[5-7] Common CLOS Blunders

This question is based on a document written by Marty Hall
<hall@aplcenmp.apl.jhu.edu>. The full text of Marty's
"Introduction to CLOS" handout is available by anonymous ftp from

This question lists a variety of common errors that occur when writing
CLOS code. It is extremely useful to glance through this list when
debugging CLOS code.

[A] Omitting a set of parentheses around the arglist in DEFMETHOD.

    For example, writing 
       (defmethod Area (Sq Square) ...)
    instead of
       (defmethod Area ((Sq Square)) ...)

    Lisp will accept the former, and think that you have two
    unspecialized arguments instead of one argument specialized as a

[B] Missing parentheses around the slot definition list in DEFCLASS.

    For example, writing
       (defclass Rectangle (Polygon)
	 (Width ...)
	 (Height ...)) 
    instead of
       (defclass Rectangle (Polygon)
	 ((Width ...)
	  (Height ...)))

    Lisp will not accept the former, but the error message doesn't
    always clearly identify the problem.

[C] Forgetting to include an empty slot definition list if you don't
    define local slots in DEFCLASS.

    For example, writing 
       (defclass Square (Rectangle)) 
    instead of
       (defclass Square (Rectangle) ())
    Lisp will not accept the former.

[D] Referring to the class name instead of the instance variable in

    For example, writing 
       (defmethod Area ((Sq Square)) 
	 (* (Width Square) (Width Square))) 
    instead of
       (defmethod Area ((Sq Square)) 
	 (* (Width Sq) (Width Sq)))

    Lisp may give you a warning about an unknown free variable, but
    probably won't even do that if you type the defmethod directly into
    the Lisp Listener (Lucid doesn't). So you might not encounter a
    problem until run-time.

[E] Forgetting that accessors are functions and thus could conflict
    with built-in function names.

    For example, writing
       (defclass Graphical-Object ()
	 ((Position :accessor Position)))
    will signal an error, since you cannot redefine the built-in
    POSITION function.  

[F] Putting the new value last instead of first in the definition of a 
    SETF method. 

    For example, writing 
       (defmethod (setf Area) ((Sq Square) (New-Area number))
	 (setf (Width Sq) (sqrt New-Area))) 
    instead of
       (defmethod (setf Area) ((New-Area number) (Sq Square))
	 (setf (Width Sq) (sqrt New-Area)))

   Lisp will accept the former, causing users to be later puzzled as
   to why (setf (Area Square-1) 10) doesn't work.

[G] Putting the new value last instead of first in a call to a :writer

    For example, given
       (defclass Circle () 
	 ((Radius :reader Radius :writer Set-Radius :initform 5)))
       (setq Circle-1 (make-instance 'Circle))
    and writing
       (Set-Radius Circle-1 10)
    instead of
       (Set-Radius 10 Circle-1)

[H] Confusion about documentation strings in DEFMETHOD. 

    People often write code like
       (defmethod Area ((Rect Rectangle))
	 "WIDTH times HEIGHT of the rectangle"
	 (* (Width Rect) (Height Rect)))
    without clearly thinking about what this might mean.  Some people
    think it will make a documentation string on the generic function that
    can be retrieved by calling (DOCUMENTATION 'Area 'function) or the
    equivalent emacs keystrokes. Others vaguely expect it to make a doc
    string on each separate method, and that the call to DOCUMENTATION
    will somehow be able to automagically figure out which method it
    applies to.

    In fact, Lisp won't complain about this code, with the result that the
    documentation is added to the method *object*, which beginners
    probably know nothing about. 

    Use the :documentation entry in defgeneric to add a documentation
    string to the generic function.

[I] Invalid :initargs are accepted by MAKE-INSTANCE.

    Many Lisp implementations will accept unknown keyword initargs without
    complaint, even at the highest safety settings. 

    So the following code, which includes a typo of :SLOT1 instead of
    :SLOT-1 in the call to make-instance
       (defclass Foo ()
	((Slot-1 :accessor Slot-1 :initarg :Slot-1 :initform 5)))

       (setq Test (make-instance 'Foo :Slot1 10)) 
    will not produce an error message, with the result that
    (Slot-1 Test) returns 5, not 10.

    This is a bug in the implementation; all implementations are
    supposed to flag this as an error. 

[J] Forgetting the class must exist before any method can specialize upon it. 

    Lisp programmers are used to being able to define functions in any
    order, where even if FOO calls BAR, FOO can be defined first. But doing
       (defmethod Area ((Rect Rectangle)) ...)
       (defclass Rectangle (Polygon) ...)
    is illegal. You have to define the class first.

    If while trying to debug the problem, you define the class by
    evaluating the definition without reordering the code to put the
    class first, you'll only run into problems later the next time you
    try to recompile the code from scratch.

    Many experienced programmers put each class definition in its own
    file at the top of the file, with methods for that class after it
    in the file. Others opt to put all the class definitions for a
    program in a single file that is loaded first, with methods in files
    that are loaded later.

[K] Changing a method to apply to a more general class does not
    supersede previous method. 

    For example, suppose a user writes
       (defmethod Half-Area ((Rect Filled-Rectangle))
	 (/ (Area Rect) 2))
    and then notices that this functionality could apply to all
    Rectangles, not just Filled-Rectangles. They might then change the
    class referenced in this method, under the specious intuition that by
    changing the old defintion, they are replacing it. In fact, they are
    actually adding a new, less-specific method.

    Now suppose that later on, they change this new method (in this
    example, by adding a call to FLOAT to avoid returning a ratio),
       (defmethod Half-Area ((Rect Rectangle))
	 (float (/ (Area Rect) 2)))
    If they test it on an instance of Filled-Rectangle, they will be
    puzzled as to why their new definition appears to have not taken
    effect. But because the old definition specialized to Filled-Rectangle 
    still exists, they're actually getting the old, more-specific definition.

    Moreover, the next time they restart a fresh Lisp and recompile
    the code, the problem will magicly disappear, since the old
    definition is no longer in the code. 

;;; *EOF*
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