Portable to use make-instance to make conditions?

Question

In Common Lisp, is it portable to use make-instance instead of make-condition to make condition objects?

For example:

(define-condition foo (condition)
  ())

(make-condition 'foo)
(make-instance 'foo)

Does this have something to do with how conditions are placed in the CLOS class hierarchy? (subtypep 'condition 'standard-class) returns false in SBCL and CLISP. However, make-instance can make conditions in both implementations. Is this guaranteed by the standard?

Answer 1

No, it is explicitly not portable to do that. From the specification:

Conforming code must observe the following restrictions related to conditions:

• define-condition, not defclass, must be used to define new condition types.
• make-condition, not make-instance, must be used to create condition objects explicitly.
• The :report option of define-condition, not defmethod for print-object, must be used to define a condition reporter.
• slot-value, slot-boundp, slot-makunbound, and with-slots must not be used on condition objects. Instead, the appropriate accessor functions (defined by define-condition) should be used

Answer 2

You can check if the condition system in a certain implementation is using CLOS and supporting CLOS operations not by

(subtypep 'condition 'standard-class)

but maybe by using something like:

(subtypep (class-of (find-class 'condition)) 'standard-class)

The Common Lisp standard does not require implementations to use CLOS to implement conditions. Thus the condition system in the standard is defined without support for CLOS: no support for make-instance required, no support for making subclasses via defclass required, ... Thus one can also not count on other features of CLOS being used.

That's one of the warts of the Common Lisp standard from 1994.

Answer 3

It is not portable, according to the 4th footnote in Practical Common Lisp chapter 19:

"In some Common Lisp implementations, conditions are defined as subclasses of STANDARD-OBJECT, in which case SLOT-VALUE, MAKE-INSTANCE, and INITIALIZE-INSTANCE will work, but it's not portable to rely on it."

Link to the particular page:

http://www.gigamonkeys.com/book/beyond-exception-handling-conditions-and-restarts.html