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

Question

I'm learning Clojure and I'm trying to define a function that take a variable number of parameters (a variadic function) and sum them up (yep, just like the + procedure). However, I don´t know how to implement such function

Everything I can do is:

(defn sum [n1, n2] (+ n1 n2))

Of course this function takes two parameteres and two parameters only. Please teach me how to make it accept (and process) an undefined 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))
  )

Solution 4

defn is a macro that makes defining functions a little simpler. Clojure supports arity overloading in a single function object, self-reference, and variable-arity functions using &

From http://clojure.org/functional_programming

Solution 5

(defn sum [& args]
  (print "sum of" args ":" (apply + args)))

This takes any number of arguments and add them up.

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Comments

[rodrigoalvesvieira]

Yes, doesn´t make sence, but it is a good illustration to your answer. Thanks.

[avichalp]

@soulcheck: is there a way to pass a seq to your sum function. For example : (sum '(1 2 3)) and the result is 6 ?

[soulcheck]

@avichalp that would be another function. just remove & from either version

[avichalp]

@soulcheck: No. I mean using the same function signature. I am a newbie in clojure hence I am able to put my point clearly here. What I would like to know is in python I can use *args and if a function is defined such that it takes *args (eg def fn(*args): pass) I can call it by giving a list like fn(*list_of_args). Can I do the same thing in clojure ?

[soulcheck]

@avichalp the call to def fn(*args): pass wouldn't be fn([1,2,3]) but fn(1,2,3) (obviously depending what you want to do exactly, but I'm trying to follow the spirit here)

[avichalp]

@soulcheck: In python I can pass a list with an star prepended to it like fn(*[1, 2, 3]) it will be equivalent to fn(1, 2, 3). Is this possible in clojure ?

[fingaz]

@avichalp the & creates a seq out of the arguments passed to the function. If you were to pass a seq, you would be passing a seq of seqs and apply would not be able to evaluate the seq because it expects a clojure.lang.Number type based on the arguments asked for by +

[nha]

Duplicate of @soulcheck answer stackoverflow.com/a/9242671/1327651

[Erik Kaplun]

Also works with lambdas: (fn ([] (+ 10 10)) ([x] (+ 10 x)) ([x y] (+ x y)))

[Yarin]

What @avichalp is asking about is Python's star operator, which unpacks a list argument into individual arguments in a function call. The answer is no, Clojure doesn't have that, you would use apply instead. See stackoverflow.com/a/10802391/165673.