diff options
Diffstat (limited to 'src/content/tils/2021/04/24/cl-generic-precedence.adoc')
| -rw-r--r-- | src/content/tils/2021/04/24/cl-generic-precedence.adoc | 149 |
1 files changed, 0 insertions, 149 deletions
diff --git a/src/content/tils/2021/04/24/cl-generic-precedence.adoc b/src/content/tils/2021/04/24/cl-generic-precedence.adoc deleted file mode 100644 index 541afb0..0000000 --- a/src/content/tils/2021/04/24/cl-generic-precedence.adoc +++ /dev/null @@ -1,149 +0,0 @@ -= 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: - -[source,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: - -[source,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: - -[source,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: - -[source,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: - -[source,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 - -:clos-wiki: https://en.wikipedia.org/wiki/Object-Oriented_Programming_in_Common_Lisp - -. {clos-wiki}[Object-Oriented Programming in Common Lisp: A Programmer's Guide - to CLOS], by Sonja E. Keene |