Question

On REPL, if I define

(def fits (map vector (take 10 (iterate inc 0))))

and then call

(== [2] (nth fits 2))

I get false.

But

(= [2] (nth fits 2))

returns true.

Is this expected ? I tried (class [2]) and (class (nth fits 2) and both return Persistent Vector.

Answer 1

== is for comparing numbers. If either of its arguments is not a number, it will always return false:

(== :a :a)
; => false

As you can see by saying (clojure.contrib.repl-utils/source ==) at the REPL (with repl-utils require‘d, of course), == calls the equiv method of clojure.lang.Numbers. The relevant bit of clojure/lang/Numbers.java (from the latest or close-to-latest commit on GitHub):

static public boolean equiv(Object x, Object y){
    return y instanceof Number && x instanceof Number
           && equiv((Number) x, (Number) y);
}

Use = for equality comparisons of things which may not be numbers. When you are in fact dealing with numbers, == ought to be somewhat faster.