precede apple
because the local ordering of superclasses must be preserved.
The class apple must precede fruit
because a class always precedes its own superclasses.
When this situation occurs, an error is signaled, as happens here
when the system tries to compute the class precedence list
of new-class.
The following might appear to be a conflicting set of definitions:
(defclass pie (apple cinnamon) ())
(defclass pastry (cinnamon apple) ())
(defclass apple () ())
(defclass cinnamon () ())
The class precedence list for pie is
(pie apple cinnamon standard-object t).
The class precedence list for pastry is
(pastry cinnamon apple standard-object t).
It is not a problem for apple to precede cinnamon in the
ordering of the superclasses of pie but not in the ordering for
pastry. However, it is not possible to build a new class that
has both pie and pastry as superclasses.