New in version 0.10.0.

The loop / recur macro gives programmers a simple way to use tail-call optimization (TCO) in their Hy code.

A tail call is a subroutine call that happens inside another procedure as its final action; it may produce a return value which is then immediately returned by the calling procedure. If any call that a subroutine performs, such that it might eventually lead to this same subroutine being called again down the call chain, is in tail position, such a subroutine is said to be tail-recursive, which is a special case of recursion. Tail calls are significant because they can be implemented without adding a new stack frame to the call stack. Most of the frame of the current procedure is not needed any more, and it can be replaced by the frame of the tail call. The program can then jump to the called subroutine. Producing such code instead of a standard call sequence is called tail call elimination, or tail call optimization. Tail call elimination allows procedure calls in tail position to be implemented as efficiently as goto statements, thus allowing efficient structured programming.

—Wikipedia (https://en.wikipedia.org/wiki/Tail_call)



loop establishes a recursion point. With loop, recur rebinds the variables set in the recursion point and sends code execution back to that recursion point. If recur is used in a non-tail position, an exception is raised.

Usage: (loop bindings &rest body)


(require [hy.contrib.loop [loop]])

(defn factorial [n]
  (loop [[i n] [acc 1]]
    (if (zero? i)
      (recur (dec i) (* acc i)))))

(factorial 1000)