Clojure detects when a number outgrows its class capacity and automatically converts the type to accomodate the bigger number a.k.a promotion:
(def sample 10)
(class sample) ;=> java.lang.Long
(class (+ sample 1000000000000000000)) ;=> java.lang.Long
(class (+ sample 100000000000000000000)) ;=> java.lang.BigInt
(class (+ sample 1.0)) ;=> java.lang.Double
Source: Joy of Clojure 69.