These notes correspond to the "August 5 92 PCL" version of PCL.

  This version of PCL is much closer than previous versions of PCL
to the metaobject protocol specified in "The Art of the Metaobject Protocol", 
chapters 5 and 6, by Gregor Kiczales, Jim des Riveres, and Daniel G. Bobrow.


[Please read the file may-day-notes.text also.  Most of that file still applies.]

Support for structures
  You can use structure-class as a metaclass to create new classes.
  Classes created this way create and evaluate defstruct forms which
  have generated symbols for the structure accessors and constructor.
  The generated symbols are used by the primary slot-value-using-class
  methods and by the primary allocate-instance method for structures.
  Defmethod optimizes usages of slot-value (when no user-defined 
  slot-value-using-class methods exist) into direct calls of the
  generated symbol accessor, which the compiler can then optimize further.
  Even when there are user-defined methods on slot-value-using-class,
  PCL does a variety of optimizations.
  
  If your implementation's version of the *-low.lisp file
  contains definitions of certain structure functions (see the end of
  low.lisp, cmu-low.lisp, lucid-low.lisp, and kcl-low.lisp), then
  structure classes are supported for all defstructs.  In this case,
  structure classes are created automatically, when necessary.

New Classes:
structure-class
structure-object
slot-class
slot-object
structure-direct-slot-definition
structure-effective-slot-definition

Improvements to slot-access
  Optimization for slot-value outsize of defmethod
  Optimization for slot-value inside of defmethod, but not of a specialized parameter.
  Optimizations that work even when there are :around methods
  on slot-value-using-class.

New types: 
   `(class ,class-object)
   `(class-eq ,class-object)

New specializer class: class-eq-specializer
  Every class has a class-eq specializer which represents all
  the direct instances of that class.
  This is useful in *subtypep.  For example, here is the way 
  generic-function initialization checks that the method-class is valid:
   (and (classp method-class)
	(*subtypep (class-eq-specializer method-class)
		   (find-class 'standard-method)))
  If you want to define methods having class-eq specializers,
  see "Initialization of method metaobjects".  The default behavior of PCL
  is to disallow this.

compute-applicable-methods-using-types

caching improvements

no magic-generic-functions list
  This simplifies some things, but complicates some other things.
  I wanted to support user-defined classes which are their own metaclass.
  You can now do:
(defclass y (standard-class) ())
(defmethod validate-superclass ((c y) (sc standard-class)) t)
(defclass y (standard-class) () (:metaclass y))

method-function changes (see the comments for make-method-lambda, below)

final dfuns

-------------------------

gfs which obey AMOP ch 6
  accessor-method-slot-definition
  add-dependent
  add-direct-method
  add-direct-subclass
  add-method
  allocate-instance
  compute-class-precedence-list
  compute-default-initargs
  compute-discriminating-function  
  compute-effective-slot-definition
[Note: compute-effective-slot-definition relys on 
 compute-effective-slot-definition-initargs and effective-slot-definition-class.
 compute-effective-slot-definition-initargs is quite useful, but is not in
 AMOP ch 6.]
  compute-slots
  direct-slot-definition-class
  effective-slot-definition-class
  ensure-class
  ensure-class-using-class
  ensure-generic-function
  ensure-generic-function-using-class
  eql-specializer-object
  extract-lambda-list
  extract-specializer-names
  finalize-inheritance
  find-method-combination
  funcallable-standard-instance-access
  {generic-function-method-class, generic-function-method-combination,
   generic-function-lambda-list, generic-function-methods, generic-function-name}
  intern-eql-specializer
  make-instance
  make-method-lambda
  map-dependents
  {method-function, method-generic-function, method-lambda-list,
   method-specializers, method-qualifiers}
  {class-default-initargs, class-direct-default-initargs, class-direct-slots,
   class-direct-subclasses, class-direct-superclasses, class-finalized-p,
   class-name, class-precedence-list, class-prototype, class-slots}
  {slot-definition-allocation, slot-definition-initargs, slot-definition-initform,
   slot-definition-initfunction, slot-definition-name, slot-definition-type}
  {slot-definition-readers, slot-definition-writers}
  {slot-definition-location}
  remove-dependent
  remove-direct-method
  remove-direct-subclass
  remove-method
  set-funcallable-instance-function
  (setf slot-value-using-class)
  slot-boundp-using-class
  slot-makunbound-using-class
  specializer-direct-generic-functions
  specializer-direct-methods
  standard-instance-access
  update-dependent

gfs which DO NOT obey AMOP ch 6

compute-applicable-methods
compute-applicable-methods-using-classes
  Handles class-eq specializers without signalling an error.  
  But see "Initialization of method metaobjects", below.

compute-effective-method
  Returns only one value. 

generic-function-argument-precedence-order
  Not yet defined.  Can get this information from the arg-info structure.

generic-function-declarations
  Not yet defined.

reader-method-class
  Not yet defined.  Some bootstrapping considerations are involved, 
  but adding this will not be very hard.

(setf class-name)
  Currently just a writer method.  Does not call reinitialize-instance or
  (setf find-class).

(setf generic-function-name)
  Currently just a writer method.  Does not call reinitialize-instance.

writer-method-class
  Not yet defined.  Some bootstrapping considerations are involved, 
  but adding this will not be very hard.

---------------------------

Initialization of method metaobjects
  The following methods are defined:
    legal-qualifiers-p (method standard-method) qualifiers
    legal-lambda-list-p (method standard-method) lambda-list
    legal-specializers-p (method standard-method) specializers
    legal-method-function-p (method standard-method) function
    legal-documentation-p (method standard-method) documentation

    legal-specializer-p (method standard-method) specializer

  You can override them if you want.
  The method for legal-specializers-p calls legal-specializer-p
  on each specializer.
  The method for legal-specializer-p allows any kind of specializer
  when the variable *allow-experimental-specializers-p* is t
  (this variable is initially nil).

---------------------------
Optimizations on slot-value
  Outside of a defmethod when define-compiler-macro is not implemented
  or the slot-name is not constant, or
  Inside a defmethod when the slot-name is not a constant:
(1)   no optimization of slot-value, slot-value-using-class is called.
      slot-value-using-class has a special dfun, though, which calls
      the slot's slot-definition-reader-function.  This function is
      a result of get-accessor-method-function.
  Outside of a defmethod when define-compiler-macro is implemented and
  the slot-name is a constant, or
  Inside a defmethod when the slot-name is a constant but the object is 
  not either (the value of a parameter specialized to a subclass of structure-object
  for which no user-defined slot-value-using-class methods apply at defmethod time), 
  or (the value of a parameter specialized to a subclass of standard-object).
(2)   PCL arranges to call an automatically created generic function
      which has one method: a reader method defined on class slot-object.
  Inside a defmethod when the slot-name is a constant and the object 
  is (the value of a parameter specialized to a subclass of structure-object
  for which no user-defined slot-value-using-class methods apply).
(3)   The slot-value form is converted to a call of the structure slot's 
      accessor function, which the compiler can then optimize further.
  Inside a defmethod when the slot-name is a constant and the object 
  is (the value of a parameter specialized to a subclass of standard-object).
(4)   The access requires two arefs, a call to (typep x 'fixnum), and a call to eq,
      in the best case.  If user defined slot-value-using-class methods apply
      at slot-value execution time, or the slot is unbound, the unoptimized 
      slot-value function (1) is called.  This was in May Day PCL; what is new here
      is that the PV (permutation vector) is looked up at defmethod load time
      rather than at run time, if the effective method is cached.

Generic functions containing only accessor methods for which no user-defined
methods on slot-value-using-class apply and which involve only standard-classes:
      A special dfun is called: one-class, two-class, one-index, or n-n.
      These were all in May Day PCL.
Generic functions excluded by the above, which contain accessor methods:
      In place of each accessor method's method-function, a function returned by
      get-accessor-method-function is used.

get-accessor-method-function (gf type class slotd) ; type is reader, writer, or boundp.
  If there is only one applicable method,
      (This method will be the system supplied one)
      the function that is returned is optimized for the current state of the
      class.  When the class changes, it must be recomputed.
  otherwise,
      a secondary dispatch function for slot-value-using-class is computed
      (using what is known about the types of the arguments) and converted 
      into an accessor function.

get-secondary-dispatch-function (gf methods types &optional method-alist wrappers)
  The types argument describes what is known about the types of the arguments.
  Method-alist is used (when supplied) to do things like replace the
  standard slot-value-using-class method function with a function optimized
  for what is known about the arguments.
  Wrappers (when supplied) means that the resulting function is guaranteed to
  be called only whith those wrappers.  Make-effective-method-function calls
  the generic-function method-function-for-caching with method-alist and
  wrappers to get a optimized method function.  (PV lookup is done at the time
  method-function-for-caching is called).

compute-applicable-methods:  Here I tried to stick with the MOP.
  The function cache-miss-values looks at the second value of the result of 
  compute-applicable-methods-using-classes.  If this value is null, we aren't 
  supposed to cache the result of camuc.  So we don't.  Instead we cache a 
  result of (default-secondary-dispatch-function gf), which in turn calls 
  compute-applicable-methods each time it is called.
---------------------------

To do:

Problem: sometimes there is no need to call a gf's dfun: the emf that is invoked
         can be cached in the caller's method closure.
1.  In expand-defmethod-internal, optimize calls to generic-functions.
  Add the support for this optimization.

2.   [When CMUCL improves its setf handling, remove the comment in
   the file macros.lisp beginning the line ";#+cmu (pushnew :setf *features*)"]



--------------
1) Generalize expand-defmethod-internal so that it can be used for non-defmethod
code.  Maybe by (a) having a parameter that says whether it is being called by 
defmethod, and (b) using the techniques used by the series package (shadowing 
defun and some others, making the shadowed definitions call e-d-i, making it 
easy for people to do the relevant package modifications)

2) Extending generic-functions by allowing the user at defgeneric time to supply
the name of a function which will be supplied (by the system) with a definition
which will return equivalent results to those returned by the generic function,
but which will (in some cases) have less checking than the generic function.
One-class, two-class, and one-index gf dfuns will map to a result of 
get-optimized-std-accessor-method-function, checking gf dfuns will map to their
function, and any other dfun will remain the same.

3) Extending expand-defmethod-internal to optimize calls to generic-functions.
There are a variety of possibilities that need to be considered; one of them
will be to arrange to call the optimized functions produced by (2) when it
is known to be safe.  
