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-= 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.