From: Jim Newton
Subject: mop incompatible meta-classes
Date: Sat, 26 Feb 2005 14:46:55 +0000
Message-ID: <38bglnF5m10jpU1@individual.net>

I do not have my copy of AMOP with me, perhaps it is explained
in there.  But can anyone see what is wrong with this simple
example?  The message says that i should declare a method on
PCL:VALIDATE-SUPERCLASS dispatching on PCL:FUNCALLABLE-STANDARD-CLASS
and STANDARD-CLASS, but that sounds it might have lots of
dangerous side effects!!!  (btw, I'm using cmucl 19a).


(defclass super-method ( STANDARD-METHOD)
   nil)

(defclass super-generic ( STANDARD-GENERIC-FUNCTION )
   nil)

(defgeneric myfun ( x)
   (:method-class super-method)
   (:generic-function-class super-generic))


The message I get is:

Error in function "DEFMETHOD SHARED-INITIALIZE :AFTER (STD-CLASS T)":
    The class #<FUNCALLABLE-STANDARD-CLASS STANDARD-GENERIC-FUNCTION 
{281A6685}>
    was specified as a super-class of the class
    #<STANDARD-CLASS SUPER-GENERIC {58004355}>, but the meta-classes
    #<STANDARD-CLASS PCL:FUNCALLABLE-STANDARD-CLASS {281A6725}> and
    #<STANDARD-CLASS STANDARD-CLASS {281A4305}> are incompatible.  Define a
    method for PCL:VALIDATE-SUPERCLASS to avoid this error.
    [Condition of type SIMPLE-ERROR]

From: Pascal Costanza
Subject: Re: mop incompatible meta-classes
Date: Sun, 27 Feb 2005 12:55:21 +0000
Message-ID: <1109508921.344658.189980@l41g2000cwc.googlegroups.com>

Jim Newton wrote:
> I do not have my copy of AMOP with me, perhaps it is explained
> in there.  But can anyone see what is wrong with this simple
> example?  The message says that i should declare a method on
> PCL:VALIDATE-SUPERCLASS dispatching on PCL:FUNCALLABLE-STANDARD-CLASS
> and STANDARD-CLASS, but that sounds it might have lots of
> dangerous side effects!!!  (btw, I'm using cmucl 19a).
>
>
> (defclass super-method ( STANDARD-METHOD)
>    nil)
>
> (defclass super-generic ( STANDARD-GENERIC-FUNCTION )
>    nil)
>
> (defgeneric myfun ( x)
>    (:method-class super-method)
>    (:generic-function-class super-generic))
>
>
> The message I get is:
>
> Error in function "DEFMETHOD SHARED-INITIALIZE :AFTER (STD-CLASS T)":
>     The class #<FUNCALLABLE-STANDARD-CLASS STANDARD-GENERIC-FUNCTION
> {281A6685}>
>     was specified as a super-class of the class
>     #<STANDARD-CLASS SUPER-GENERIC {58004355}>, but the meta-classes
>     #<STANDARD-CLASS PCL:FUNCALLABLE-STANDARD-CLASS {281A6725}> and
>     #<STANDARD-CLASS STANDARD-CLASS {281A4305}> are incompatible.
Define a
>     method for PCL:VALIDATE-SUPERCLASS to avoid this error.
>     [Condition of type SIMPLE-ERROR]

Make this:

(defclass super-generic (standard-generic-function)
  ()
  (:metaclass funcallable-standard-class))

BTW, PCL is very touchy wrt generic functions: According to AMOP,
funcallable-standard-class and standard-class should be compatible by
default, and furthermore you can only subclass
standard-generic-function once. However, the second subclass of s-g-f
gives you a crash in PCL.

Currently, clisp has the most compatible MOP implementation, so it may
be a better idea to prototype your stuff in clisp and then see how to
port it to other CL implementations.


Pascal