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Transformer typeclasses for cats.

Provides transformer typeclasses for cats' Monads, Applicatives and Functors.

As well, there are some abstractions thrown in that allow you to generically lift MTL typeclasses through transformers.

You can have multiple cats-mtl transformer typeclasses in scope at once without implicit ambiguity, unlike in pre-1.0.0 cats or Scalaz 7.


libraryDependencies += "org.typelevel" %% "cats-mtl-core" % "0.7.0"

If your project uses Scala.js, replace the double-% with a triple. Note that cats-mtl has an upstream dependency on cats-core version 2.x.

Cross-builds are available for Scala 2.11–2.13, Scala.js major version 0.6.x.

If you're not sure where to start or what Cats-mtl even is, please refer to the getting started guide.

Supported Classes

  • EitherT
  • Kleisli
  • IorT
  • OptionT
  • ReaderWriterStateT
  • StateT
  • WriterT


The cats-mtl-laws artifact provides Discipline-style laws for all of the type classes defined in cats-mtl. It is relatively easy to use these laws to test your own implementations of these typeclasses. Take a look here for more.

libraryDependencies += "org.typelevel" %% "cats-mtl-laws" % "0.7.0" % Test

These laws are compatible with both Specs2 and ScalaTest.



  1. Website:
  2. ScalaDoc:

Related Cats links (the core):

  1. Website:
  2. ScalaDoc:

Migrating from Cats pre-1.0.0

cats-core used to provide various mtl-classes which were moved to cats-mtl and split up. You can find the migration guide here


People are expected to follow the Scala Code of Conduct when discussing cats-mtl on the Github page, Gitter channel, or other venues.

We hope that our community will be respectful, helpful, and kind. If you find yourself embroiled in a situation that becomes heated, or that fails to live up to our expectations, you should disengage and contact one of the project maintainers in private. We hope to avoid letting minor aggressions and misunderstandings escalate into larger problems.


All code is available to you under the MIT license, available at and also in the COPYING file.

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