To continue on my previous post, here is the implementation of map
which returns both reducible and (lazy)seqable collection:

(defn my-map
    [f c]
    (let [coll (clojure.core.reducers/map f c)
          d (delay (clojure.core/map f c))]
      (reify
        clojure.core.protocols/CollReduce
        (coll-reduce [_ f1]
          (clojure.core.protocols/coll-reduce coll f1))
        (coll-reduce [_ f1 init]
          (clojure.core.protocols/coll-reduce coll f1 init))
        clojure.core.reducers/CollFold
        (coll-fold [_ n combinef reducef]
          (clojure.core.reducers/coll-fold coll n combinef reducef))
        clojure.lang.Seqable
        (seq [_] @d))))

user>   (def x (my-map inc (range 100)))
#'user/x
user> x
#<user$my_map$reify__1358 user$my_map$reify__1358@71f2a681>
user> (seq x)
(1 2 3 4 5 6 7 8 9 10 11 12 . . . 100)
user> (last x)
100
user> (reduce + x)
5050
user> (clojure.core.reducers/fold + x)
5050

Best,
JW

On Thursday, August 8, 2013 4:46:37 PM UTC+2, Jozef Wagner wrote:
>
> Wow, thank you very much! A perfect solution.
>
> At the end, wouldn't be good if the reducers would implement alongside 
> CollReduce also a Seqable interface, so that reducers could be used as a drop 
> in replacement for today's sequence functions (map, filter, ...)? CollReduce 
> implements 'eager' computations (when calling reduce and fold) while the 
> Seqable would implement a lazy ones (when calling seq).
>
> Functions working with seqs (e.g. first, rest) always call seq before working 
> with the collection, so this would be transparent for them. It may be a 
> breaking change and complects things a bit though...
>
> JW
>
> On Thursday, August 8, 2013 3:34:31 PM UTC+2, Christophe Grand wrote:
>>
>> ArrayDeque based versions:
>>
>> (defn drop-last [n coll]
>>   (reducer coll
>>     (fn [f1]
>>       (let [buffer (java.util.ArrayDeque. (int n))]
>>         (fn self
>>           ([] (f1))
>>           ([ret x]
>>             (.add buffer x)
>>             (if (<= (count buffer) n)
>>               ret
>>               (f1 ret (.pop buffer)))))))))
>>
>> (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
>>         (doto (clojure.core.protocols/coll-reduce
>>           coll
>>           (fn [^java.util.Deque q x]
>>             (.add q x)
>>             (when (> (count q) n)
>>               (.pop q))
>>             q) (java.util.ArrayDeque. (int n))) prn)
>>         f1 init))))
>>
>>
>>
>> On Thu, Aug 8, 2013 at 3:16 PM, Christophe Grand <chris...@cgrand.net> wrote:
>>>
>>> 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....@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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>>>>
>>>
>>>
>>>
>>>
>>> --
>>> On Clojure http://clj-me.cgrand.net/
>>> Clojure Programming http://clojurebook.com
>>> Training, Consulting & Contracting http://lambdanext.eu/
>>
>>
>>
>>
>> --
>> On Clojure http://clj-me.cgrand.net/
>> Clojure Programming http://clojurebook.com
>> Training, Consulting & Contracting http://lambdanext.eu/

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