You need to use a buffer to defer calls to the reduced function

(defn drop-last [n coll]
  (reducer coll
    (fn [f1]
      (let [buffer
            (atom clojure.lang.PersistentQueue/EMPTY)]
        (fn self
          ([] (f1))
          ([ret x]
            (let [b (swap! buffer conj x)]
              (if (<= (count @buffer) n)
                ret
                (do
                  (swap! buffer pop)
                  (f1 ret (peek b)))))))))))

An array or a ring buffer should be used instead of  the atom and
persistent queue combo to reduce allocation.

take-last is harder because you can't know when the reduction is over when
using #'reducer, so you have to implement CollReduce yourself:

(defn take-last [n coll]
  (reify clojure.core.protocols.CollReduce
    (coll-reduce [this f1]
      (clojure.core.protocols/coll-reduce this f1 (f1)))
    (coll-reduce [_ f1 init]
      (clojure.core.protocols/coll-reduce
        (clojure.core.protocols/coll-reduce
          coll
          (fn [q x]
            (let [q (conj q x)]
              (if (<= (count q) n)
                q
                (pop q)))) clojure.lang.PersistentQueue/EMPTY)
        f1 init))))

again, use of a mutable array/buffer would be preferable.

hth,

Christophe

On Thu, Aug 8, 2013 at 1:00 PM, Jozef Wagner <jozef.wag...@gmail.com> wrote:

> Is it possible to implement efficient butlast (and drop-last, take-last)
> with reducers? The only solution I can think of needs additional reduce to
> compute count, which may often be undesirable.
>
> Or is it OK to say that reducers are not designed for such cases?
>
> JW
>
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