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Confusion about deftype and CLOS classes.
- To: email@example.com
- Subject: Confusion about deftype and CLOS classes.
- From: Peter Szolovits <firstname.lastname@example.org>
- Date: Fri, 14 Jun 91 00:33:52 EDT
- Sender: email@example.com
I need a bit of help figuring out something unclear in CLtL2, and/or
maybe a MCL2.0 bug. I do the following:
(deftype foo () 'fixnum)
(defmethod zorch ((x foo)) ...)
and get the error
> Error: Argument foo is not of the required type.
> While executing: find-class
This seems consistent with the statement on p. 781 that "The form
deftype does not create any classes." However, to be able to use
deftype as a means of abstracting from fixnum, I need to be able to
define a class for it. So, I try
(setf (find-class 'foo) (find-class 'fixnum))
but alas get another error:
> Error: Cannot set (find-class 'foo) because type foo is already
defined by deftype
> While executing: ccl::set-find-class
I also can't just do
(defclass foo (fixnum) ())
because fixnum is s built-in class, and thus can't be a base class for
After an hour of playing around, I realized that the (setf ...) DOES
work if I simply don't do the (deftype ...) first! Indeed, then
(typep 123 'foo) works, just as hoped for. Is this correct CL
behavior? I.e., are deftype and (setf (find-class ...) ...) simply
In any case, this does not seem sufficient, however. In another example,
I need to define
(deftype bar () (integer 0 *)), i.e., the non-negative integers.
Alas, I can't then see a way to use bar for method selection. I.e., I
can't do (defmethod zorch ((x bar)) ...), though (typep x 'bar) works.
I realize that CLOS does not define classes for types specified by
type specifier lists such as (integer 0 *), but I can't even see how
to get method selection based on the "integer" part of the spec.
Have I simply run into an awkward corner of CLOS/type interaction, or
is there some trick I'm overlooking? Do I need to do something ugly
(deftype bar () (integer 0 *))
(setf (find-class 'bar-class) (find-class 'integer))
(defmethod zorch ((x bar-class))
(when (typep x 'bar) ...))
Any enlightenment would be much appreciated. Thank you. --Pete Szolovits