recursion in implementation of "partition" function

Question

I was randomly reading through Clojure source code and I saw that partition function was defined in terms of recursion without using "recur":

(defn partition
  ... ...
  ([n step coll]
     (lazy-seq
       (when-let [s (seq coll)]
         (let [p (doall (take n s))]
           (when (= n (count p))
             (cons p (partition n step (nthrest s step))))))))
  ... ...)

Is there any reason of doing this?

Answer 1

Partition is lazy. The recursive call to partition occurs within the body of a lazy-seq. Therefore, it is not immediately invoked but returned in a special seq-able object to be evaluated when needed and cache the results realized thus far. The stack depth is limited to one invocation at a time.

A recur without a lazy-seq could be used to create an eager version, but you would not want to use it on sequences of indeterminate length as you can with the version in core.