Community Announcements

• Feb 15 2019 Cats 2019 roadmap is published.
• Jan 31 2019 Cats 1.6.0 is released.
• Jan 31 2019 Cats ecosystem community survey 2018 results is published

Cats

Overview

Cats is a library which provides abstractions for functional programming in the Scala programming language. The name is a playful shortening of the word category.

Scala supports both object-oriented and functional programming, and this is reflected in the hybrid approach of the standard library. Cats strives to provide functional programming abstractions that are core, binary compatible, modular, approachable and efficient. A broader goal of Cats is to provide a foundation for an ecosystem of pure, typeful libraries to support functional programming in Scala applications.

For more detail about Cats' motivations, go here.

You can read the API Documentation, here.

We have an exciting 2019 roadmap.

Getting Started

Cats is currently available for Scala 2.10 (up to 1.2.x), 2.11, 2.12, 2.13.0-M4, and Scala.js.

Cats relies on improved type inference via the fix for SI-2712, which is not enabled by default. For Scala 2.11.9 or later you should add the following to your build.sbt:

scalacOptions += "-Ypartial-unification"

Or, if you need to support older versions of Scala you can use the sbt-partial-unification plugin which extends support back through Scala 2.10.6 or later, to add it, simply add this line to your plugins.sbt:

addSbtPlugin("org.lyranthe.sbt" % "partial-unification" % "1.1.2")

And then create the Cats dependency, by adding the following to your build.sbt:

libraryDependencies += "org.typelevel" %% "cats-core" % "1.6.0"

This will pull in the cats-core module. If you require some other functionality, you can pick-and-choose from amongst these modules (used in place of "cats-core"):

• cats-macros: Macros used by Cats syntax (required).
• cats-kernel: Small set of basic type classes (required).
• cats-core: Most core type classes and functionality (required).
• cats-laws: Laws for testing type class instances.
• cats-free: Free structures such as the free monad, and supporting type classes.
• cats-testkit: lib for writing tests for type class instances using laws.
• alleycats-core: Cats instances and classes which are not lawful.

There are several other Cats modules that are in separate repos so that they can maintain independent release cycles.

• cats-effect: standard IO type together with Sync, Async and Effect type classes
• cats-mtl: transformer typeclasses for Cats' Monads, Applicatives and Functors.
• mouse: a small companion to Cats that provides convenient syntax (aka extension methods)
• kittens: automatic type class instance derivation for Cats and generic utility functions
• cats-tagless: Utilities for tagless final encoded algebras
• cats-collections: Data structures which facilitate pure functional programming

Release notes for Cats are available in CHANGES.md.

Cats is still under active development. While we don't anticipate any major redesigns, changes that are neither source nor binary compatible are to be expected in upcoming RC1 and 1.0 releases.

The Cats ecosystem

By sharing the same set of type classes, instances and data types provided by Cats, projects can speak the same "Cats language", and integrate with each other with ease.

General purpose libraries to support pure functional programming

• cats-par: Abstract type member Parallel instances
• cats-retry: composable retry logic for Cats and Cats Effect
• droste: recursion schemes for Cats
• Dsl.scala: The !-notation for creating Cats monadic expressions
• eff: functional effects and effect handlers (alternative to monad transformers)
• Freestyle: pure functional framework for Free and Tagless Final apps & libs
• iota: Fast [co]product types with a clean syntax
• Monocle: an optics library for Scala (and Scala.js) strongly inspired by Haskell Lens.
• newts: Defines newtypes compatible with Cats type classes
• origami: monadic folds
• refined: simple refinement types for Scala
• shims: seamless interoperability for cats and scalaz typeclasses and datatypes

Libraries with more specific uses

• atto: friendly little text parsers
• cats-scalacheck: cats typeclass instances for scalacheck
• cats-time: cats typeclass instances for java time
• circe: pure functional JSON library
• Ciris: Lightweight, extensible, and validated configuration loading in Scala
• cormorant: CSV handling library for FP
• decline: A composable command-line parser
• doobie: a pure functional JDBC layer for Scala
• extruder: Populate case classes from any data source
• fastparse-cats: cats Monad and Alternative instances for fastparse
• Fetch: efficient data access to heterogeneous data sources
• finch: Scala combinator library for building Finagle HTTP services
• Frameless: Expressive types for Spark
• FS2: compositional, streaming I/O library
• fuuid: functional uuid's
• github4s: wrapper around the GitHub API
• grafter: dependency-injection library using the Reader pattern
• gsheets4s: wrapper around the Google Sheets API
• hammock: Purely functional HTTP client
• henkan: Type safe conversion between case class instances with similar fields
• http4s: A minimal, idiomatic Scala interface for HTTP
• linebacker: functional thread pool management
• log4cats: functional logging
• monadic-html: Tiny DOM binding library for Scala.js
• Monix: high-performance library for composing asynchronous and event-based programs
• pureconfig: A boilerplate-free library for loading configuration files
• rainier: Bayesian inference in Scala
• scala-forex: exchange rate lookups
• scala-maxmind-ip-lookups: IP geolocation through the Maxmind database
• scala-referer-parser: referer parsing
• scala-weather: weather lookups
• scanamo: simpler DynamoDB access for Scala
• seals: tools for schema evolution and language-integrated schemata
• sup: Composable, purely functional healthchecks in Scala
• tsec: Typesafe, functional, general purpose cryptography and security library

Your project talks cats too? Submit a PR to add it here!

The full-size Cats logo is available for use for Cats related projects, contents, souvenirs, etc.

We offer a Cats Friendly Badge to let others know your project works with Cats!

Below are quick html and markdown snippets to use the badge in your own project.

<a href="https://typelevel.org/cats/"><img src="https://typelevel.org/cats/img/cats-badge-tiny.png" alt="Cats friendly" /></a>

![Cats Friendly Badge](https://typelevel.org/cats/img/cats-badge-tiny.png) 

How can I contribute to Cats?

We welcome contributions to Cats and would love for you to help build Cats. See our contributor guide for more information about how you can get involved.

Community

Discussion around Cats is currently happening on Github issue and PR pages as well as in two Gitter channels:

Gitter channel cats is for general user questions and discussions, and

Gitter channel cats-dev is dedicated for Cats development related discussions. For people who wants to follow closely and/or to participate in the decisions in Cats development, this is the room to join.

You can get an overview of who is working on what via Waffle.io.

People are expected to follow the Scala Code of Conduct when discussing Cats 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.

If you are being harassed, please contact one of us immediately so that we can support you.

Binary compatibility and versioning

After 1.0.0 release, we decided to use MAJOR.MINOR.PATCH Semantic Versioning 2.0.0 going forward, which is different from the EPOCH.MAJOR.MINOR scheme common among Java and Scala libraries (including the Scala lang).

Cats strives to provide a solid and stable foundation for an ecosystem of FP libraries. Thus, we treat backward binary compatibility maintenance with a high priority. In semantic versioning, backward breaking change is ONLY allowed between MAJOR versions. We will maintain backward binary compatibility between PATCH AND MINOR versions. For example, when we release Cats 1.1.0, it will be backward binary compatible with the previous 1.0.x versions. I.E. the new JAR will be a drop-in replacement for the old one. This is critical when your application has a diamond dependency on Cats - depending on two or more libraries that all depend on Cats. If one library upgrades to the new 1.1.0 Cats before the other one does, your application still runs thanks to this backward binary compatibility.

Also worth noting is that according to semantic versioning, MINOR version Y (x.Y.z | x > 0) MUST be incremented if new, backwards compatible functionality is introduced to the public API. It MUST be incremented if any public API functionality is marked as deprecated.

Any binary breaking changes will require a MAJOR version bump, which we will be very cautious about. We will also consider using organization and package name for major versioning in the future. But that decision is yet to be made.

Adopters

Here's a (non-exhaustive) list of companies that use Cats in production. Don't see yours? You can add it in a PR!

• Abacus Protocol
• Anduin Transactions
• Apple Inc. (FEAR team)
• AutoScout24
• Avast
• BabylonHealth
• Banno Group inside of Jack Henry & Associates
• Basefarm
• buildo
• Codacy
• Codecentric
• Colisweb
• DriveTribe
• Dwolla
• Earnest
• eBay Inc.
• Eloquentix
• eSailors
• Evotor Marketplace
• e.near
• E.ON
• formation.ai
• Free2Move
• HomeAway
• iHeartRadio
• ImmobilienScout24
• ITV
• Lookout
• Metacommerce
• Nezasa
• NCR Edinburgh
• Ocado Technology
• Packlink
• Raiffeisenbank Russia
• Rakuten
• REA Group
• Reality Games
• Rudder
• Scalac
• Scala Center
• Sigma
• Snowplow Analytics
• Spiceworks
• Spotahome
• Spotify
• SpringerNature
• SRF
• Stripe
• Tecsisa
• Teikametrics
• The Guardian
• Underscore Consulting
• Wegtam GmbH
• WeWork
• Wix.com
• Zalando
• 47 Degrees

Maintainers

The current maintainers (people who can merge pull requests) are:

• ceedubs Cody Allen
• rossabaker Ross Baker
• johnynek P. Oscar Boykin
• travisbrown Travis Brown
• adelbertc Adelbert Chang
• LukaJCB Luka Jacobowitz
• peterneyens Peter Neyens
• tpolecat Rob Norris
• non Erik Osheim
• mpilquist Michael Pilquist
• milessabin Miles Sabin
• djspiewak Daniel Spiewak
• fthomas Frank Thomas
• julien-truffaut Julien Truffaut
• kailuowang Kailuo Wang

We are currently following a practice of requiring at least two sign-offs to merge PRs (and for large or contentious issues we may wait for more). For typos or other small fixes to documentation we relax this to a single sign-off.

Copyright and License

All code is available to you under the MIT license, available at http://opensource.org/licenses/mit-license.php and also in the COPYING file. The design is informed by many other projects, in particular Scalaz.

Copyright the maintainers, 2015-2019.