Developing a Principled Approach to IO Monads in Scala

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Developing a principled approach to programming IO in Scala Raymond Tay

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A little about me

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A little bit more about me Contributed to cats, dotty, eff- monad, akka, akka-http, scala compiler.

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Effects What is it? Anything but you are probably thinking about side-effect(s). Controlling how side-effects are effected is the purpose of the IO Monad.

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Effects (Scala 2)

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What is needed … - Sequencing computations - Stack safety - Support evaluation modes - Lazy - Asynchronous - Strict

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What is needed … - Sequencing computations - Stack safety - Support evaluation modes - Lazy - Asynchronous - Strict

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Stack execution

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Trampoline

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What is needed … - Sequencing computations - Stack safety - Support evaluation modes - Lazy - Asynchronous - Strict

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Getting started with IO

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What exactly is IO? A value of type IO[A] is a computation which, when evaluated, can perform effects before returning a value of type A IO values are pure, immutable values and thus preserves referential transparency, being usable in functional programming. An IO is a data structure that represents just a description of a side effectful computation.

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IO Typeclass Hierarchy

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Asynchronous & Cancelable

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Asynchronous & Cancelable in action ===== Start =======> => Concurrent-[scala-execution-context-global-180] Yes:5 => Concurrent-[scala-execution-context-global-178] Yes:2 => Concurrent-[scala-execution-context-global-181] Yes:4 Task-5 is no longer active canceled Task-3 is no longer active canceled Task-4 is no longer active canceled Task-6 is no longer active canceled => Concurrent-[scala-execution-context-global-179] Yes:3 Task-2 is no longer active canceled Task-8 is no longer active canceled Task-7 is no longer active canceled Task-9 is no longer active canceled Task-10 is no longer active canceled Task-1 is no longer active canceled … => Concurrent-[scala-execution-context-global-179] Yes:8 Left(Hello!_2)

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Asynchronous & Non-Cancelable

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Asynchronous & Non-Cancelable ===== Start =======> => Async-[scala-execution-context-global-179] Yes:2 => Async-[scala-execution-context-global-179] Yes:3 … => Async-[scala-execution-context-global-179] Yes:4 => Async-[scala-execution-context-global-179] Yes:7 => Async-[scala-execution-context-global-179] Yes:8 Left(Hello!_2) ===== End =======>

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Cancelable

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Cancelable

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Error Handling

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Resource management

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Safely Acquire & Release

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Concurrent Modelling

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Concurrent Datatypes (I) - Buffered Channel

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Encoding the Datatype

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Modelling Concurrent Algos

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Buffered Channel in Action

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Concurrent Datatypes (II) - Skip Channel

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Skip Channel in action

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Logging with Cats-effect

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Setting the stage…

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Logging …

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Do I have to do this every time ?

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Gotchas

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Stack safety

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Cancellation assumes no-blocking

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Just because it compiles …

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Thank You JCConf 2019 ! @RaymondTayBL @https:/ /www.linkedin.com/in/ raymondtayboonleong

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References => code repo : https://github.com/raymondtay/JCConf_2019 => cats effect : https://typelevel.org/cats-effect => cats effect tutorial : https://typelevel.org/cats- effect/tutorial/tutorial.html => monix : http://monix.io/ => fs2 : https://fs2.io => log4cats : https://christopherdavenport.github.io/ log4cats/ => concurrent Haskell paper : https://www.microsoft.com/ en-us/research/wp-content/uploads/1996/01/concurrent- haskell.pdf