Object identity asks whether two values refer to one specific allocated instance, rather than merely containing equal data. Clojure exposes this test through identical?. Equality primarily compares values, while types can add semantic distinctions; identity compares the instance itself. section 6.0 section 6.5
identical? accepts exactly two arguments of any type, including nil, and returns true only when both references designate the same object instance. Supplying any other arity raises clojure.lang.ArityException. section 6.5
Use identity deliberately rather than as a replacement for ordinary equality:
- Use = when logically equal values should compare equal, even when collections have different concrete types. section 6.0
- Use identical? when a particular instance carries meaning, such as a unique sentinel used to stop a consumer loop. section 6.5
- Interpret results carefully because interning can make separately written basic values share an instance, producing surprising true results. section 6.5
A private sentinel
Create a unique object and have both producer and consumer recognize that exact instance: (def SENTINEL (Object.)) (if (identical? SENTINEL event) (println "done") (process event)) A separately created object is not the sentinel, even if it has the same role or contents. This lets a daemon-like consumer distinguish a stop signal from ordinary business data. section 6.5
Bindings and instances
Two bindings can point to one object: (let [x #{1 2 3} y x] (identical? x y)) ;; true By contrast, independently produced values may not share identity, even when they print alike. Java value construction and caching can change the result, so identity is not a general-purpose value comparison.
