trampoline.clj

akar.try-out=> (defn tail-recursive-sum [x running-total]
          #_=>   (if (zero? x) running-total #(tail-recursive-sum (dec x) (+ running-total x))))
#'akar.try-out/tail-recursive-sum

akar.try-out=> (alter-var-root #'tail-recursive-sum (fn [cur] (partial trampoline cur)))
#object[clojure.core$partial$fn__4759 0x6188158f "clojure.core$partial$fn__4759@6188158f"]

akar.try-out=> (tail-recursive-sum 10000 0)

StackOverflowError   clojure.lang.Util.equiv (Util.java:30)

trampoline.scala

scala> import util.control.TailCalls._
import util.control.TailCalls._

scala> def tailRecursiveSum(x: Int, running: Int): TailRec[Int] = x match {
     |   case 0 => done(running)
     |   case _ => tailcall(tailRecursiveSum(x - 1, x + running))
     | }
tailRecursiveSum: (x: Int, running: Int)util.control.TailCalls.TailRec[Int]

scala> tailRecursiveSum(100000, 0).result
res2: Int = 705082704

Not sure what you're doing with the alter-var-root. I would do something like:

(defn tramp-sum [x t] 
  (trampoline tail-recursive-sum 10000 0))

(tramp-sum 10000 0)
;;=> 50005000

Of course in this case, the idiomatic way to get TCO in Clojure is to just use recur. trampoline is only needed for mutual recursion.

(defn tail-recursive-recur [x running-total]
  (if (zero? x) running-total (recur (dec x) (+ running-total x))))

(tail-recursive-recur 10000 0)
;;=> 50005000

@puredanger Thanks a lot, mate! I figured out what was wrong. alter-var-root also changes the reference to tail-recursive-sum in its original definition. :-) Mutability... Still bites me after all these years!