Clojure Q&A - Recent questions and answers in Transducers

Answered: The behavior of `filter` on infinite sequences in Transducers

Sun, 24 May 2026 08:31:02 +0000

take converts its collection argument into a seq, and making a seq out of an eduction is done via the java.lang.Iterable route. That route moves the data around in chunks of size 32. So (take ... (eduction ...)) will try to fetch 32 elements regardless of the number passed to take.

So, it's not about transducers, it's about how eduction and converting it into a seq work.

You'll see the same behavior with sequence - but that's because it creates a chunked sequence internally.

Answered: Optimize `eduction`

Sun, 05 Apr 2026 21:50:04 +0000

Logged as https://clojure.atlassian.net/browse/CLJ-2953

Answered: There is currently no option for `partition` to use the advantage of transducers.

Tue, 22 Aug 2023 18:54:45 +0000

Logged as https://clojure.atlassian.net/browse/CLJ-2797

Answered: Could `run!` gain another arity to cover the common case of feeding into `eduction`?

Tue, 22 Aug 2023 14:50:49 +0000

Logged as https://clojure.atlassian.net/browse/CLJ-2796

Transducer arity for vec

Fri, 28 Jul 2023 04:26:18 +0000

In light of Metosin's recent comments on the costs of laziness, what do you think about adding a transducer arity to vec? In essence it becomes an eager version sequence. Semantically I think (vec (map inc) coll) integrates fairly well with the idea of vec. That tiny bit of terseness over (into [] (map inc) coll) I think is worth having personally.

https://clojure-goes-fast.com/blog/clojures-deadly-sin/

calling take-nth with 0 causes an OutOfMemoryError, and kills the repl.

Wed, 08 Mar 2023 07:59:54 +0000

calling take-nth with 0 causes an OutOfMemoryError, and kills the repl. Perhaps a validation > 0 required here?

to reproduce,

user=> (take-nth 0 [1 2])
OutOfMemoryError Java heap space
        reply.eval-modes.nrepl/session-responses (nrepl.clj:51)
        reply.eval-modes.nrepl/session-responses (nrepl.clj:51)
        reply.eval-modes.nrepl/session-responses/fn--1340 (nrepl.clj:56)
        clojure.lang.LazySeq.sval (LazySeq.java:42)
        clojure.lang.LazySeq.seq (LazySeq.java:51)
        clojure.lang.RT.seq (RT.java:535)
        clojure.core/seq--5467 (core.clj:139)
        clojure.core/filter/fn--5962 (core.clj:2826)
        clojure.lang.LazySeq.sval (LazySeq.java:42)
        clojure.lang.LazySeq.seq (LazySeq.java:58)
        clojure.lang.RT.seq (RT.java:535)
        clojure.core/seq--5467 (core.clj:139)
Bye for now!
Error printing return value (OutOfMemoryError) at clojure.core/take-nth (core.clj:4289).

Answered: Why doesn't transduce call initializers?

Sun, 15 Jan 2023 05:38:58 +0000

I'm not sure if you're on Slack but there was a really good answer to this in a thread about six months ago: https://clojurians.slack.com/archives/C053AK3F9/p1657570994323509?thread_ts=1657568229.677049&cid=C053AK3F9

The TL;DR is that "you can use a transducer to build a reducing function, so that's why transducers must support the init arity" with the following example give:

(def useless-xform
  (fn [rf]
    (fn
      ([] (rf))
      ([result] (rf result))
      ([result input]
       (rf result input)))))

(transduce identity
           (useless-xform +)
           [1 2])
;; 3

This is the most plausible explanation I've read so far on the otherwise puzzling init arity of transducers "not being used".

Answered: group-by reducing function

Sat, 06 Aug 2022 15:11:55 +0000

Logged as https://clojure.atlassian.net/browse/CLJ-2716

Answered: Does not eduction work with reduce anymore?

Thu, 07 Oct 2021 03:55:44 +0000

Nothing has changed here - eduction has always been implemented only with IReduceInit. The clojuredocs example is wrong.

Answered: What would transducers-first Clojure look like?

Mon, 04 Oct 2021 15:25:34 +0000

https://github.com/pixie-lang/pixie

Answered: halt-when transducer throws class cast exception when used with into

Mon, 23 Aug 2021 01:39:29 +0000

We have an existing ticket for this at https://clojure.atlassian.net/browse/CLJ-2556

Answered: clojure.walk/walk can use transducers and protocols

Fri, 13 Nov 2020 20:34:15 +0000

Also see https://ask.clojure.org/index.php/2376/faster-more-flexible-dispatch-for-clojure-walk / https://clojure.atlassian.net/browse/CLJ-1239

Answered: Reducing to a collection

Fri, 25 Sep 2020 23:08:32 +0000

"...why this function doesn't work." Short answer: it is not a function. def just binds a symbol to the result of the enclosed form.

If you want a function, you can change the first line to

(defn countToFive []

Answered: Infinite loop with stateful transducer on core.async channel

Sat, 08 Aug 2020 17:52:29 +0000

Have a look at partition-by:
https://github.com/clojure/clojure/blob/clojure-1.10.1/src/clj/clojure/core.clj#L7160
Especially the comment ";;clear first!", one of the two comments in Clojure core that has an exclamation point.

Answered: Stateful transducers, thread safety and performance

Tue, 18 Feb 2020 07:09:48 +0000

The answer can be found in this discussion https://groups.google.com/d/msg/clojure/VQj0E9TJWYY/nXVf_H6sAgAJ

So yes, we can use unsafe transducers when we are sure about single-threaded reducing context. But not in general case.

Answered: Why does eduction take xforms rather than a single xform?

Mon, 14 Oct 2019 11:04:46 +0000

The motivation appears to be to aid conversion from lazy-seq code using ->>, e.g.

(->> s (interpose 5) (partition-all 2))

to this:

(->> s (eduction (interpose 5) (partition-all 2)))

Example lifted here from the above link.

Answered: Transducers and Maps

Wed, 02 Oct 2019 09:25:45 +0000

In xforms (https://github.com/cgrand/xforms) there is a x/str transducing context to directly build a string from a transduction:

user=> (x/str 
         (comp
           (x/for [{:keys [dataPoints]} %
                   {{:keys [lat lng]} :location} dataPoints]
             (str lat " " lng))
           (interpose ", "))
         (-> trip :tripData :segments))
"1 2, 3 4"

Answered: What are other possible transducer contexts?

Wed, 14 Aug 2019 22:30:44 +0000

clojure.java.jdbc/reducible-query
next.jdbc/plan
lines-reducible

Answered: Consider kv support for transducers (similar to reducers fold)