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Copyright (c) Xerox Corporation 1988. All rights reserved.
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These notes correspond to the "8/24/88 (beta) AAAI PCL" version of PCL.
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Please read this entire document carefully.
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There have been a number of changes since the 8/2/88 version of PCL. As
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usual, these changes are part of our efforts to make PCL conform with
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the CLOS specicification (88-002R). This release contains the big
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changes which the 7/7 through 8/2 releases were really getting ready
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This version of PCL has been tested at PARC in the following Common
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Xerox Medley (aka EnvOS Medley)
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KCL (October 15, 1987)
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Most of the changes in this version of PCL fall into one of two
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The first major set of changes makes the order of arguments to setf
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generic functions and methods conform with the spec. In addition, these
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changes allow the first argument to defmethod to be of the form (SETF
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The second major set of changes have to do with slot access and instance
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structure. Importantly, PCL now checks to see if a slot is bound, and
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calls slot-unbound if the slot is unbound. This is a major change from
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previous releases in which slot access just returned NIL for slots which
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had not yet been set. These changes affect all the functions which
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access the slots of an instance. In addition, the generic functions
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which are called by the slot access functions in exceptional
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circumstances are affected. This set of changes also include the
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implemenentation of the real initialization protocol as specified by
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In addition, there are a number of other changes. The most significant
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of these has to do with the symbols which the PCL package exports by
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The rest of this document goes on to first describe the slot access
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changes, then describe the setf generic function changes, and finally
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describe some of the other minor changes.
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At the very end of this file is a new section which lists PCL features
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which are scheduled to disappear in future releases. Please read this
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section and take it to heart. This features will be disappearing.
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*** Changes to slot access and instance structure ***
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This release includes a number of changes to the way slot access works
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in PCL. Some of these changes are incompatible with old behavior. Code
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which was written with the actual CLOS spec in mind should not be
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affected by these incompatible changes, but some older code may be
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The basic thrust of the changes to slot access is to bring the following
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functions and generic functions in line with the specification:
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slot-boundp-using-class
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slot-exists-p-using-class
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slot-makunbound-using-class
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slot-value-using-class
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(setf slot-value-using-class)
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make-instances-obsolete
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make-instance (temporarily called *make-instance)
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initialize-instance (temporarily called *initialize-instance)
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update-instance-for-different-class
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update-instance-for-redefined-class
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In this release, these functions accept the specified number of
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arguments, return the specified values, have the specified effects, and
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are called by the rest of PCL in the specified way at the specified
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times (with the exception that PCL does not yet call *make-instance to
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create its own metaobjects). Because PCL now checks for unbound slots,
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you may notice a slight performance degradation in certain applications.
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For complete information, you should of course see the CLOS specification.
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The rest of this note is a short summary of how this new behavior is
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different from the last release.
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- Dynamic slots are no longer supported. Various functions like
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slot-value-always and remove-slot no longer exist. Also,
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slot-value-using-class now only accepts the three arguments as
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described in the spec. The two extra arguments having to do with
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dynamic slots are no longer accepted.
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Shortly, we will release a metaclass which provides the now missing
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dynamic slot behavior.
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- slot-missing now receives and accepts different arguments.
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- slot-unbound is now implemented, and is called at the appropriate
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- the initialization protocol specified in 88-002R is now almost
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completely implemented. The only difference is that the current
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implementation does not currently check the validity of initargs.
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So, no errors are signalled in improper initargs are supplied.
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Because of name conflicts with the two other initialization protocols
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PCL currently supports, some of the specified initialization functions
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do not have their proper name. The mapping between names in the
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specification and names in this version of PCL is as follows:
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make-instance *make-instance
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initialize-instance *initialize-instance
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reinitialize-instance <has proper name>
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update-instance-for-different-class <has proper name>
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update-instance-for-redefined-class <has proper name>
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shared-initialize <has proper name>
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In a future release of PCL, these functions will have their proper
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names, and all the old, obsolete initialization protocols will
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Convert to using this new wonderful initialization protocol soon.
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Sometime soon we will release a version of PCL which does significant
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optimization of calls to make-instance. This should speed up instance
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creation dramatically, which should significantly improve the
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performance of some programs.
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- The function all-slots no longer exists. There is a new generic
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function called slots-to-inspect, which controls the default behavior
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of describe. It also controls the default behavior of the inspector
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in ports which have connected their inspectors to PCL. It specifies
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which slots of a given class should be inspected. See the definition
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in the file high.lisp for more.
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- the metaclass obsolete-class no longer exists. The mechanism by which
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instances are marked as being obsolete is now internal, as described
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in the spec. The generic-function make-instances-obsolete can be used
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to force the instances of a class to go through the obsolete instance
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update protocol (see update-instance-for-redefined-class).
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- all-std-class-readers-miss-1, a generic function which was part of
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the database interface code I sent out a few weeks ago, has a slightly
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different argument list. People using the code I sent out a few weeks
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ago should replace the definition there with:
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(defmethod all-std-class-readers-miss-1
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((class db-class) wrapper slot-name)
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(declare (ignore wrapper slot-name))
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- The implementation of the slot access generic functions have been
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considerably streamlined. The impenetrable macrology which used to be
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- Because the behavior of the underlying slot access generic functions
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has changed, it is possible that some user code which hacks the
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underlying instance structure may break. Most of this code shouldn't
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break though. There have been some questions on the mailing list
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about what is the right way to modify the structure of an instance.
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I am working on that section of chapter 3 right now, and will answer
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those questions sometime soon.
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*** Changes to SETF generic functions ***
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This release of PCL includes a significant change related to the order
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of arguments of setf generic functions. To most user programs, this
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change should be invisible. Your program should run just fine in the
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new version of PCL. Even so, there is some conversion you should do to
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your program, since DEFMETHOD-SETF is now obsolete and will be going
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Some programs may take some work to adapt to this change. This will
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be particularly true of programs which manipulated methods for setf
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generic-functions using make-instance, add-method and friends.
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Included here is a brief overview of this change to PCL. Most people
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will find that this is all they need to know about this change.
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The CLOS specification assumes a default behavior for SETF in the
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absence of any defsetf or define-modify-macro. The default behavior is
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to expand forms like:
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(FUNCALL #'(SETF FOO) a x y)
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The key point is that by default, setf expands into a call to a function
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with a well-defined name, and that in that call, the new value argument
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comes before all the other arguments.
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This requires a change in PCL, because previously, PCL arranged for the
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new-value argument to be the last required argument. This change
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affects the way automatically generated writer methods work, and the way
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that defmethod with a first argument of the form (SETF <symbol>) works.
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An important point is that I cannot implement function names of the form
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(SETF <symbol>) portably in PCL. As a result, in PCL, I am using names
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of the form |SETF FOO|. Note that the symbol |SETF FOO| is interned in
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the home package of the symbol FOO. (See the description of the
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GET-SETF-FUNCTION and GET-SETF-FUNCTION-NAME).
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The user-visible changes are:
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- DEFMETHOD will accept lists of the form (SETF FOO) as a first
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argument. This will define methods on the generic function named
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by the symbol |SETF FOO|. As specified in the spec, these methods
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should expect to receive the new-value as their first argument.
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Calls to defmethod of this form will also arrange for SETF of FOO to
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expand into an appropriate call to |SETF FOO|.
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- Automatically generated writer methods will expect to receive the new
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value as their first argument.
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- DEFMETHOD-SETF will also place the new-value as the first argument.
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This is for backward compatibility, since defmethod-setf itself will
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be obsolete, and you should convert your code to stop using it.
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- GET-SETF-FUNCTION is a function which takes a function name and
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returns the setf function for that function if there is one. Note
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that it doesn't take an environment argument. Note that this function
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is not specified in Common Lisp or CLOS. PCL will continue to support
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it as an extra export indefinetely.
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- GET-SETF-FUNCTION-NAME is a function which takes a function name
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and returns the symbol which names the setf function for that
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function. Note that this function is not specified in Common Lisp
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or CLOS. PCL will continue to support it as an extra export
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- For convenience, PCL defines a macro called DO-STANDARD-DEFSETF which
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can be used to do the appropriate defsetf. This may be helpful for
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programs which have calls to setf of a generic-function before any
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of the generic function's method definitions. A use of this macro
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(do-standard-defsetf position-x)
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Afterwards, a form like (SETF (POSITION-X P) V) will expand into a
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form like (|SETF POSITION-X| V P).
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The reason you may have to use do-standard-defsetf is that I cannot
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portably change every implementations SETF to have the new default
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behavior. The proper way to use this is to take an early file in
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your system, and put a bunch of calls to do-standard-defsetf in it.
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Note that as soon as PCL sees a defmethod with a name argument of
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the form (SETF FOO), or it sees a :accessor in a defclass, it will
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do an appropriate do-standard-defsetf for you.
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In summary, the only things that will need to be changed in most
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programs is that uses of defmethod-setf should be converted to
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appropriate uses of defmethod.
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Here is an example of a typical user program which is affected by this
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(defclass position ()
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((x :initform 0 :accessor pos-x)
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(y :initform 0 :accessor pos-y)))
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(defclass monitored-position (position)
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(defmethod-setf pos-x :before ((p monitored-position)) (new)
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(format *trace-output* "~&Changing x coord of ~S to ~D." p new))
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(defmethod-setf pos-y :before ((p monitored-position)) (new)
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(format *trace-output* "~&Changing y coord of ~S to ~D." p new))
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To bring this program up to date, you should convert the two
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defmethod-setf forms as follows:
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(defmethod (setf pos-x) :before (new (p monitored-position))
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(format *trace-output* "~&Changing x coord of ~S to ~D." p new))
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(defmethod (setf pos-y) :before (new (p monitored-position))
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(format *trace-output* "~&Changing y coord of ~S to ~D." p new))
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*** Other changes in this release ***
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* The symbols exported by the PCL package have now changed. The PCL
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package now exports the symbols listed in the table of contents of
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chapter 2 of the spec. This list of symbols is the value of the
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variable pcl::*exports*.
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Following is the list of symbols which were exported in the 8/2/88
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version but which are not exported in the 8/18/88 version.
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DEFMETHOD-SETF DEFGENERIC-OPTIONS DEFGENERIC-OPTIONS-SETF
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CLASS-CHANGED CLASS-NAMED SYMBOL-CLASS
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CBOUNDP GET-METHOD GET-SETF-GENERIC-FUNCTION
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Following is the list of symbols which are exported in the 8/18/88
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version, but which were not exported in previous versions:
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CALL-METHOD CLASS-NAME COMPUTE-APPLICABLE-METHODS
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DEFGENERIC ENSURE-GENERIC-FUNCTION FIND-METHOD
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FUNCTION-KEYWORDS GENERIC-FLET GENERIC-LABELS
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INITIALIZE-INSTANCE MAKE-INSTANCES-OBSOLETE NO-APPLICABLE-METHOD
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NO-NEXT-METHOD REINITIALIZE-INSTANCE SHARED-INITIALIZE
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SLOT-BOUNDP SLOT-EXISTS-P SLOT-MAKUNBOUND
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SLOT-MISSING SLOT-UNBOUND SYMBOL-MACROLET
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UPDATE-INSTANCE-FOR-DIFFERENT-CLASS
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UPDATE-INSTANCE-FOR-REDEFINED-CLASS
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It should be noted that not all of these newly exported symbols have
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been "implemented" yet.
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* Any program written using PCL will need to be completely recompiled
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to run with this release of PCL.
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* The generic-function generic-function-pretty-arglist now returns a
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nice arglist for any generic function. It combines all the keyword
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arguments accepted by the methods to get the combined set of keywords.
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In some ports, the environment specific ARGLIST function has been
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connected to this, and so the environments will print out nice arglists
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for generic functions.
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* Some bugs in trace-method have been fixed. Trace-method should now
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work in all ports of PCL.
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* NO-MATCHING-METHOD has been renamed to NO-APPLICABLE-METHOD. In
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addition, it now receives arguments as specified.
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* defmethod has been modified to allow macros which expand into
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* The :documentation slot option is now accepted in defclass forms. The
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documentation string put here cannot yet be retrieved using the
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documentation function. That will happen in a later release.
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* The :writer slot option is now implemented.
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* Some brain damage in high.lisp which caused method lookup to work
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incorrectly for built in classes. In addition, it caused the
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class-local-supers and class-direct-subclasses of the built in classes
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to be strange. People using CLOS browsers should notice this change
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dramatically, as it will make the browse of the built in part of the
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class lattice look right.
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*** Older Changes ***
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Following are changes which appeared in release of PCL from 7/7/88 to
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8/2/88. Each change is marked with the release it appeared in.
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Loading defclass forms should be much faster now. The bug which caused
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all the generic functions in the world to be invalidated whenever a
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class was defined has now been fixed.
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Loading defmethod forms should also be much faster. A bug which caused
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a tremendous amount of needles computation whenever a method was also
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A bug which caused several slots of the classes T, OBJECT, CLASS and
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STANDARD-CLASS to be unbound has been fixed.
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load-pcl now adds the symbols :PCL and :PORTABLE-COMMONLOOPS to
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PCL still doesn't do any sort of call to PROVIDE because of the total
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lack of uniformity in the behavior of require and provide in the various
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common lisp implementations.
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This version of PCL finally fixes the horrible bug that prevented
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the initform for :class allocation slots from being evaluated when the
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PCL now converts the function describe into a generic function of one
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argument. This is to bring it into conformance with the spec as
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In Symbolics Genera, it is actually a function of one required and one
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optional argument. This is because the 3600 sometimes calls describe
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with more than one argument.
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In Lucid Lisp, describe only takes an optional argument. This argument
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defaults to the value of *. PCL converts describe to a generic function
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of one required argument so it is not possible to call describe with
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class-named and symbol-class have been replaced by find-class.
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find-class is documented in 88-002R.
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with-slots and with-accessors now conform to 88-002R.
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The old definition of with-slots is now called obsolete-with-slots. The
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same is true for with-accessors.
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with-slots ---> obsolete-with-slots
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with-accessors --> obsolete-with-accessors
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The temporary correct definition of with-slots, with-slots* is now
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called with-slots. The same is true for with-accessors*.
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with-slots* --> with-slots
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with-accessors* -> with-accessors
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The class-precedence list of the class null now conforms to 88-002R.
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In previous releases of PCL, the class precedence-list of the class
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null was: (null list symbol sequence t). In this release the class
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precedence list of the class null is: (null symbol list sequence t).
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This change was made to bring PCL into conformance with the spec.
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print-object now takes only two arguments.
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This changes was made to begin bringing print-object in conformance with
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88-002R. print-object conforms to the spec to the extent that is is
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called at the approrpiate times for PCL instances. In most
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implementations, it is not called at the appropriate times for other
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instances. This is not under my control, encourage your vendor to
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provide the proper support for print-object.
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This version of PCL now includes a beta test version of a new iteration
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package. This iteration package was designed by Pavel Curtis and
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implemented by Bill vanMelle. This iteration package is defined in the
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file iterate.lisp. Please feel free to experiment with it. We are all
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very interested in comments on its use.
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*** PCL Features that will be disappearing ***
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This section describes features in PCL that will be disappearing in
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future releases. For each change, I try to give a release date after
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which I will feel free to remove this feature. This list should not be
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considered complete. Certain other PCL features will disappear as well.
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The items on this list are the user-interface level items that it is
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possible to give a lot of warning about. Other changes will have more
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subtle effects, for example when the lambda-list congruence rules are
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- :accessor-prefix in defclass
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Can disappear anytime after 8/29.
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Warning that this is obsolete has been out for some time. You should
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use :accessor in each of the slot specifications of the defclass form.
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It is true that this is slightly more cumbersome, but the semantic
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difficulties associated with :accesor-prefix are even worse.
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- :constructor in defclass
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Can disappear anytime after 8/29.
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Warning that this is obsolete has been out for some time. It will be
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disappearing shortly because the intialization protocol which it goes
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with will be disappearing. A future release of PCL will support a
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special mechanism for defining functions of the form:
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(defun make-foo (x y &optional z)
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(make-instance 'foo 'x x :y y :z z))
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In the case where there are only :after methods on initialize-instance
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and shared-initialize, these functions will run like the wind. We hope
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to release this facility by 9/15.
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- old definition of make-instance, intialize, initialize-from-defaults,
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initialize-from-init-plist
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Can disappear anytime after 8/29.
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Convert to using the new initialization protocol as described in the
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- mki, old definition of initialize-instance
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Can disappear anytime after 8/29.
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Convert to using the new initialization protocol as described in the
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Can disappear anytime after 9/15.
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Convert to using (defmethod (setf foo) ...
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pcl/notes/8-28-88-notes.text
