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#
msgid ""
msgstr ""
msgid ""
"title: Common Lisp argument precedence order parameterization of a generic "
"function"
msgstr ""
msgid "date: 2021-04-24"
msgstr ""
msgid "layout: post"
msgstr ""
msgid "lang: en"
msgstr ""
msgid ""
"ref: common-lisp-argument-precedence-order-parameterization-of-a-generic-"
"function"
msgstr ""
msgid ""
"When CLOS dispatches a method, it picks the most specific method definition "
"to the argument list:"
msgstr ""
msgid ""
"\n"
"* (defgeneric a-fn (x))\n"
"#<STANDARD-GENERIC-FUNCTION A-FN (0) {5815ACB9}>\n"
"\n"
"* (defmethod a-fn (x) :default-method)\n"
"#<STANDARD-METHOD A-FN (T) {581DB535}>\n"
"\n"
"* (defmethod a-fn ((x number)) :a-number)\n"
"#<STANDARD-METHOD A-FN (NUMBER) {58241645}>\n"
"\n"
"* (defmethod a-fn ((x (eql 1))) :number-1)\n"
"#<STANDARD-METHOD A-FN ((EQL 1)) {582A7D75}>\n"
"\n"
"* (a-fn nil)\n"
":DEFAULT-METHOD\n"
"\n"
"* (a-fn \"1\")\n"
":DEFAULT-METHOD\n"
"\n"
"* (a-fn 0)\n"
":A-NUMBER\n"
"\n"
"* (a-fn 1)\n"
":NUMBER-1\n"
msgstr ""
msgid ""
"CLOS uses a similar logic when choosing the method from parent classes, when"
" multiple ones are available:"
msgstr ""
msgid ""
"* (defclass class-a () ())\n"
"\n"
"#<STANDARD-CLASS CLASS-A {583E0B25}>\n"
"* (defclass class-b () ())\n"
"\n"
"#<STANDARD-CLASS CLASS-B {583E7F6D}>\n"
"* (defgeneric another-fn (obj))\n"
"\n"
"#<STANDARD-GENERIC-FUNCTION ANOTHER-FN (0) {583DA749}>\n"
"* (defmethod another-fn ((obj class-a)) :class-a)\n"
"; Compiling LAMBDA (.PV-CELL. .NEXT-METHOD-CALL. OBJ):\n"
"; Compiling Top-Level Form:\n"
"\n"
"#<STANDARD-METHOD ANOTHER-FN (CLASS-A) {584523C5}>\n"
"* (defmethod another-fn ((obj class-b)) :class-b)\n"
"; Compiling LAMBDA (.PV-CELL. .NEXT-METHOD-CALL. OBJ):\n"
"; Compiling Top-Level Form:\n"
"\n"
"#<STANDARD-METHOD ANOTHER-FN (CLASS-B) {584B8895}>\n"
msgstr ""
msgid ""
"Given the above definitions, when inheriting from `class-a` and `class-b`, "
"the order of inheritance matters:"
msgstr ""
msgid ""
"* (defclass class-a-coming-first (class-a class-b) ())\n"
"#<STANDARD-CLASS CLASS-A-COMING-FIRST {584BE6AD}>\n"
"\n"
"* (defclass class-b-coming-first (class-b class-a) ())\n"
"#<STANDARD-CLASS CLASS-B-COMING-FIRST {584C744D}>\n"
"\n"
"* (another-fn (make-instance 'class-a-coming-first))\n"
":CLASS-A\n"
"\n"
"* (another-fn (make-instance 'class-b-coming-first))\n"
":CLASS-B\n"
msgstr ""
msgid ""
"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:"
msgstr ""
msgid ""
"* (defgeneric yet-another-fn (obj1 obj2))\n"
"#<STANDARD-GENERIC-FUNCTION YET-ANOTHER-FN (0) {584D9EC9}>\n"
"\n"
"* (defmethod yet-another-fn ((obj1 class-a) obj2) :first-arg-specialized)\n"
"#<STANDARD-METHOD YET-ANOTHER-FN (CLASS-A T) {5854269D}>\n"
"\n"
"* (defmethod yet-another-fn (obj1 (obj2 class-b)) :second-arg-specialized)\n"
"#<STANDARD-METHOD YET-ANOTHER-FN (T CLASS-B) {585AAAAD}>\n"
"\n"
"* (yet-another-fn (make-instance 'class-a) (make-instance 'class-b))\n"
":FIRST-ARG-SPECIALIZED\n"
msgstr ""
msgid ""
"For that, we use the `:argument-precedence-order` option when declaring a "
"generic function:"
msgstr ""
msgid ""
"* (defgeneric yet-another-fn (obj1 obj2) (:argument-precedence-order obj2 obj1))\n"
"#<STANDARD-GENERIC-FUNCTION YET-ANOTHER-FN (2) {584D9EC9}>\n"
"\n"
"* (yet-another-fn (make-instance 'class-a) (make-instance 'class-b))\n"
":SECOND-ARG-SPECIALIZED\n"
msgstr ""
msgid ""
"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."
msgstr ""
msgid "Comparison with Clojure"
msgstr ""
msgid "Clojure has an equivalent, when using `defmulti`."
msgstr ""
msgid ""
"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."
msgstr ""
msgid "Conclusion"
msgstr ""
msgid ""
"Making the argument precedence order configurable for generic functions but "
"not for class definitions makes a lot of sense."
msgstr ""
msgid ""
"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."
msgstr ""
msgid "TIL."
msgstr ""
msgid ""
"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?"
msgstr ""
msgid ""
"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."
msgstr ""
msgid "References"
msgstr ""
msgid ""
"[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"
msgstr ""
#~ msgid ""
#~ "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 to the"
#~ " choice to be based on the second argument first?"
#~ msgstr ""
#~ msgid ""
#~ "When declaring a class, we can choose the precedence order, and that is "
#~ "about it. But when defining a generic function, the order of argumentws is "
#~ "more important to the function semantics, and the argument precedence being "
#~ "left-to-right is just the default behaviour."
#~ msgstr ""
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