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////
export sort=1
////
= Clojure auto curry
: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.
|
