Re: Caching of effective methods in custom method combinations

On 21 Jul 2015, at 09:46, Edi Weitz edi@weitz.de wrote:

Hi everybody,

Here's some behavior I see on SBCL as well as LispWorks, so I'm assuming for now they're right in doing so. My question would be why they are.

I start with the following code which I compile and load:

;;;;;;;;;;;;;;;;;;;;;;;;

(defvar *current-qualifiers* '(:a :b :c))

(define-method-combination test () ((methods *)) (let ((selected-methods (loop for method in methods when (intersection (method-qualifiers method) *current-qualifiers*) collect method))) `(call-method ,(first selected-methods) ,(rest selected-methods))))

(defgeneric foo (thing) (:method-combination test)) (defmethod foo :a (thing) '(:red)) (defmethod foo :b (thing) '(:blue))

;;;;;;;;;;;;;;;;;;;;;;;;;;

Now, in the REPL I do the following:

CL-USER> (foo 42) (:BLUE) CL-USER> (setq *current-qualifiers* '(:a :c)) (:A :C) CL-USER> (foo 42) (:BLUE)

I almost expected this. The effective method obviously isn't computed anew but was cached. But even if I now re-evaluate the DEFINE-METHOD-COMBINATION form, (FOO 42) will still return (:BLUE). Only if I re-evaluate the DEFGENERIC form will the return value change to (:RED).

There is no protocol, neither in Common Lisp, nor in AMOP, that specifies what happens when a method combination is redefined. I believe an implementation would even be right to completely ignore redefinitions. What you’re seeing when you evaluate the defgeneric form again is that this triggers a reinitialize-instance of the generic function metaobject with the new method combination object.

My question is if the standard somewhere allows this caching to happen. That would for example mean that any kind of "dynamic method combination" (for example based on the time of the day, just for grins) is impossible.

The standard doesn’t /disallow/ this form of caching to happen. You can indeed not express something like “dynamic method combinations” - you should rather define your own generic function metaobject class to do something like this.

Pascal

-- Pascal Costanza The views expressed in this email are my own, and not those of my employer.