How to call a method object with standard functions

Question

How does one call a method object as a function?

Closer-mop and clos packages both provide method-function for turning a method object into a function. However, is there a way to do it without including another package? And if not, which package? (Using SBCL), but if a package is needed then how does the discrimination function do it?

Here is an example of using find-method to get a method object. The question is then how to call method-to-be-called.

(defclass a () ((x :accessor x :initform 0)))
(defgeneric inc (i))
(defmethod inc ((i a)) (incf (x i)))
(defvar r (make-instance 'a))

;; ... in a land far far away:    
(defvar method-to-be-called (find-method #'inc '() '(a)))

(funcall method-to-be-called r);; crashes and burns

As a secondary question, the docs say that the discrimination function first tries to compute-applicable-methods-by-class to find a method object, and if that fails, it uses compute-applicable-methods. Why do this two layer approach? Is it correct to assume the find-method is doing this two layer approach, so it is better to use find-method ?

-- Appendix -- In the comments below Rainer Joswig pointed out that this find-method form is implementation dependent:

(find-method #'inc '() '(a))) ; works on sbcl 1.3.1

He says the specifier list should be classes and suggests instead:

(find-method #'inc '() (list (find-class 'a))))

So I thought to just put my class in there:

(find-method #'inc '() (list a))  ; crashes and burns

Apparently (defclass a ... ) does not set a to a class. In fact it doesn't set it to anything!

* (defclass a () ((x :accessor x :initform 0)))
#<STANDARD-CLASS COMMON-LISP-USER::A>
* a

... The variable A is unbound.

However, this works:

* (defvar ca (defclass a () ((x :accessor x :initform 0))))
CA
* (defmethod inc ((i a)) (incf (x i)))
WARNING: Implicitly creating new generic function COMMON-LISP-USER::INC.
#<STANDARD-METHOD COMMON-LISP-USER::INC (A) {1005EE8263}>
enter code here
* (find-method #'inc '() (list ca))   
#<STANDARD-METHOD COMMON-LISP-USER::INC (A) {1005EE8263}>
* 

So a class is the return value from the defclass, not the value of the symbol that is provided to defclass.

Answer 1

For the particular case of method-function, closer-mop for SBCL simply reexport the existing symbol from sb-pcl, as seen in closer-mop-packages.lisp. The whole file make use of read-time conditionals (see 1.5.2.1 Use of Implementation-Defined Language Features). That means that if you are working with SBCL, you might call sb-pcl:method-function (PCL means Portable Common Loops).

The generic function compute-applicable-methods-by-class allows you to know which methods are applicable given classes. This is useful if you don't have actual instances on which you can operate. It seems also that compute-applicable-methods-using-classes allows the implementation to memoize the applicable methods when the second return value is true. This generic method does not allow you to find applicable methods specialized with eql specializers.

I am speculating here, but it makes sense to fall back on compute-applicable-methods to allow for example eql-specializers or because it is slightly easier to define a method for compute-applicable-methods. Note the paragraph about consistency:

The following consistency relationship between compute-applicable-methods-using-classes and compute-applicable-methods must be maintained: for any given generic function and set of arguments, if compute-applicable-methods-using-classes returns a second value of true, the first value must be equal to the value that would be returned by a corresponding call to compute-applicable-methods. The results are undefined if a portable method on either of these generic functions causes this consistency to be violated.

I don't think there is a find-method-using-classes generic function specified anywhere.

Answer 2

(find-method #'inc '() '(a))

Above does not work. We need a list of classes, not a list of symbols.

(funcall (method-function (find-method #'inc
                                       '()
                                       (list (find-class 'a))))
         r)

Since the function method-function belongs to the MOP, many implementations provide it and it is in some implementation specific package. CLOSER-MOP makes it available, too.

But usually, if you are already trying extracting method functions, then you are probably using CLOS the wrong way or you are really knowing what you are doing...

Answer 3

How does one call a method object as a function?

Honest question: why do you want to do that? Did you specify how the method's function is built in the first place, or not?

Even with closer-mop, I believe that the function returned by closer-mop:method-function is, at most, consistent with closer-mop:make-method-lambda in terms of its lambda-list, so perhaps you can use a package to know what you can count on portably.

A method's function does not have to be a function with the same lambda-list as the generic function, and usually it isn't due to next-method-p and call-next-method. Some implementations might use dynamic bindings for the next method list, so these might have a method lambda-list congruent with the generic function. Just don't count on it, generically.

I believe SBCL is not one of these implementations, the next method list is passed to the method's function to support next-method-p and call-next-method.

Why do this two layer approach?

Because it allows memoizing (or caching) based on the list of classes, when possible. If the generic function is called again with arguments of the same classes, and the generic function has not been updated (see "Dependent Maintenance Protocol" in the MOP), it can reuse the last result without further processing, for instance, by keeping the results in a hash table which keys are lists of classes.

However, if compute-applicable-methods-using-classes returns a false second value, then compute-applicable-methods is used. The reason is that no method could be found using classes alone, and this means some method has a non-class specializer.

This is different than saying there are no applicable methods, for instance, if all methods are specialized on classes and there are no applicable methods, compute-applicable-methods-using-classes should return an empty list and a true second value. There's no point in calling compute-applicable-methods, it won't (or rather, it shouldn't, if well implemented) find anything further.

It's still possible to perform memoization when compute-applicable-methods is used, but the memoization is no longer as trivial as, for instance, using a list of classes as a key in a hash table. Perhaps you could use a tree structure where you'd try to look up a method for each specializer (instance, then class) sequentially for each argument, until a tree node matched the whole specializable parameter list.

With non-standard specializers, you'd have to change the search order for each node. Unless such specializer's priority is not strictly before, between or after eql and a class, then you're in uncharted areas.

Actually, you'll have to change compute-applicable-methods-using-classes to recognize the non-standard specializers and return false early, and you'll have to change compute-applicable-methods to process these specializers, anyway, so perhaps you'll have a good knowledge on, if possible, how to memoize with the results of compute-applicable-methods anyway.

Is it correct to assume the find-method is doing this two layer approach, so it is better to use find-method ?

No, the purpose of find-method is to find a specific method, not an applicable method. It does not use compute-applicable-methods-using-classes or compute-applicable-methods at all. In fact, it couldn't use the latter ever, as it takes actual arguments instead of specializers.