Comments on: Clojure: A few things I've been tripping up on http://www.markhneedham.com/blog/2009/11/20/clojure-a-few-things-ive-been-tripping-up-on/ Thoughts on Software Development Sun, 14 Mar 2010 09:50:28 +0000 http://wordpress.org/?v=2.8.4 hourly 1 By: Steve Gilardi http://www.markhneedham.com/blog/2009/11/20/clojure-a-few-things-ive-been-tripping-up-on/comment-page-1/#comment-27044 Steve Gilardi Sat, 21 Nov 2009 19:01:34 +0000 http://www.markhneedham.com/blog/?p=1851#comment-27044 Regarding "edge cases" for truthiness, http://clojure.org/special_forms#if gives the full details. The potential gotcha is covered there: all instances of class java.lang.Boolean other than the canonical false instance Boolean/FALSE are logically true. ; clojure false is Boolean/FALSE user=> (identical? false Boolean/FALSE) true ; other Boolean instances are logically true user=> (Boolean. false) false user=> (if (Boolean. false) "truthy" "not truthy") "truthy" ; always use Boolean/valueOf rather than the constructor user=> (Boolean/valueOf false) false user=> (if (Boolean/valueOf false) "truthy" "not truthy") "not truthy" ; clojure.core/boolean will fix up any Boolean instance user=> (if (boolean (Boolean. false)) "truthy" "not truthy") "not truthy" ; so will Boolean/valueOf user=> (if (Boolean/valueOf (Boolean. false)) "truthy" "not truthy") "not truthy" user=> Pure Clojure code will always produce and consume canonical false. This subtlety only comes up when using Java interop either directly (calling a Boolean constructor, something you can and should always avoid) or when interfacing with Java libraries. When in doubt about a Boolean value returned from Java, casting using either clojure.core/boolean or Boolean/valueOf will ensure no surprises. Note that many (most?) Java libraries will return a boolean (primitive) value rather than a Boolean (object) value. Primitive false always works without surprises. user=> (identical? Boolean/FALSE (.booleanValue (Boolean. false))) true Regarding "edge cases" for truthiness, http://clojure.org/special_forms#if gives the full details. The potential gotcha is covered there: all instances of class java.lang.Boolean other than the canonical false instance Boolean/FALSE are logically true.

; clojure false is Boolean/FALSE
user=> (identical? false Boolean/FALSE)
true
; other Boolean instances are logically true
user=> (Boolean. false)
false
user=> (if (Boolean. false) "truthy" "not truthy")
"truthy"
; always use Boolean/valueOf rather than the constructor
user=> (Boolean/valueOf false)
false
user=> (if (Boolean/valueOf false) "truthy" "not truthy")
"not truthy"
; clojure.core/boolean will fix up any Boolean instance
user=> (if (boolean (Boolean. false)) "truthy" "not truthy")
"not truthy"
; so will Boolean/valueOf
user=> (if (Boolean/valueOf (Boolean. false)) "truthy" "not truthy")
"not truthy"
user=>

Pure Clojure code will always produce and consume canonical false. This subtlety only comes up when using Java interop either directly (calling a Boolean constructor, something you can and should always avoid) or when interfacing with Java libraries. When in doubt about a Boolean value returned from Java, casting using either clojure.core/boolean or Boolean/valueOf will ensure no surprises.

Note that many (most?) Java libraries will return a boolean (primitive) value rather than a Boolean (object) value. Primitive false always works without surprises.

user=> (identical? Boolean/FALSE (.booleanValue (Boolean. false)))
true

]]>