title: Clojure auto curry
date: 2021-04-24 1
updated_at: 2021-04-27
layout: post
lang: en
ref: clojure-auto-curry
Here's a simple macro defined by Loretta He to create Clojure functions that are curried on all arguments, relying on Clojure's multi-arity support:
(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:
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:
(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:
* (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 funcalls, 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.