Clojure "apply" throws ClassCastException in simple sum function

Question

This function

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

should sum up all args sequence elements throws an exception. Why?

user> (defn sum [& args] (apply + args))
#'user/sum
user> (sum [1 2 3])
ClassCastException   java.lang.Class.cast (Class.java:2999)
user> (sum (range 1 10))
ClassCastException   java.lang.Class.cast (Class.java:2999)

(also used as example (which does not compile for me) in this question How to make a Clojure function take a variable number of parameters?)

I'm using nrepl in Emacs 24.2 Live with Clojure 1.5.1

score 3 · Accepted Answer · answered May 22 '13 at 16:20

3

When you specify a function parameter with & args, that means that args will contain a list of all further function parameters. In this case, sum is getting a list of all of its parameters--but its parameter is already a list, so effectively what sum is doing here is

(apply + [[1 2 3]])

If you get rid of the & in your definition of sum's parameters (so that the list is just [args]) you'll get what you want.

answered May 22 '13 at 16:20

bdesham

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D'oh! Thanks :) It's obvious now :D – Saša Tomislav Mataić May 22 '13 at 16:25
Glad to help, and welcome to Stack Overflow! If I've answered your question, you can mark it as "accepted" by clicking the outline of the checkmark next to my answer :-) – bdesham May 22 '13 at 16:32

score 0 · Answer 2 · answered May 22 '13 at 16:25

0

Try the following. In Clojure variable arguments are lists itself

(sum 1 2 3)

answered May 22 '13 at 16:25

Abimaran Kugathasan

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Clojure "apply" throws ClassCastException in simple sum function

2 Answers2