Question

With the following code, I get #<CompilerException java.lang.UnsupportedOperationException: Can only recur from tail position (NO_SOURCE_FILE:4)> despite the fact that all recurs are in tail positions. If I remove the recur from the one-argument version, it stops complaining. Why is this happening?

(defn remove-duplicates "Removes duplicate elements of lst. 

For example, given (1 2 3 1 4 1 2), remove-duplicates returns a sequence 
containing the elements (1 2 3 4), in some order."
  [lst] (recur (rest lst) (set (first lst)))
  [lst uniques] (cond (zero? (count lst)) uniques
                      :else (cond
                              (some (partial = (first lst)) uniques)
                              (recur (rest lst) uniques)
                              :else 
                              (recur (rest lst) (first lst)))))

Answer 1

You haven’t split up the multi-arity bodies right. Should read (defn foo ([x] (...)) ([x y] (...))). This causes the compiler to think you’re doing totally different stuff, which probably accounts for your issue.