instance? with one argument

[Phillip Lord]

The instance? function does not crash with a single argument, when I
think that it should throw an arity exception.

I asked on SO about this a while back, so I now know why, but I got hit
by this again yesterday. I think this is a bug, but I am not sure
whether I have missed a good reason why it is not considered so.


http://stackoverflow.com/questions/13176400/clojure-instance-single-argument

[Herwig Hochleitner]

This special handling technique is known as a compiler macro in clojurescript (and elsewhere). Essentially, all direct calls to instance? get emitted as an instanceof expression for performance.

Higher order uses behave as expected:

user=> (apply instance? Long [])

ArityException Wrong number of args (1) passed to: core$instance-QMARK-  clojure.lang.AFn.throwArity (AFn.java:437)

Unfortunately the handler in the compiler doesn't do arity checks, hence the behavior you see. IMO this is a bug. 

I can't speak for clojure/core, but I believe ticket and patch would be welcome in this case.

[Herwig Hochleitner]

Also direct calls are only handled, if the first arg is a symbol that directly resolves to a class:

user=> (def C Long)

user=> (instance? C)

ArityException Wrong number of args (1) passed to: core$instance-QMARK-  clojure.lang.AFn.throwArity (AFn.java:437)

Starting from this, I just discovered another, much more severe bug:

user=> (let [Long String] (instance? Long "abc"))

false

I've opened a ticket: http://dev.clojure.org/jira/browse/CLJ-1171

[Phillip Lord]

Yes, you are right about the lexical binding; I suspect that is less
likely to hit people though. I don't know why I keep typing

(instance? String)

but I've done it a lot. Possibly because I am using a Java API with a
lot of very similar classes; by the time I have managed to find out
which class I need to check, I've forgotten what I was doing in the
first place.

Thanks for opening the ticket!

> --

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Phillip Lord, Phone: +44 (0) 191 222 7827
Lecturer in Bioinformatics, Email: philli...@newcastle.ac.uk
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