Clojure: = vs ==

Question

I'm struggling with understanding why

(= 1 1.0)

evaluates to false whereas

(== 1 1.0)

evaluates to true. According to the docs, == seems to be only working on numbers but other than that there doesn't seem much difference. So, what am I missing?

Answer 1

== checks for mathematical equivalence. = with numbers checks for equivalence in a way that is agnostic to size where applicable, but is strict about representation:

user> (= (float 1.0) (double 1.0))
true
user> (= (int 1) (byte 1))
true
user> (= (int 1) (double 1))
false
user> (= 0.5 (/ 1 2))
false
user> (== 0.5 (/ 1 2))
true

assumedly, the reasoning is that the representation of floating point can lose precision, and should not be treated as equivalent to integral or exact representations.