On May 4, 1:53 am, Laurent PETIT <laurent.pe...@gmail.com> wrote:
> 2009/5/4 Christophe Grand <christo...@cgrand.net>
>
>
>
>
>
> > Janico Greifenberg a écrit :
> > > Hi,
>
> > > I encountered unexpected behavior of the 'if' form in clojure when using
> > > instances of java.lang.Boolean as the condition. I wanted to convert
> > > input strings to booleans and used the constructor of the Boolean class
> > > with the string parameter. However, when I pass these values as a
> > > condition to if, the true-branch always gets executed. For example:
>
> > > > (if (Boolean. "true") 1 2)
> > > 1
> > > > (if (Boolean. "false") 1 2)
> > > 1
>
> > > It seems to me that this has to do with the identity of the objects, as
> > > (Boolean. "false") is not identical (although equal) to the clojure
> > > literal false. Is this behavior intentional or a bug?
>
> > > The problem does not occur when I use Boolean/parseBoolean which returns
> > > a lower case boolean.
>
> > It's intentional, Rich said "for efficiency only canonic false is
> > logical false in
> > Clojure." inhttp://groups.google.com/group/clojure/msg/81ba3175da9a877c
>
> > Workaround:
> > > (if (boolean (Boolean. "false")) 1 2)
> > 2
>
> At the cost of a lesser experience concerning java interoperability, then ?
>
> Isn't that "premature optimization" ? ;-)
Not at all. The cost would apply to all usage of 'if' (i.e. all
conditionals).
The Boolean ctor is documented as a 'do not use' item:
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Boolean.html#Boolean(boolean)
I'm not going to make all of Clojure slower in order to support people
using something they are supposed to avoid.
The right thing is to use valueOf, as Nick suggested:
user=> (if (Boolean/valueOf "false") 1 2)
2
Rich
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