knutwalker / transducers-scala

Transducers are a way to build reusable transformations.


Build Status Coverage Maven Apache License


Transducers are a way to build reusable transformations.

Put simply, transducers are like operations on collections (e.g. map and filter) but without being bound to any collection or result type. Thus, they can be reused in many contexts and carry only the simple processing logic.

Getting it

transducers-scala is published to sonatype:

libraryDependencies += "de.knutwalker" %% "transducers-scala" % "0.5.0"

Detailed instructions are available the installation notes.

Snapshot version

A snapshot is available at sonatype:

resolvers += "sonatype snapshots" at ""

libraryDependencies += "de.knutwalker" %% "transducers-scala" % "0.6.0-SNAPSHOT"

Alternatively, clone this repo and run

./sbt +publishLocal # for ivy-style repositories
./sbt +publishM2    # for maven-style repositories

Using it

Add "de.knutwalker" %% "transducers-scala" % "0.5.0" to your dependencies. The full artifact names are transducers-scala_2.10 and transducers-scala_2.11, so this library is available for Scala 2.10 and 2.11.

After that, simply use scalax.transducers._

import scalax.transducers._

val xform = filter[Int](_ % 2 == 0).map(_.toString).flatMap(_.toList).take(10)
val data = Stream.from(13)
val result = into[List].run(xform)(data)
assert(result == List('1', '4', '1', '6', '1', '8', '2', '0', '2', '2'))

For more examples and a better introduction, see the Usage Guide.


In short, transducers transform reducers. Reducers combine some result and a value together to a new result. Reducers are similar to the binary operations, one uses with folds. A Transducer takes a reducer and return a transformed reducer. The crux is, that neither reducers nor transducers are in any form aware of their concrete input and output types.

Type simplification

type Reducer[A, R] = ((R, A) => R)

type Transducer[A, B] = (Reducer[B, R] => Reducer[A, R]) forSome { type R }

Reducers are just functions, that combine a result and an input value to a new result. This implementation is restricted in such a way, that the R is identical (same R in and out). This is mostly not a problem, since transducers may change types and most reducers have access to a down-stream reducer of the target type (that can be different from A). Reducers also have two additional operations. The first is a setup operation, that is called with the current result before and items flow through the reducer. The second is a finish operation, that allows the reducer to do something whenever a stream of data is finished (although it might never finish).

A Transducer[A, B] is a function, that takes an Reducer[B, _] and returns an Reducer[A, _]. Transducers transform their reducers from right to left. When finally applied, the data flows from left to right. So, a Transducer[A, B] can roughly be seen as a function A => B, in that it describes a transformation from A to B, but it is not specified, how many B can be produced. A transducer may provide 0, 1, or an arbitrary number of Bs, not just one (as A => B would have to).

An important note is, that Transducer[A, B] is a rank-2 type to quantify R without needing to know about it. This makes transducers completely unaware of their input or output.

Some other projects with partially similar traits

Besides Clojure and transducers-java there exist a number of projects, that share some traits with transducers-scala.

RxJava (RsScala)

Probably the most similar concept are RxJava's Operators. Operators transform Subscribers where transducers transform reducers. While these operators are bound to be used with Subscribers, the Subscriber/Observer/Observable itself can be plugged in easily on different sources and targets.

psp-view (psp-std)

Paul Phillips' take on a scala collection/standard library includes collections, that are thoroughly non-strict and thus, achieve the same traits on terms of laziness and early termination. These are, however, regular collections where the operations are bound to the specific collection.


This stuff is build on top of Rich Hickeys transducers for Clojure and Java. See this talk and this post for more info. Also, there is a Java implementation.

The sbt launcher is provided by Paul Phillips.

Stuff todo

  • documentation
  • some more tests
  • contrib modules, that included
    • support for scalaz type classes
    • support for functionaljava classes
    • support for guava classes


Copyright 2014 Paul Horn

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.