Couple of things to watch out for, Clojure doesn't pattern match on the
args in the way you seem to be expecting, so this will blow the stack (i.e.
calling qsort with the empty list won't call the 0-argument method you have
there.)
The error you're running into is that pop/peek isn't defined for lazy
sequences. doall will force your lazy-sequence, but it's still of type
LazySeq:
>(type (lazy-seq [1 2 3]))
clojure.lang.LazySeq
So putting the base-case into an if/else branch, and switching pop/peek for
rest/first would look like so:
(defn qsort
([list]
(if (seq list)
(let [piv (first list)
f-half (filter (fn [n] (<= n piv)) (rest list))
s-half (filter (fn [n] (> n piv)) (rest list))]
(concat (qsort f-half) (cons piv (qsort s-half))))
nil)))
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