STREAMP and CLOS

[berni@iml.fhg.de (Stefan Bernemann)]

    Date: Mon, 24 Feb 1992 20:49 MET
    From: York%Chuck-Jones.West.Dialnet.ILA.COM@fhg.de (William M. York)

	Date: Mon, 24 Feb 1992 08:25 PST
	From: barmar@think.com (Barry Margolin)

	    Date: Mon, 24 Feb 1992 12:51 EST
	    From: bwild@azurro.fzi.de (Bernd Wild)

	    We got a surprising result when applying STREAMP to an arbitrary CLOS  
	    instance. It always returns T. This behavior has changed from Genera  
	    8.0 to 8.1. While in 8.0 only "real" streams resulted in a T-value in  
	    8.1 this can be observed for every instance.

	I can't reproduce this.  I did

	(clos:defclass foo () ())
	(setq a (clos:make-instance 'foo))

	(streamp a) => NIL

	(send a :which-operations) => (:SEND-IF-HANDLES :WHICH-OPERATIONS :OPERATION-HANDLED-P :PRINT-SELF)

	I'm running Genera 8.1.1, in case you're running plain 8.1.  I don't
	think it should make a difference, though; none of the CLOS patches in
	the ECO seem to relate to this.

    Gack!  This surprising behavior can be obtained simply by loading
    CLIM!  Bern, are you trying this in an environment with CLIM 1.0
    loaded?

    This bug is caused by a team effort of several random Genera/CLIM
    facilities.

    1) STREAMP is implemented in Genera by seeing if the object
    handles the :TYI or :TYI message (as opposed to, say, checking to
    see if it is a subclass of STREAM, or has a STREAMP method that
    returns T).  This is due to the long and convoluted history of
    the Genera stream implementation.

    2) The WHICH-OPERATIONS implementation for CLOS objects works by
    mapping over all the generic functions that are mapped to Flavor
    messages via the :SELECTOR option to DEFGENERIC.  For each
    function, it maps over all the methods, looking at the class of
    the method's specialization argument.  If that class is in the
    class precedence list of the class of the argument to
    WHICH-OPERATIONS, it adds that message to the operations list.

    3) CLIM uses the :SELECTOR mechanism to map STREAM-WRITE-CHAR to
    :TYO, so STREAM-WRITE-CHAR is on the list of "selector" GF's
    checked by WHICH-OPERATIONS.

    4) CLIM also defines a method for STREAM-WRITE-CHAR on the class
    T (so that STREAM-WRITE-CHAR method invocations on old Genera
    stream objects will forward to a call to WRITE-CHAR).

    So, the fact that STREAM-WRITE-CHAR is in the list of generic
    functions with selectors (in this case, to the associated message
    :TYO), and that it has a method defined on class T, and that
    class T is a superclass of your FOO class defined above, all
    means that WHICH-OPERATIONS thinks that FOO supports the :TYO
    message, and therefore STREAMP thinks that an instance of FOO is
    a stream.

    (Pant, pant)

    I am going to be pretty embarrassed if Mr. Wild is not using
    CLIM.

    I am open to suggestions for the right place to fix this.

I'm running clim 1.0 under genera 8.1

No real suggestions for a fix, but the implementation of the method 
STREAM-WRITE-CHAR on the class T seems arguable: calling
(clim:stream-write-char (clos:make-instance 'foo) #\x)
runs into an infinte loop on the symbolics.

Using clim on the Macintosh under MCL 2.0b4, calling STREAM-WRITE-CHAR with the same arguments 
results in a "reasonable" error message saying STREAM-TYO is not applicable.
Guessing from the disassembled code of the method STREAM-WRITE-CHAR on the class T 
the method basically calls STREAM-TYO on the arguments, and there is no
method for STREAM-TYO on class T (which is ok). 

Stefan.

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