Monix vs Cats-Effect

I’m the author of Monix and a major contributor to Cats-Effect.

This post describes the relationship between Monix and Cats-Effect, their history and when each should be used.

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Monix: Version 3.0.0-RC1

The 3.0.0 release is near and it’s going to be huge. This is the first of the two release candidates planned, see below for the timeline.

The theme of the 3.x series are:

  1. deep integration with Typelevel Cats
  2. deep integration with cats-effect
  3. a much improved Task, in concert with its cats-effect integration
  4. improvements across the board to Observable, CancelableFuture, etc.
  5. the introduction of monix.tail.Iterant, a purely functional, pull-based streaming alternative

Featured presentation:

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Monix: Version 2.3.3, Critical Bug Fixes

This is a maintenance release, that is binary backwards compatible with series 2.3.x.

Upgrade to 2.3.3 is recommended, as it contains critical bug fixes! See list below for more details.

A release for the 3.0.0-Mx series will follow either today or tomorrow.

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Monix: Version 2.3.0, New Features, Bug Fixes

Version 2.3.0 is out now.

It is a major release that remains binary and source compatible with 2.2.x.

In summary we’ve got a replacement for Scala’s NonFatal, a new Task.deferAction builder for defering tasks with an injected scheduler, Task and Coeval run-loop optimizations, the Observable.observeOn operation after popular request, a Consumer.transformInput for transforming consumers by transforming their input stream, fixes for the reactive streams integration and more. See details below.

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Monix: Version 2.2.4, Bug Fixes, Upgraded Scala.js

Version 2.2.4 is out now.

It is a minor release that’s binary and source compatible with 2.2.x, fixing a bugs and upgrading Scala.js:

  • Bug #338: Fix broken test that can yield errors in Scala’s community build
  • Bug #333: Observable.firstStartedOf fails due to off-by-one error
  • Bug #335: Observable concatMap / mapTask trigger invalid state warning on raiseError children


  • Issue #334: Add CompositeCancelable.getAndResetTo, needed for the new firstStartedOf implementation
  • Upgrade Scala.js to latest version 0.6.15, handing deprecation warnings in TaskApp

Upgrade should be safe, being binary compatible with all previous 2.2.x versions.

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