How to reduce code duplication using method combination but keeping possible early return

Question

I got a set of classes which represent a message that has to be handled. But there is only a limited amount of open spots for handlers. Therefore any "dispatch" of a handler handling an message object has to check first whether there is a free spot.

If there is -> dispatch.

If there is not -> do not dispatch and return corresponding message

As this part of the code will be the same in any dispatch method I figured it would be best to use the method combination facility to enforce that, but I cannot figure out how.

In my current code base I tried to use a :before method, but apparently you cannot use return in such context:

(defclass message () ((msg :initarg :msg :reader msg)))

(defclass message-ext (message) 
    ((univ-time :initarg :univ-time :reader univ-time)))

(defparameter *open-handler* nil)

(defgeneric handle (message)
  (:documentation "handle the given message appropriately"))

(defmethod handle :before ((message message))
  (when (> (length *open-handler*) 1)
    (return :full)))

(defmethod handle ((message message))
  (push (FORMAT nil "dispatched handler") *open-handler*))

(defmethod handle ((message-ext message-ext))
  (push (FORMAT nil "dispatched ext handler") *open-handler*))

(handle (make-instance 'message :msg "allemeineentchen"))

(handle (make-instance 'message-ext 
                       :msg "rowrowrowyourboat" 
                       :univ-time (get-universal-time)))

(handle (make-instance 'message-ext 
                       :msg "gentlydownthestreet" 
                       :univ-time (get-universal-time)))

Execution of a form compiled with errors.
Form:
  (RETURN-FROM NIL FULL)
Compile-time error:
  return for unknown block: NIL
   [Condition of type SB-INT:COMPILED-PROGRAM-ERROR]

Restarts:
 0: [RETRY] Retry SLIME interactive evaluation request.
 1: [*ABORT] Return to SLIME's top level.
 2: [TERMINATE-THREAD] Terminate this thread (#<THREAD "worker" RUNNING {100594F743}>)

Backtrace:
  0: ((SB-PCL::FAST-METHOD HANDLE :BEFORE (MESSAGE)) #<unavailable argument> #<unavailable argument> #<unavailable argument>)
  1: ((SB-PCL::EMF HANDLE) #<unavailable argument> #<unavailable argument> #<MESSAGE-EXT {1005961733}>)
  2: (SB-INT:SIMPLE-EVAL-IN-LEXENV (HANDLE (MAKE-INSTANCE 'MESSAGE-EXT :MSG "gentlydownthestreet" :UNIV-TIME (GET-UNIVERSAL-TIME))) #<NULL-LEXENV>)
  3: (EVAL (HANDLE (MAKE-INSTANCE 'MESSAGE-EXT :MSG "gentlydownthestreet" :UNIV-TIME (GET-UNIVERSAL-TIME))))
  4: ((LAMBDA () :IN SWANK:INTERACTIVE-EVAL))

Is this approach even sane, and if yes how can I do it in a working fashion? (I did already try return-from with the same result)

Answer 1

I think you should be using the :around method qualifier instead:

(defmethod handle :around ((message message))
  (if (cddr *open-handler*)
      :full
      (call-next-method)))

However, a more "lispy" approach is to use the CL Condition System, e.g., something like this:

(define-condition too-many-messages (...) (...) ...)
(defun add-message (message)
  (when (cddr *open-handler*)
    (signal 'too-many-messages))
  (push message *open-handler*))
(defmethod handle ((message message))
  (add-message (FORMAT nil "dispatched handler")))

You will have to handle the condition (using, e.g., handler-bind) in addition to checking the return values of your handle function.

PS. Calling length on a list to check that it is long enough is not a very good idea - although in your case, when the list is guaranteed to be short, this might be more of a style issue.

PPS. It is not a very good idea to use the word handle as a name of your function because CL has functions which contain it (e.g., handler-case). This will complicate the search in your code in addition to confusing people reading your code.