This threw me off the other day

09:43 $ clj
Clojure 1.8.0
(defmulti foo :bar)
(defmethod foo :qix [quux znoot] (println 'hi))
#'user/foo
user=> #object[clojure.lang.MultiFn 0x205d38da "[email protected]"]
user=> (foo {:bar :qix})
ArityException Wrong number of args (1) passed to: user/eval5/fn--6  clojure.lang.AFn.throwArity (AFn.java:429)
user=>

Albeit in a more convoluted setting.

Reading through some code, I see that defmethod does amongst other things a call (fn [email protected]) which gives us a function named, in my example user/eval5/fn--6

If we change the call to fn to be (fn ~multifn [email protected]) the error becomes:

user=> (defmulti foo :bar)
(defmethod foo :qix [quux znoot] (println 'hi))
(foo {:bar :qix})#'user/foo
user=> #object[clojure.lang.MultiFn 0x4e928fbf "[email protected]"]
user=>
ArityException Wrong number of args (1) passed to: user/eval5/foo--6  clojure.lang.AFn.throwArity (AFn.java:429)
user=>

Which at least gives the user a clue. Would a ticket be in order ofr this?

Another way to get to this would be to let AFn have a specific name (which would default to getClass().getSimpleName() and have MultiFn set the name of the AFn when addMethod is called.

The latter approach leads to:

Clojure 1.8.0
user=> (defmulti foo :bar)
(defmethod foo :qix [quux znoot] (println 'hi))
(foo {:bar :qix})#'user/foo
user=> #object[clojure.lang.MultiFn 0x4e928fbf "[email protected]"]
user=>
ArityException Wrong number of args (1) passed to: foo  clojure.lang.AFn.throwArity (AFn.java:441)
user=>

Problem here is that we loose the ns of the function.

Ticket would be fine

https://dev.clojure.org/jira/browse/CLJ-2263