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)
I think you should be using the
:around
method qualifier instead:However, a more "lispy" approach is to use the CL Condition System, e.g., something like this:
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.