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----
-
-title: Common Lisp argument precedence order parameterization of a generic function
-
-date: 2021-04-24 2
-
-layout: post
-
-lang: en
-
-ref: common-lisp-argument-precedence-order-parameterization-of-a-generic-function
-
----
-
-When CLOS dispatches a method, it picks the most specific method definition to the argument list:
-
-```lisp
-
-* (defgeneric a-fn (x))
-#<STANDARD-GENERIC-FUNCTION A-FN (0) {5815ACB9}>
-
-* (defmethod a-fn (x) :default-method)
-#<STANDARD-METHOD A-FN (T) {581DB535}>
-
-* (defmethod a-fn ((x number)) :a-number)
-#<STANDARD-METHOD A-FN (NUMBER) {58241645}>
-
-* (defmethod a-fn ((x (eql 1))) :number-1)
-#<STANDARD-METHOD A-FN ((EQL 1)) {582A7D75}>
-
-* (a-fn nil)
-:DEFAULT-METHOD
-
-* (a-fn "1")
-:DEFAULT-METHOD
-
-* (a-fn 0)
-:A-NUMBER
-
-* (a-fn 1)
-:NUMBER-1
-```
-
-CLOS uses a similar logic when choosing the method from parent classes, when multiple ones are available:
-
-```lisp
-* (defclass class-a () ())
-
-#<STANDARD-CLASS CLASS-A {583E0B25}>
-* (defclass class-b () ())
-
-#<STANDARD-CLASS CLASS-B {583E7F6D}>
-* (defgeneric another-fn (obj))
-
-#<STANDARD-GENERIC-FUNCTION ANOTHER-FN (0) {583DA749}>
-* (defmethod another-fn ((obj class-a)) :class-a)
-; Compiling LAMBDA (.PV-CELL. .NEXT-METHOD-CALL. OBJ):
-; Compiling Top-Level Form:
-
-#<STANDARD-METHOD ANOTHER-FN (CLASS-A) {584523C5}>
-* (defmethod another-fn ((obj class-b)) :class-b)
-; Compiling LAMBDA (.PV-CELL. .NEXT-METHOD-CALL. OBJ):
-; Compiling Top-Level Form:
-
-#<STANDARD-METHOD ANOTHER-FN (CLASS-B) {584B8895}>
-```
-
-Given the above definitions, when inheriting from `class-a` and `class-b`, the order of inheritance matters:
-
-```lisp
-* (defclass class-a-coming-first (class-a class-b) ())
-#<STANDARD-CLASS CLASS-A-COMING-FIRST {584BE6AD}>
-
-* (defclass class-b-coming-first (class-b class-a) ())
-#<STANDARD-CLASS CLASS-B-COMING-FIRST {584C744D}>
-
-* (another-fn (make-instance 'class-a-coming-first))
-:CLASS-A
-
-* (another-fn (make-instance 'class-b-coming-first))
-:CLASS-B
-```
-
-Combining the order of inheritance with generic functions with multiple arguments, CLOS has to make a choice of how to pick a method given two competing definitions, and its default strategy is prioritizing from left to right:
-
-```lisp
-* (defgeneric yet-another-fn (obj1 obj2))
-#<STANDARD-GENERIC-FUNCTION YET-ANOTHER-FN (0) {584D9EC9}>
-
-* (defmethod yet-another-fn ((obj1 class-a) obj2) :first-arg-specialized)
-#<STANDARD-METHOD YET-ANOTHER-FN (CLASS-A T) {5854269D}>
-
-* (defmethod yet-another-fn (obj1 (obj2 class-b)) :second-arg-specialized)
-#<STANDARD-METHOD YET-ANOTHER-FN (T CLASS-B) {585AAAAD}>
-
-* (yet-another-fn (make-instance 'class-a) (make-instance 'class-b))
-:FIRST-ARG-SPECIALIZED
-```
-
-CLOS has to make a choice between the first and the second definition of `yet-another-fn`, but its choice is just a heuristic.
-What if we want the choice to be based on the second argument, instead of the first?
-
-For that, we use the `:argument-precedence-order` option when declaring a generic function:
-
-```lisp
-* (defgeneric yet-another-fn (obj1 obj2) (:argument-precedence-order obj2 obj1))
-#<STANDARD-GENERIC-FUNCTION YET-ANOTHER-FN (2) {584D9EC9}>
-
-* (yet-another-fn (make-instance 'class-a) (make-instance 'class-b))
-:SECOND-ARG-SPECIALIZED
-```
-
-I liked that the `:argument-precedence-order` option exists.
-We shouldn't have to change the arguments from `(obj1 obj2)` to `(obj2 obj1)` just to make CLOS pick the method that we want.
-We can configure its default behaviour if desired, and keep the order of arguments however it best fits the generic function.
-
-## Comparison with Clojure
-
-Clojure has an equivalent, when using `defmulti`.
-
-Since when declaring a multi-method with `defmulti` we must define the dispatch function, Clojure uses it to pick the method definition.
-Since the dispatch function is required, there is no need for a default behaviour, such as left-to-right.
-
-## Conclusion
-
-Making the argument precedence order configurable for generic functions but not for class definitions makes a lot of sense.
-
-When declaring a class, we can choose the precedence order, and that is about it.
-But when defining a generic function, the order of arguments is more important to the function semantics, and the argument precedence being left-to-right is just the default behaviour.
-
-One shouldn't change the order of arguments of a generic function for the sake of tailoring it to the CLOS priority ranking algorithm, but doing it for a class definition is just fine.
-
-TIL.
-
-## References
-
-1. [Object-Oriented Programming in Common Lisp: A Programmer's Guide to CLOS](https://en.wikipedia.org/wiki/Object-Oriented_Programming_in_Common_Lisp), by Sonja E. Keene