Sean Corfield <[email protected]> writes:
> On Wed, Aug 18, 2010 at 10:28 AM, Shantanu Kumar
> <[email protected]> wrote:
>> Would suggest to consider this:
>>
>> (let [ x (true? (new java.lang.Boolean false)) ] (if x "trouble"
>> "ok"))
>
> Ah, but that wouldn't work for a new Boolean set to true:
>
> (let [ x (true? (new java.lang.Boolean true)) ] (if x "ok" "trouble"))
>
> Expect "ok"? Nope, you'll get "trouble".
How about something along these lines?
(defmacro careful-if [test then else]
`(let [result# ~test]
(cond (nil? result#) ~else
(identical? Boolean/FALSE result#) ~else
(identical? Boolean/TRUE result#) ~then
(instance? Boolean result#) (if (Boolean/valueOf result#)
~then
~else)
:else ~then)))
In the case of a "proper" boolean value I think you just get one more
reference check than with normal "if"; you only hit reflection if both
those reference checks fail. It satisfies the original author's
challenge, anyway:
(map #(careful-if %1 :truthy :falsey)
[true false (new Boolean true) (new Boolean false)])
;;; => (:truthy :falsey :truthy :falsey)
I'll leave it to someone more skilled in Clojure than me to tell us what
would be the impact of either using this as a replacement "if" or
somehow embedding similar logic into the Java interop functions...
--
Mark Shroyer
http://markshroyer.com/contact/
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