7

If i just use Option in for-comprehension everything goes as expected:

val a = Some(1)
val b = None
val c = Some(3)

val r = for {
  aa <- a
  bb <- b
  cc <- c
} yield aa + bb + cc

println(r) // None, because b is None

but how to achieve the same behaviour using cats IO?

import cats.effect.IO
// in reality this will be a methods with side effect
val a = Some(1)
val b = None
val c = Some(3)

val r = for {
  _ <- IO{println("a"); a}
  _ <- IO{println("b"); b} // want to stop execution here
  _ <- IO{println("c"); c}
} yield ()

r.unsafeRunSync()

In result i get a b c, but i expect only a b.

Is it possible to achieve? Is it a right way to do it?

erip
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Nikita Ryanov
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1 Answers1

9

You can do this with monad transformers; specifically, you'll want to use OptionT[IO, T] here:

import cats.effect._
import cats.data.OptionT
import cats.implicits._
import cats.effect.IO

val a = Some(1)
val b = None
val c = Some(3)

val r = for {
  _ <- OptionT[IO, Int](IO {println("a"); a})
  _ <- OptionT[IO, Int](IO {println("b"); b})
  _ <- OptionT[IO, Int](IO {println("c"); c})
} yield ()

r.value.unsafeRunSync()

See it in action here.

erip
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