Should there be an IO Monad?

[rongcuid]

I noticed that Cats does not have an IO monad, while Scalaz does. Is there a reason not to include it in the current implementation? Or is it just "unimplemented"?

[adelbertc]

I believe the omission of an IO or Task monad in Cats is on purpose. I've tried to list the reasons below:

1. There can only be one IO monad since it sits "at the bottom" and it is sketchy to interpret from one IO/Task monad to another due to execution semantics. That being said, between libraries like FS2 and Monix which provide their own IO/Task-y monads, it is perhaps not desirable to add another one into the mix.
2. It is not entirely straightforward if we should have a pure IO monad, or an IO monad that also captures async like Task does. Historically folks have gone for the latter, but this introduces complications as Cats wishes to support both the JVM and Scala.JS. Due to differences in the runtimes it is not easy to support both in this particular domain.

[rongcuid]

Does it have something like an IO monad, then? Or is it just not a common pardigm used in Scala?

If so, what is used to explicitly "mark" impure functions (although I know Scala is an impure language)?

[yilinwei]

@CarlDong

If I want what @adelbertc calls a pure IO Monad I simply alias a type IO = Reader[World, A] where World has the subset of features which delegates to a suitable library such as java nio. This is (usually) sufficient to mark effects.

[adelbertc]

@CarlDong If stack safety is not a concern a plain encoding can be used:

sealed abstract class IO[A] {
  def unsafePerformIO(): A

  def flatMap[B](f: A => IO[B]): IO[B] = f(unsafePerformIO())

  def map[B](f: A => B): IO[B] =flatMap(a => IO.pure(f(a)))
}

object IO {
  def apply[A](a: => A): IO[A] = new IO[A] { def unsafePerformIO(): A = a }

  def pure[A](a: A): IO[A] = new IO[A] { def unsafePerformIO(): A = a }
}

Short of that, for now you'll have to use something from an existing lib like FS2's Task or Doobie's IOLite: https://github.com/tpolecat/doobie/blob/series/0.3.x/yax/core/src/main/scala/doobie/util/iolite.scala

[rongcuid]

I am more used to Haskell programming so I guess stack safety is pretty important... I will try, though.

[ceedubs]

FYI I've created #1227 to add a note about IO/Task (or the lack thereof) to the FAQ.

[adelbertc]

@CarlDong If you're going to end up using streams, then bringing in something like scalaz-stream (aka FS2) (or Monix) is probably the way to go. If you also end up doing some database or HTTP work, there's also libraries like Doobie and HTTP4S that have bindings to scalaz-stream.

[ceedubs]

I'm going to go ahead and close this out since there's now an item in the FAQ for this. Thanks for reminding me that it should be added @CarlDong.