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

Question

I've encountered a problem while I was doing a task from 4clojure.com. Here is the description of a task:

Write a function which returns the last element in a sequence.

I've solved it using the following code:

#(first (reverse %))

When I wanted to change the first function with a number of an index. like so:

#(0 (reverse %))

I've received an error:

java.lang.ClassCastException: java.lang.Long cannot be cast to clojure.lang.IFn

My question is: Why am I receiving this error? I cannot get it, because for instance ([1 2 3 4] 0) is perfectly valid and returns the first element of a sequence so why I cannot use index of an array in the function?

EDIT1: Even the following code does not work and I suppose APersistentVector is first there.

#((reverse %) 0)

EDIT2: I managed to make it work by converting the list which is returned from reverse function to vector. Thanks @Josh

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

Josh · Accepted Answer · 2017-07-28 17:00:53Z

8

If you look at the code for APersistentVector, you will see:

public abstract class APersistentVector extends AFn ...

AFn implements IFn, which extends java's Callable and Runnable interfaces, which means that a clojure persistent vector can be called as a function, with the argument being used as the index to retrieve. You can see this here:

public Object invoke(Object arg1) {
    if(Util.isInteger(arg1))
        return nth(((Number) arg1).intValue());
    throw new IllegalArgumentException("Key must be integer");
}

The same is true for maps and sets; they can all be invoked as functions:

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

However, a Long (your number zero) does not implement IFn:

(ancestors (class 42))
=>
#{java.lang.Comparable
  java.lang.Number
  java.lang.Object
  java.io.Serializable}

Hence, it cannot be invoked as a function.

edited Jul 28, 2017 at 17:00

answered Jul 28, 2017 at 16:51

Josh

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5 Comments

FieryCod Over a year ago

But wait ([1 2 3 4] 0) works fine, so how it is possible that in this particular example it is invoked as a function?

Josh Over a year ago

@FieryCod Not sure I understand -- any clojure persistent vector that is the first element of a form (as in your example ([1 2 3 4] 0) ) will be invoked as a function, always. What do you mean, "this particular example"? first is a clojure core function, just to be clear.

FieryCod Over a year ago

By particular example I meant ([1 2 3 4] 0). Ok if you say that any persistent vector that is the first element of a form will be invoked as a function please enlighten me why this #(((reverse %) 0)) throws an error? If I understood you correctly it should work because PersistentVector is first.

Josh Over a year ago

You will notice that the error given when you try to apply that function to a sequence: ClassCastException clojure.lang.PersistentList cannot be cast to clojure.lang.IFn. reverse does not return a vector; it returns a PersistentList, and lists do not implement IFn (one reason being that lists are not associative, i.e., they cannot be accessed via an index like vectors) @FieryCod

FieryCod Over a year ago

Thank you, you've perfectly explained it. Have a nice day:)

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