Essential Effects

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What people are saying:

If you’re at all interested in FP in Scala, buy Essential Effects by @arosien — it’s a fantastic overview of Cats Effect!

I can confirm that Essential Effects is really good. A great practical guide to Cats Effect.

If you do #Scala this is a *must buy*. Go for it. If you miss it, you'll regret it!

Can confirm this book is legit. I'd easily recommend it to anybody getting into Scala or wanting to learn pure functional programming with Cats Effect.

I've been working through a reviewer copy and it's great. Full of stuff I wish I had in one place when I was learning this. Plenty of things I missed and have now learned.

This [...] is exactly the kind of introduction to concurrency I wish I had had :)

Describing IO like that really helped me understand it!

Cats Effect is a Scala library that makes it easy to write code that effectively uses multiple cores and doesn’t leak resources. This makes building complex applications, such as highly concurrent services, much more productive. Essential Effects aims to introduce the core concepts in Cats Effect, giving you the knowledge you need to go further with the library in your own applications.

This book is for you if:

• You've heard about "programming with effects" but want to learn more.
• You want to know the basics of Cats Effect.
• You, your team, or your organization is refactoring your code from Future to IO, or is considering doing so.
• Your existing parallel and concurrent code is a tangled mess, or a confusing black box, and you're not sure how to approach fixing it.

Essential Effects will teach you to:

• Understand the meaning and role of side-effects and effects.
• Understand how to encapsulate side-effects in a safer form.
• Use parMapN and other combinators to run effects in parallel.
• Fork independent work into concurrent tasks, then cancel or join them.
• Learn how to separate CPU-bound work from blocking, I/O-bound work.
• Integrate callback-based code, like scala.concurrent.Future, into a safer, effect-based interface.
• Build and combine resource-leak-proof dependencies for applications.
• Test code that performs multiple effects like concurrency and I/O.

Training

Want help learning about effects? Inner Product LLC offers a companion training course to the book.

About the Author

Programming in Scala since 2011, Adam focuses on building systems, teaching, and mentoring using functional programming as a Principal at Inner Product LLC. He previously helped various startups in many domains develop back-end systems and implement continuous deployment practices, and also spent five years as a developer at Xerox PARC. This is his first book.

Follow @arosien

Table of Contents

1. Preface

• Acknowledgements
• About this book
• Cats Effect versions
• Source code for examples and exercises
• Prerequisites

Chapter 1. Effects: evaluation and execution

• The substitution model of evaluation
• Dealing with side-effects
• The Effect Pattern
• Capturing arbitrary side-effects as an effect
• Composing effects
• Summary

Chapter 2. Cats Effect IO

• Constructing IO values
• Transforming IO values
• Executing IO values
• IO as an effect
• Executing effects in applications with IOApp
• Summary

Chapter 3. Parallel execution

• Does IO support parallelism?
• The Parallel typeclass
• Inspecting parallelism
• parMapN
• parTraverse
• parSequence
• Summary

Chapter 4. Concurrent control

• Decomposing the behavior of parMapN
• Gaining control with Fiber
• Canceling a running Fiber
• Racing multiple effects
• Summary

Chapter 5. Shifting contexts

• How much parallelism can we get?
• The need for multiple contexts
• Contexts for I/O-bound actions
• How do you know something is blocking?
• Finer-grained control of contexts
• Example: contexts for database access in Doobie
• Summary

Chapter 6. Integrating asynchrony

• Asynchronous callbacks
• Integrating with Future
• Summary

Chapter 7. Managing resources

• Creating a Resource to manage state
• Composing managed state
• Resources for dependency management
• Summary

Chapter 8. Testing Effects

• Assertions on effectful values
• Testing effect scheduling by controlling its dependencies
• Summary

Chapter 9. Concurrent coordination

• Atomic updates with Ref
• Write-once synchronization with Deferred
• Concurrent state machines
• Summary

Chapter 10. Case study: job scheduler

• Jobs
• Job Scheduler
• Reacting to job state changes
• Putting everything together
• Summary

Chapter 11. Conclusion

• Next steps

Glossary

Appendix A: Cheatsheets

Appendix B: Abstracting effects with typeclasses

Appendix C: Solutions to selected exercises

References