On-the-fly collections with Reducibles

digest large datasets with ease

On-the-fly collections with Reducibles


Lazy Sequences

Lazy Sequences are great for exploring a problem space. However you hit a few snags as data volumes ramp up:

It is all too easy to create head retention

leading to an Out Of Memory Error

Each lazy sequence maintains its own header tank of chunked data

which can add up to alot of memory in a long pipeline

Difficult to release resources predictably

For system resources like Files, there is no reliable way to release resources that are lazily crawled

Exception are deferred to The Future

Any latent exceptions will be exposed in a lazy manner; meaning you could have to handle an exception when you are least expecting it, or have the Virtual Machine quietly end that thread.

Awkward to interrupt processing

if you want to abort processing 2% of the way through a 1'000'000'000 element sequence you may find that it takes a surprisingly long time to finish and return control.

Enter reducible

A reducible is a neat way to avoid these problems.

A reducible is a direct reify of clojure.lang.IReduceInit or clojure.lang.IReduce that conceals a loop/recur behind a reduce friendly facade.

It runs eagerly and can plug into reduce/transduce/into thereby avoiding problems described above.

Consider IReduceInit

because this is the more general purpose of the two.

public interface IReduceInit{
    Object reduce(IFn f, Object start) ;

this reduce method will get called by clojure.core/reduce function. A general implementation might look like

(defn iterator-reducible
  "expresses an iterator through the medium of IReduceInit
   if first-val is nil it will be ignored"
  [first-val ^java.util.Iterator it]
  (reify IReduceInit
    (reduce [this f init]
      (loop [acc (if first-val
                   (f init first-val)
        (if (or (reduced? acc)
                (not (.hasNext it)))
          (unreduced acc)
          (recur (f acc (.next it))))))))
  1. It’s a plain old loop/recur

  2. We invoke the reducing function f to handle a value

  3. We always check the accumulator for the reduced short circuit. This supports early-termination constructs like (take 2 The iterator may go on for ever, so we need a way to break out of the loop.

  4. the first parameter to the reduce function is the Java this reference, it typically does not carry any value her and is usually ignored.

You use reducibles directly with

  • into

  • reduce

  • transduce

(into [] (take 10) (iterator-reducible :starting-value (.iterator (range))))
=> [:starting-value 0 1 2 3 4 5 6 7 8]

You CANNOT plug them into sequence

(sequence (take 10) (iterator-reducible :starting-value (.iterator (range))))

IllegalArgumentException Don't know how to create ISeq from: dev$iterator_reducible$reify__54612  clojure.lang.RT.seqFrom (RT.java:550)

although you can of course (into '()

It gets useful when

using system resources; for example if we need to crawl zip files but have to make sure that the file resources get released immediately after use, then we can write

(defn zipfile-reducible
  "expects to be passed a .zip or .jar file
   crawls through the file, presenting entries to the reducing function
   ensures the file is closed afterwards"
  (reify IReduceInit
    (reduce [this f init]
      (with-open [is (clojure.java.io/input-stream zf)
                  zis (ZipInputStream. is)]
        (loop [acc init]
          (let [next-entry (.getNextEntry zis)]
            (if (and (some? next-entry)
                  (not (reduced? acc)))
              (recur (f acc (assoc (bean next-entry)
                              :input-stream zis)))
              (unreduced acc)))

and we can experiment with

(into []
  (comp (drop 10) (take 1))
    (jio/file (System/getProperty "java.home") "lib/charsets.jar")))

to take this further, we can then crawl every zip entry in every .jar file in /usr/lib/jvm like this

(defn preserving-reduced
  "copy-and-pasted from clojure.core, which declares it as private"
  #(let [ret (rf %1 %2)]
     (if (reduced? ret)
       (reduced ret)

(defn chaining-reducible
  "like concat but for reducibles
  takes a coll of colls.
  Returns reducible that chains call to reduce over each coll"
  (reify IReduceInit
    (reduce [_ f init]
      (let [prf (preserving-reduced f)]
        (reduce (partial reduce prf)

(defn iszipped-suffix?
  [^File f]
  (let [n (.getName f)]
    (some #(string/ends-with? n %)
      #{".jar" ".zip"})))

(time (reduce
        (fn [acc _] (inc acc))
          (map zipfile-reducible
            (filter iszipped-suffix?
              (file-seq (.getParentFile (.getParentFile (jio/file (System/getProperty "java.home"))))))))))
"Elapsed time: 16922.336304 msecs"
=> 367051

Note that we are using lazy sequences of zipfile reducibles; If this created a problem, there is scope for turning this into a reducible too

Careful of those arities

  • The 2 argument reduce will only ever use clojure.lang.IReduce

  • Everything else uses clojure.lang.IReduceInit including

    • BOTH arities of transduce

    • into

    • eduction


(reduce + (iterator-reducible nil (.iterator (range 10))))

ClassCastException dev$iterator_reducible$reify__54612 cannot be cast to clojure.lang.IReduce  clojure.core.protocols/fn--7831 (protocols.clj:75)

will throw you a ClassCastException


(transduce identity + (iterator-reducible nil (.iterator (range 10))))
=> 45


We could of course extend the iterator-reducible definition to support this but in practice I find the 2 argument form of reduce to be little used.


If you have large volumes of data to process, it is worth considering whether reducibles can get you better performance with lower system resource usage.