clojure - Error while trying to get first element of sequence by index in a function call -

i've encountered problem while doing task 4clojure.com. here description of task:

write function returns last element in sequence.

i've solved using following code:

#(first (reverse %)) 

when wanted change first function number of index. so:

#(0 (reverse %)) 

i've received error:

java.lang.classcastexception: java.lang.long cannot cast clojure.lang.ifn

my question is: why receiving error? cannot it, because instance ([1 2 3 4] 0) valid , returns first element of sequence why cannot use index of array in function?

edit1: following code not work , suppose apersistentvector first there.

#((reverse %) 0) 

edit2: managed make work converting list returned reverse function vector. @josh

(#((vec (reverse %)) 0)[1 2 3]) 

if @ code apersistentvector, see:

public abstract class apersistentvector extends afn ... 

afn implements ifn, extends java's callable , runnable interfaces, means clojure persistent vector can called function, argument being used index retrieve. can see here:

public object invoke(object arg1) {     if(util.isinteger(arg1))         return nth(((number) arg1).intvalue());     throw new illegalargumentexception("key must integer"); } 

the same true maps , sets; can invoked functions:

({:a 1 :b 2} :b)  ;; 2 (#{:a :b} :a)     ;; :a ([1 2 3 4] 0)     ;; 1 

however, long (your number zero) not implement ifn:

(ancestors (class 42)) => #{java.lang.comparable   java.lang.number   java.lang.object   java.io.serializable} 

hence, cannot invoked function.