defprotocol should throw error when signatures include variable number of parameters


I tried to use & in the signature for a method in defprotocol. Apparently (see below), this is compiled so that & becomes a simple parameter name, and there is no special handling for variable number of parameters. I think the use of & in a protocol signature ought to be detected and immediately cause an exception (I also think this restriction on the signatures ought to be documented; I couldn't find it specified in the current documentation, though of course it is implied (as I later realized) by the fact that defprotocol creates a Java interface).

user=> (defprotocol Applier (app [this f & args]))
user=> (deftype A [] Applier (app [_ f & args] (prn f & args) (apply f args)))
user=> (app (A.) + 1 2)
#<core$PLUS clojure.core$PLUS@5d9d0d20> 1 2
IllegalArgumentException Don't know how to create ISeq from: java.lang.Long
clojure.lang.RT.seqFrom (




Andy Fingerhut
October 26, 2013, 12:25 AM

I have not investigated the reason yet, but patch 0001-Forbid-vararg-declaration-in-defprotocol-definterfac.patch no longer applies cleanly after the latest commits to Clojure master on Oct 25 2013.

Tassilo Horn
October 28, 2013, 8:21 AM

I've rebased the patch onto the current master so that it applies cleanly again.

Tassilo Horn
October 28, 2013, 8:25 AM

Stu, I've assigned this issue to you because you've been assigned to CLJ-1165 which I have closed as duplicate of this issue.

One minor difference between my patch to this issue and CLJ-1165 is that here I use a CompilerException with file/line/column info whereas in CLJ-1165 I've used `ex-info`. I think the CE is more appropriate/informative, as the error is already triggered during macro expansion.

Michael Blume
August 16, 2015, 10:36 PM

Rebased onto master

Michael Blume
August 17, 2015, 6:46 PM

Be nice if we got this merged – I just got a pull request using a varargs protocol that seemed to work by accident because the only time the varargs arity was called, it was called with two additional arguments, matching the '& and the 'args in the interface. Confused the hell out of me before I worked out what was going on.




Greg Chapman





Code and Test

Affects versions