This document summarizes a presentation about demystifying functional effect systems in Scala. It discusses building a basic functional effect system using an IO monad to represent effects. It describes how to add capabilities like handling failures, asynchrony using async effects, and concurrency using fibers. The presentation provides examples for each capability and discusses how they are implemented in the runtime. It concludes by discussing potential next steps like adding locking effects, fiber interruption, and resource management.

1. Demystifying
functional effect systems in Scala
Dmitry Karlinsky
Backend Engineer at Wix.com, Data Streams team
twitter: @drkarlinsky linkedin/dkarlinsky github.com/darlinsky

2. We use ZIO
Demystifying
functional effect systems in Scala
FUNCTIONAL
EFFECT SYSTEMS
Cats Effect

3. Let’s build a
functional effect system in Scala
FUNCTIONAL
EFFECT SYSTEMS

4. What are
Functional Effect Systems?
FUNCTIONAL
EFFECT SYSTEMS

5. non pure
operations such
as IO
Effects
FUNCTIONAL
EFFECT SYSTEMS

6. Allow describing
effects as values
that can be easily
and safely
composed into
descriptions of whole
programs
Effect Systems
FUNCTIONAL
EFFECT SYSTEMS

7. How to build an effect system?
Start with
basic Effects
and Runtime
Add
capabilities
Concurrency
with Fibers
FUNCTIONAL
EFFECT SYSTEMS

8. Agenda
Creating and
combining
effects
1 Failing and
Recovering
from Errors
2 Asynchrony3 Concurrency
with Fibers
4

9. Creating and
combining effects
01

10. IO Monad
Data structure for
describing effects
Composition describes
sequencing
TIO

11. IO Monad
Data structure for
describing effects
Composition describes
sequencing
Derived Operators
TIO

12. IO Monad
Hello World
EXAMPLEEXAMPLE

13. IO Monad
Declarative Encoding
(Free Monad)
TIO

14. Runtime
Minimal Recursive
Implementation
RUNTIME

15. Runtime
Running Hello World
RUNTIMEEXAMPLE

16. Failing and Recovering
from Errors
02

17. Failure
Representing Errors
TIO

18. Failure
Recovering from Errors
Recover Effect
TIO

19. Example
Helpers
Example: Fail and
Recover
EXAMPLE

20. Failure
Recovering from Errors
Runtime
RUNTIME

21. Example
Example: Fail and
Recover
EXAMPLE

22. Limitations
Fully synchronous - no way to represent
async effects
Not stack safe

23. Stack Safety
Example: Foreach
EXAMPLE

24. STACK SAFETY

25. Asynchrony
03

26. Asynchrony:
A process that continues its execution in a
different place or time than the one it started in.
Fabio Labella: “How do Fibers Work? A Peek Under the Hood”

27. Async Effects
Effect Constructor
TIO

28. Async Effects
Example
Non Blocking Sleep
EXAMPLE

29. Async Effects
Runtime
runAsync
RUNTIME

30. Async Effects
Runtime
Implementation
RUNTIME

31. Async Effects
Example: Sleep
EXAMPLE

32. Stack Safety
And Now?
EXAMPLE

33. Limitations
Not stack safe

34. Stack Safety
Helper
Executor
STACK SAFETY

35. Stack Safety
Scheduling effects on a
thread pool
RUNTIME

36. Stack Safety
And Now?
EXAMPLE

37. Concurrency with
Fibers
04

38. Concurrency:
A property of systems where several processes
execute at the same time.
CONCURRENCY
WITH FIBERS

39. Fibers
TIO.flatMap combinator describes sequences of computations.
Logical threads of execution are not tied to specific OS threads.
New combinators:
fork creates new Fibers
CONCURRENCY
WITH FIBERS
join lets one fiber await the result of another

40. Concurrency
Fiber Cominators
TIO

41. Concurrency
Example: Fork Join
EXAMPLE

42. Concurrency
FiberRuntime
● start()
● onDone()
RUNTIME

43. Concurrency
FiberRuntime
● eval()
RUNTIME

44. Concurrency
Example: Fork Join
EXAMPLE

45. Concurrency
Bonus Example
foreachPar()
EXAMPLE

46. Start with
a basic IO
monad
Add
capabilities
Concurrency
with Fibers
FUNCTIONAL
EFFECT SYSTEMS
What’s Next

47. ❏ Locking effects to alternative Executors (e.g for blocking effects)
❏ Fiber interruption
❏ Resource management (e.g Bracket)
❏ Fiber supervision - auto interruption of child fibers
❏ Interruption safe resource management
What’s Next

48. More Resources
● My blog post (that this talk is based on) - goes into more details
● Fabio Labella — How do Fibers Work? A Peek Under the Hood
● FIBER SUPERVISION IN ZIO — Wiem Zine El Abidine, Adam Fraser | Scalar 2020

49. Thank You
twitter: @drkarlinsky linkedin/dkarlinsky github.com/darlinsky

50. Q&A
twitter: @drkarlinsky linkedin/dkarlinsky github.com/darlinsky