My first thought it that this is because (class nil) => nil so even
tho' the 'correct' Java signature is matched and called, the
multi-method dispatch is still invoked under the hood.
So (String) null selects the [String] String signature, (class
valueHolder) => nil so the :default implementation is called, but I
would expect (nil? valueHolder) to be true and to have it return "c".
Similarly, (Object) null selects [Object] String, but again calls the
:default implementation... although I would expect "c" back.
And (Integer) null selects [Integer] Integer, calls the :default
implementation and fails when it tries to return "c" (or more likely
"d" given the previous two results).
I'd add println calls to each implementation and see what valueHolder
seems to be in each case...
Sean
On Sat, Jan 5, 2013 at 12:49 AM, rjf89 <robert....@gmail.com> wrote:
> I'm kind of new to Clojure at the moment, and have been playing around
> re-writing some utility libraries from work in Clojure for import into other
> pieces of our infrastructure. I'm having a blast using it, but I've gotten a
> bit stuck on trying to implement something with Multimethods. I have a
> multimethod dispatching based upon class – for example:
>
>> (ns interop.core
>> (:gen-class
>> :methods [[doSomething [String] String]
>> [doSomething [Integer] Integer]
>> [doSomething [Object] String]]))
>> (defmulti -doSomething (fn [this valueHolder & default] (class
>> valueHolder)))
>> (defmethod -doSomething String [this valueHolder]
>> (if (nil? valueHolder) "a" "b"))
>> (defmethod -doSomething Integer [this valueHolder]
>> (if (nil? valueHolder) (Integer. 1) (Integer. 2)))
>> (defmethod -doSomething :default [this valueHolder]
>> (if (nil? valueHolder) "c" "d"))
>
>
> If I compile and export this, and attempt to use it in a Java project, it
> works fine for most use cases:
>
>> core c = new core();
>> System.out.println(c.doSomething(new Integer(1))); // 2 --> Correct
>> System.out.println(c.doSomething("")); // 'b' --> Correct
>> System.out.println(c.doSomething(new Object())); // 'd' --> Correct
>> // System.out.println(c.doSomething(null)); // The method
>> doSomething(String) is ambiguous
>> System.out.println(c.doSomething((String) null)); // 'd' --> Wrong
>> System.out.println(c.doSomething((Object) null)); // 'd' --> Correct
>> System.out.println(c.doSomething((Integer) null)); // ClassCastExc: String
>> cannot be cast to Integer
>
>
> The oddity is the last line. It looks like it's: Calling the Object =>
> String implementation (marked default), but its trying to cast the result
> after the fact to an Integer (to match the Integer => Integer
> implementation). If I had to make a guess, I'd say the return type is being
> correctly matched, but the input parameter is causing the :default
> implementation to always be called. Anyone have any suggestions or ideas on
> either what I'm doing wrong, or even just whats happening behind the scenes?
> Any ideas on how to fix whats probably an egregious mistake on my behalf?
>
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--
Sean A Corfield -- (904) 302-SEAN
An Architect's View -- http://corfield.org/
World Singles, LLC. -- http://worldsingles.com/
"Perfection is the enemy of the good."
-- Gustave Flaubert, French realist novelist (1821-1880)
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