tl;dr: for a recur call to a variadic function, wrap the variadic parameters in a (list ..)

I am writing this for my future self because I’ll stub my toe on this again if I don’t. Future self, you’re welcome.

So, I was editing a function to use recur. It was my first time using recur with a variadic parameter.

WTF. The recur invocation threw an error. Walking in the debugger confused me further. What saved me from frustration was this in the middle of the community documentation for recur.

; Note that recur can be surprising when using variadic functions.

And in the official docs

The recur expression must match the arity of the recursion point exactly. In particular, if the recursion point was the top of a variadic fn method, there is no gathering of rest args - a single seq (or null) should be passed.

Well, good to know.

And here’s the function I was refactoring.

(defn cleave-repeat
  "Finds a repeating pattern in string s using r as the next attempt to try."
  [s & [r]]               ; http://clojuredocs.org/clojure.core/recur 
                          ; recur with variadic does not wrap r in
                          ; a list. So a normal function call and recur'd one will otherwise differ.
  (if (empty? s)
    r
    (if (empty? r)
      (cleave-repeat (apply str (rest s)) (str (first s))) ; This calls the variadic "normally"
      (if (empty?                       ; We have the pattern in r if this is true
           (clojure.string/split
             (apply str (drop-last (mod (count s) (count r)) s))
             (re-pattern r)))
        r                            ; Return the pattern
        ; 'recur' calls with varadic not wrapped by a list, so (list ...) is needed.
        (recur (apply str (rest s))
          (list (clojure.string/join [r (str (first s))])))))))

Damn you JVM.