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Extensible effects are an alternative to monad transformers for computing with effects in a functional way. This library is based on the "free-er" monad and extensible effects described in Oleg Kiselyov in Freer monads, more extensible effects.

You can learn more in the User Guide:

You can also check out this presentation at flatMap Oslo 2016 (slides).


Eff is published for Scala 2.12, 2.13 and 3.

core is available for the JVM, ScalaJS and scala-native. Sbt dependency:
// check maven badge above for latest version
libraryDependencies += "org.atnos" %% "eff" % "5.21.0"

// to write types like Reader[String, *] addCompilerPlugin("org.typelevel" %% "kind-projector" % "0.13.2" cross CrossVersion.full)

// to get types like Reader[String, *] (with more than one type parameter) correctly inferred for scala 2.12.x scalacOptions += "-Ypartial-unification"


eff is a Typelevel project. This means we embrace pure, typeful, functional programming, and provide a safe and friendly environment for teaching, learning, and contributing as described in the Scala Code of Conduct.

Feel free to open an issue if you notice a bug, have an idea for a feature, or have a question about the code. Pull requests are also gladly accepted.


Modules in this repository


  • eff-zio eff extension for ZIO effects.

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