diff options
Diffstat (limited to 'src/content/tils/2021/04/24')
| -rw-r--r-- | src/content/tils/2021/04/24/cl-generic-precedence.adoc | 149 | ||||
| -rw-r--r-- | src/content/tils/2021/04/24/clojure-autocurry.adoc | 135 | ||||
| -rw-r--r-- | src/content/tils/2021/04/24/scm-nif.adoc | 61 |
3 files changed, 0 insertions, 345 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 diff --git a/src/content/tils/2021/04/24/clojure-autocurry.adoc b/src/content/tils/2021/04/24/clojure-autocurry.adoc deleted file mode 100644 index a2c2835..0000000 --- a/src/content/tils/2021/04/24/clojure-autocurry.adoc +++ /dev/null @@ -1,135 +0,0 @@ -= Clojure auto curry -:sort: 1 -:updatedat: 2021-04-27 - -:defcurry-orig: https://lorettahe.github.io/clojure/2016/09/22/clojure-auto-curry - -Here's a simple macro defined by {defcurry-orig}[Loretta He] to create Clojure -functions that are curried on all arguments, relying on Clojure's multi-arity -support: - -[source,clojure] ----- -(defmacro defcurry - [name args & body] - (let [partials (map (fn [n] - `(~(subvec args 0 n) (partial ~name ~@(take n args)))) - (range 1 (count args)))] - `(defn ~name - (~args ~@body) - ~@partials))) ----- - -A naive `add` definition, alongside its usage and macroexpansion: - -[source,clojure] ----- -user=> (defcurry add - [a b c d e] - (+ 1 2 3 4 5)) -#'user/add - -user=> (add 1) -#object[clojure.core$partial$fn__5857 0x2c708440 "clojure.core$partial$fn__5857@2c708440"] - -user=> (add 1 2 3 4) -#object[clojure.core$partial$fn__5863 0xf4c0e4e "clojure.core$partial$fn__5863@f4c0e4e"] - -user=> ((add 1) 2 3 4 5) -15 - -user=> (((add 1) 2 3) 4 5) -15 - -user=> (use 'clojure.pprint) -nil - -user=> (pprint - (macroexpand - '(defcurry add - [a b c d e] - (+ 1 2 3 4 5)))) -(def - add - (clojure.core/fn - ([a b c d e] (+ 1 2 3 4 5)) - ([a] (clojure.core/partial add a)) - ([a b] (clojure.core/partial add a b)) - ([a b c] (clojure.core/partial add a b c)) - ([a b c d] (clojure.core/partial add a b c d)))) -nil ----- - -This simplistic `defcurry` definition doesn't support optional parameters, -multi-arity, `&` rest arguments, docstrings, etc., but it could certainly evolve -to do so. - -I like how `defcurry` is so short, and abdicates the responsability of doing the -multi-arity logic to Clojure's built-in multi-arity support. Simple and -elegant. - -Same Clojure as before, now with auto-currying via macros. - -== Comparison with Common Lisp - -My attempt at writing an equivalent for Common Lisp gives me: - -[source,lisp] ----- -(defun partial (fn &rest args) - (lambda (&rest args2) - (apply fn (append args args2)))) - -(defun curry-n (n func) - (cond ((< n 0) (error "Too many arguments")) - ((zerop n) (funcall func)) - (t (lambda (&rest rest) - (curry-n (- n (length rest)) - (apply #'partial func rest)))))) - -(defmacro defcurry (name args &body body) - `(defun ,name (&rest rest) - (let ((func (lambda ,args ,@body))) - (curry-n (- ,(length args) (length rest)) - (apply #'partial func rest))))) ----- - -Without built-in multi-arity support, we have to do more work, like tracking the -number of arguments consumed so far. We also have to write `#'partial` -ourselves. That is, without dependending on any library, sticking to ANSI -Common Lisp. - -The usage is pretty similar: - -[source,lisp] ----- -* (defcurry add (a b c d e) - (+ a b c d e)) -ADD - -* (add 1) -#<FUNCTION (LAMBDA (&REST REST) :IN CURRY-N) {100216419B}> - -* (funcall (add 1) 2 3 4) -#<FUNCTION (LAMBDA (&REST REST) :IN CURRY-N) {100216537B}> - -* (funcall (add 1) 2 3 4 5) -15 - -* (funcall (funcall (add 1) 2 3) 4 5) -15 - -* (macroexpand-1 - '(defcurry add (a b c d e) - (+ a b c d e))) -(DEFUN ADD (&REST REST) - (LET ((FUNC (LAMBDA (A B C D E) (+ A B C D E)))) - (CURRY-N (- 5 (LENGTH REST)) (APPLY #'PARTIAL FUNC REST)))) -T ----- - -This also require `funcall`s, since we return a `lambda` that doesn't live in -the function namespace. - -Like the Clojure one, it doesn't support optional parameters, `&rest` rest -arguments, docstrings, etc., but it also could evolve to do so. diff --git a/src/content/tils/2021/04/24/scm-nif.adoc b/src/content/tils/2021/04/24/scm-nif.adoc deleted file mode 100644 index 2ea8a6f..0000000 --- a/src/content/tils/2021/04/24/scm-nif.adoc +++ /dev/null @@ -1,61 +0,0 @@ -= Three-way conditional for number signs on Lisp -:categories: lisp scheme common-lisp -:sort: 2 -:updatedat: 2021-08-14 - -:on-lisp: https://www.paulgraham.com/onlisptext.html -:sicp: https://mitpress.mit.edu/sites/default/files/sicp/index.html - -A useful macro from Paul Graham's {on-lisp}[On Lisp] book: - -[source,lisp] ----- -(defmacro nif (expr pos zero neg) - (let ((g (gensym))) - `(let ((,g ,expr)) - (cond ((plusp ,g) ,pos) - ((zerop ,g) ,zero) - (t ,neg))))) ----- - -After I looked at this macro, I started seeing opportunities to using it in many -places, and yet I didn't see anyone else using it. - -The latest example I can think of is section 1.3.3 of {sicp}[Structure and -Interpretation of Computer Programs], which I was reading recently: - -[source,scheme] ----- -(define (search f neg-point pos-point) - (let ((midpoint (average neg-point pos-point))) - (if (close-enough? neg-point post-point) - midpoint - (let ((test-value (f midpoint))) - (cond ((positive? test-value) - (search f neg-point midpoint)) - ((negative? test-value) - (search f midpoint pos-point)) - (else midpoint)))))) ----- - -Not that the book should introduce such macro this early, but I couldn't avoid -feeling bothered by not using the `nif` macro, which could even remove the need -for the intermediate `test-value` variable: - -[source,scheme] ----- -(define (search f neg-point pos-point) - (let ((midpoint (average neg-point pos-point))) - (if (close-enough? neg-point post-point) - midpoint - (nif (f midpoint) - (search f neg-point midpoint) - (midpoint) - (search f midpoint pos-point))))) ----- - -It also avoids `cond`'s extra clunky parentheses for grouping, which is -unnecessary but built-in. - -As a macro, I personally feel it tilts the balance towards expressivenes despite -its extra cognitive load toll. |