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Java ships with some robust parallel Streams already implemented. If you don't believe me, check the Files.lines() stream in JDK9 for yourself. clambda is a little Clojure library to help you reduce/transduce over Java Streams (including parallel ones). Helpers for conveniently creating Lamdas from plain Clojure functions were inevitable.




This code started out as an attempt to write a parallel dictionary-attack tool (for great fun and NO profit) that could handle really big files without memory linearly growing. Laziness, or even better, something reducible will do a great job at that, but constrained me in one thread. I quickly realised that I could use Java Streams. In fact, when I found out that Files.lines returned a Stream that could be turned to parallel rather easily - I was simply was ecstatic (turns out that's only true for JDK 9 and above). Long story short, once the potential clicked, I simply wrote some convenience utilities, and some interesting things simply fell out.

All that said, the value of this library is most-likely NOT password-cracking, but rather Java interop (consuming Streams).




In some respects, I feel this is the centerpiece when it comes to Java interop. Everything else builds on top of this, and it does exactly what it says on the tin. It turns a java Stream into something reducible.


A 'collecting' transducing context (along the lines of clojure.core/into), for Streams. No intermediate collections are involved, and in the case of a sequential Stream, it can/will be done fully mutably (via transients). The same cannot be said for a parallel Stream, which can do the 'outer' combining using transients (via into), but the 'inner' reductions must not mutate their arguments, so conj has to be used (as opposed to conj!).

(use 'clambda.core)
(import '[ LongStream]
        '[java.util Random])

(let [test-stream (LongStream/range 0 10)
         expected (range 0 10)]
      (= expected (stream-into [] test-stream))) ;; => true

(let [test-stream (-> (Random.) (.ints 10 1 50))]
      (every? odd? (stream-into [] (filter odd?) test-stream)) ;; => true

(let [test-stream (.parallel (LongStream/range 0 500))
          expected (range 1 501)]
      (= expected (stream-into [] (map inc) test-stream)))  ;; => true


When dealing with a parallel Stream, you have to be extra careful with what you use as init value, because that may (and probably will) be used in multiple reductions (each in its own thread). An empty vector, or an empty anything will be fine, but depending on your use-case, an non-empty target collection may not produce the results you expect. Again, perhaps your logic does account for duplicates as the result of using the same (non-empty) init in multiple reductions, but maybe it doesn't. Be aware and cautious!


An abortive transducing context for Streams. For sequential Streams, rather similar to .findFirst() in terms of Streams, or clojure.core/some in terms of lazy-seqs. For parallel Streams, more like .findAny(), with the added bonus of being able to abort the search on the 'other' threads as soon as an answer is found on 'some' thread.

Even though the idea of (first (filter some-pred some-seq)) is sort of an anti-pattern when dealing with chucked lazy-seqs, it's actually a great patten when adapted for transducers/reducibles.

(use 'clambda.core)
(import '[ LongStream]
        '[java.util Random])

(let [test-stream (LongStream/range 1 11)]
      ;; find first even number in the stream
      (= 2 (stream-some (filter even?) test-stream))) ;; => true

At this point implementing a dictionary-attacker based on the (parallel) Stream returned by Files/lines is trivial:

;; where `try-fn` is some (undefined) process accepting a candidate
;; and returning something truthy upon match
(stream-some (map try-fn) parallel-line-stream)

Mind you, 2GB is the maximum size of a file that the JVM will let you mmap. In other words, you can safely forget about parallelism if you're dealing with files larger than 2GB (on the JVM). If you're curious and would like to know more about that limitation see here. Essentially, it all comes down to int being 32-bit and being used as the type for array indexing. The irony is that this decision was made around a time (mid 90s) when 64-bit CPUs were, somewhat, visible on the horizon.


An alternative to clojure.core/line-seq. Returns something reducible, rather than a lazy-seq.

If one is ok with a single-threaded dictionary-attack, it's worth considering the following.

(transduce     ;; no reason for Streams
  (map try-fn) ;; `try-fn` tries (somehow) each candidate
  (lines-reducible (io/reader "/path/to/dict.txt")))

I would be (pleasantly) surprised to find some other single-threaded approach that would be faster than the above, as it basically boils down to a loop/recur against a BufferedReader. In fact, from what I can observe the above approach is showing very similar timings as the sequential Files/lines Stream (via .findFirst()), something which I find rather encouraging, and just goes to show that quite often there is very little to be gained dropping down to Java. You should (always) benchmark for yourself though ;).


An alternative to clojure.core/iterator-seq. Returns something reducible, rather than a lazy-seq.


Diverging from the overall spirit of this library (which is reducible-streams), stream-seq lets you turn a java Stream into a clojure Seq, via its plain old sequential Iterator (see iterator-seq). It is a very simple function which essentially wraps iterator-seq, and is provided for completeness, but make sure you understand the trade-offs before using it. I won't say "don't use it", because there are cases where you may need to. That said, I feel obliged to say that stream-reducible should almost always be your first choice.


If you're writing Clojure, then you can probably ignore this function. I don't see why you would want to convert a native Clojure data-structure to a Java Stream. However, if you're writing Java, it is conceivable that your code needs to consume a Clojure data-structure. In such (admittedly rare) cases, it is only natural to want to express your code in terms of streams.

Here is roughly what you need to do in order to invoke seq-stream from Java.

import clojure.lang.IFn;
import clojure.lang.ISeq;

private static final IFn require = Clojure.var("clojure.core", "require");

public Stream seqStream (ISeq s, long splitSize, boolean parallel){

    IFn toSeqStream = Clojure.var("clambda.core", "seq-stream");
    return toSeqStream.invoke(s, splitSize, parallel);

Parallelism is supported, but is enabled by default only for vectors. If you wish to enable it for other types, or disable it for vectors, use the 3-arg overload as shown above. First argument is obviously the Seq in question, second is the partition size for lazy-seqs (if parallelism is enabled), and third is whether we want to allow for parallelism.

Some rudimentary benchmarking shows set/map being much slower (especially set) than vector, in both parallel and sequential execution. That said, it needs to be noted that when considered in isolation, there seems to be great benefit from going parallel (when the numbers grow). Vectors perform respectably in either context, especially when parallelised, (that's why it's done by default for them). Lazy-seqs can technically be parallelised but there is usually very little (to no) benefit in terms of performance, as they cannot be split cheaply. In any case, it will always be cheaper not to involve (yet) another abstraction (Stream) if you can afford it. Again, be aware and cautious!


jlambda (multi-method dispatching on the first arg)

You don't need to require this namespace if you're not adding implementations. A wrapper function is provided in clambda.core/jlambda for convenience.

The following implementations are provided:

  • [:predicate f] where f is a fn of 1 arg
  • [:function f] where f is a fn of 1 arg
  • [:bi-function f] where f is a fn of 2 args
  • [:consumer f] where f is a fn of 1 arg
  • [:long-consumer f] where f is a fn of 1 arg
  • [:int-consumer f] where f is a fn of 1 arg
  • [:double-consumer f] where f is a fn of 1 arg
  • [:bi-consumer f] where f is a fn of 2 args
  • [:supplier f] where f is a fn of 0 args
  • [:long-supplier f] where f is a fn of 0 args
  • [:int-supplier f] where f is a fn of 0 args
  • [:double-supplier f] where f is a fn of 0 args
  • [:unary f] where f is a fn of 1 arg
  • [:long-unary f] where f is a fn of 1 arg
  • [:int-unary f] where f is a fn of 1 arg
  • [:double-unary f] where f is a fn of 1 arg
  • [:binary f] where f is a fn of 2 args

Primitive casts are implicitly added where needed.


Be wary of parallel streams

init values

As a general rule of thumb, be extra careful with what you hand off as the init value to a parallel reduction. There is no one-size-fits-all recipe here - it all depends on the function that's reducing. In the case of stream-into, the reducing-fn is (unsurprisingly) conj or conj! (when possible). So, simply put, two threads receiving the same non-empty init value, will produce duplicates (unless init is a set of course). The duplicates will be N copies of the initial init elements, where N will end up being the number of threads that used that init for their internal reduction.

collecting mutably

Collecting mutably (via transients) is no problem for sequential streams. Parallel reductions can NOT be done (fully) mutably - only their (eventual) combining, and that's exactly what stream-into does. It uses conj for the 'inner' reductions, and conj! (via into) for combining their results.


If you want to (idiomatically) consume Java streams from Clojure, or Clojure seqs from Java, then you've come to the right place. For Clojure users, see stream-reducible, stream-into, stream-some, and the jlambda constructor-fn. For Java users, check-out seq-stream (see the 'How' section for instructions). Do NOT expect that processing a SeqStream with Lamdas will give you anywhere near the performance of a native (mutable) Stream (or even that of processing the same seq directly in Clojure), unless perhaps in the case of vectors (that happen to parallelize nicely too).

All the aforementioned functions exist in the clambda.core namespace.


There is quite a bit of 'rogue' code out on the internet showcasing how to consume Java Streams from Clojure. In terms of an actual library however, I was only able to find ike.cljj, which does a fine job, if you're willing to sacrifice the potential for parallelism (that a Stream might otherwise give you).


Copyright © 2018 Dimitrios Piliouras

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.

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