On 30 Sep 2010, at 11.36 am, K. wrote:
> Can somebody explains me why the instance? function accepts one
> argument whereas the documentation states "Usage: (instance? c x)"
What an interesting question. If you try and emulate the behaviour yourself by
creating a separate fn with a different name, it works as you might expect:
(def
^{:arglists '([^Class c x])
:doc "Evaluates x and tests if it is an instance of the class
c. Returns true or false"
:added "1.0"}
eggs? (fn eggs? [^Class c x] (. c (isInstance x))))
#'user/eggs?
user=> (eggs?)
IllegalArgumentException Wrong number of args (0) passed to: user$eggs-QMARK-
clojure.lang.AFn.throwArity (AFn.java:439)
user=> (eggs? Integer)
IllegalArgumentException Wrong number of args (1) passed to: user$eggs-QMARK-
clojure.lang.AFn.throwArity (AFn.java:439)
user=> (eggs? java.lang.Long 1)
true
This fn must be created deep down in the guts of the Clojure implementation
where things are murky and magical...
I assume that x defaults to nil in the case where the number of args passed to
instance? is less than the required arity (2). However, that's just a guess...
Can anyone shed some more light?
Sam
---
http://sam.aaron.name
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