How to make a Clojure function take a variable number of parameters?

Solution 1:

In general, non-commutative case you can use apply:

(defn sum [& args] (apply + args))

Since addition is commutative, something like this should work too:

(defn sum [& args] (reduce + args))

& causes args to be bound to the remainder of the argument list (in this case the whole list, as there's nothing to the left of &).

Obviously defining sum like that doesn't make sense, since instead of:

(sum a b c d e ...)

you can just write:

(+ a b c d e ....)

Solution 2:

Yehoanathan mentions arity overloading but does not provide a direct example. Here's what he's talking about:

(defn special-sum
  ([] (+ 10 10))
  ([x] (+ 10 x))
  ([x y] (+ x y)))

(special-sum) => 20

(special-sum 50) => 60

(special-sum 50 25) => 75

Solution 3:

 (defn my-sum
  ([]  0)                         ; no parameter
  ([x] x)                         ; one parameter
  ([x y] (+ x y))                 ; two parameters
  ([x y & more]                   ; more than two parameters


    (reduce + (my-sum x y) more))
  )