Common APIs: Cats Effect IO [Part-2]

Common APIs: Cats Effect IO [Part-2]


In the part 1 of this series, we looked at Cats Effect 3 and how to run a simple app written using it. In this part, let's look at some of the common APIs and methods used with IO datatype.

map, flatMap and for-comprehension

Since IO is monad, we can use the common operations like map and flatMap to chain IOs. Alternately, we can use for-comprehension as well.

val io1 = IO("Scala")
val io2 = IO("Cats")
val mapPgm: IO[String] =
val flatMapPgm: IO[String] = io1.flatMap(a => + _))
val forComp: IO[String] = for {
  a <- io1
  b <- io2
} yield a + b

We can also use flatten to avoid nested IOs:

val aIO: IO[String] = IO("Hello")
val anotherIO: IO[String] = IO(aIO).flatten

Cats Effect further provides another utility method to do this using defer:

val deferredIO: IO[String] = IO.defer(IO("String"))

Please note that IO.delay(anotherIO) will create a nested IO, while IO.defer(IO) will return a flattened IO.

Other Simple Methods

To discard a result of an IO, we can use void.

val strIO: IO[String] = IO("Cats Effect")
val voidIO: IO[Unit] = strIO.void

To replace the value in an IO with another value, we can use as instead of map:

val asIntIO: IO[Int] = strIO as 100

To create an effect that prints content to console, we can use the helper method IO.println():

val printIO = IO.println("Hello World")

Handling Exceptions

Since IO describes an effect, there is a chance that the evaluation might fail at runtime. IO datatype can capture such failures as well. We can use raiseError to fail an IO.

Creating IO with Errors

val aFailedIO: IO[String] = IO.raiseError[String](new Exception("Failed IO"))

If we want to create a failed IO based on some condition, we can use IO.raiseWhen(). It raises the provided exception if the condition matches else return IO[Unit]. This is especially useful to handle some undesired situations.

val raisedIO: IO[Unit] = IO.raiseWhen(num == 0)(new RuntimeException("Number can not be 0"))

Similarly, IO.raiseUnless() raises an exception when the condition doesn't match.

Handling Errors

We can handles the errors from an IO using handleError() method. This is similar to the recover on Future.

val handledIO: IO[Int] = aFailedIntIO.handleError(ex => 0)

We can also use handleErrorWith in the same way we use recoverWith on Future. The handler inside the handleErrorWith should return another IO.

val handledWithIO: IO[Int] = aFailedIntIO.handleErrorWith(_ => IO.pure(0))

We can also use redeem method to handle both success and failure cases together. This is similar to the transform method on Future. However, redeem takes the exception handling first and then the success handling.

val redeemedIO: IO[String] = intIO.redeem(_ => "failed", _ => "success")

Lifting Common Types to IO

IO provides some utility methods to lift most common Scala types into IO.

  • IO.fromOption lifts Option[A] to an IO[A]
  • IO.fromTry lifts Try[A] to an IO[A]
  • IO.fromEither lifts Either[Throwable, A] to an IO[A]

We can also lift Future to an IO using the method fromFuture. However, to suspend Future execution, we need to wrap the Future in an IO first.

lazy val aFuture: Future[Int] = Future(100)
val ioFromFuture: IO[Int] = IO.fromFuture(IO(aFuture))

Chaining IOs

We can use map, flatMap and for-comprehension to chain IOs. However, there are some other combinators that are available to chain different IOs.

Let's define two IOs to explain the chaining as:

val firstIO: IO[Int] = IO(100)
val secondIO: IO[String] = IO("Millions")

Now, let's apply chaining using *> combinator:

val firstSecond: IO[String] = firstIO *> secondIO

It will execute firstIO, then the secondIO and discard the result of firstIO and returns the result of the secondIO.

Now, let's use <* combinator. It runs the IOs in the same order, but keeps the result of the first and ignores the result of the second:

val secondFirst: IO[Int] = firstIO <* secondIO

There is another combinator >> which is almost similar to *>. The only difference is that >> is lazily evaluated and hence stack safe. Consider >> if recursion is involved:

val anotherCombinator: IO[String] = firstIO >> secondIO

There is another version &> and <& which are similar to >> and <<, but execute the IOs in parallel instead of sequential.

val combinedParallel: IO[String] = firstIO &> secondIO

IO Sleep

Cats Effect provides a very clean way to asynchronously make an IO sleep for specified time. It is called as Semantic Blocking.

val sleepingIO = IO.sleep(100.millis)

Please note that, unlike Thread.sleep, IO.sleep will NOT block the thread. Cats Effect will internally manage threads and handle sleep asynchronously.

IO Never

We can create a never terminating IO using the method never.

val neverEndingIO = IO.println("Start") >> IO.never >> IO.println("Done")

In the above code, the last IO which prints "Done" will never be executed since there is an IO.never just before it.


In this part, we looked at some of the common and most useful combinators and methods on IO data structure. The sample code used here is available in GitHub under the package part2.