How to make a Clojure function take a variable num

问题:

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.

回答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 ....)

回答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

回答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))
  )

回答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

回答5:

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

This takes any number of arguments and add them up.