user=> (defn truthy? [x] (if x true false)) #'user/foo user=> (truthy? nil) false user=> (truthy? Boolean/FALSE) false user=> (Boolean. false) false user=> (truthy? (Boolean. false)) true
Wait, what? Looks like Clojure's truthy check is looking for something to be either nil (= Java null) or the specific java.lang.Boolean instance Boolean/FALSE. A novel Boolean instance with the value false is treated as truthy! This may not be desirable behavior. I doubt checking against Boolean/FALSE with .equals instead of identical? will be much more expensive -- it's likely, after JIT optimization, to boil down to a dereference and a comparison to a constant in place of a direct pointer comparison, so one extra instruction. The biggest case where this is liable to cause problems is with Java serialization: user=> (def q (java.io.ByteArrayOutputStream.)) #'user/q user=> (.writeObject (java.io.ObjectOutputStream. q) Boolean/FALSE) nil user=> (truthy? (.readObject (java.io.ObjectInputStream. (java.io.ByteArrayInputStream. (.toByteArray q))))) true As you can see, round-tripping Boolean/FALSE through serialization does not readResolve() it to Boolean/FALSE but rather constructs a de novo false instance, so boxed booleans that pass through serialization become truthy to Clojure. The other potential source of trouble is interop with Java code that for any reason calls the Boolean constructor instead of using the class constants. Autoboxing of the primitive boolean false seems to produce the specific instance Boolean/FALSE, fortunately: user=> (truthy? (.contains (java.util.HashSet.) 42)) false Of course, if it didn't, this would have been noticed a long time ago. -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
