Monadic IO with ScalaZ

Posted on January 27, 2013

I just recently scratched the surface with scalaz. Think of it as an additional standard library for scala that’s FP oriented. It provides a bunch of type classes, instances for pretty much everything, some fancy data types, pimps(Pimp My Library) for standard library collections, actor implementation and probably some stuff I’m not aware of. I could really use a “map of scalaz” - but I’ll probably dive into source and scaladoc anyway. One fancy feature that’s not noted on theirGoogle Code page is IO monad implementation.

Denali Landscape
Real World Tm - the thing your programs have to interact with (Photo credit: blmiers2)

I’ve written a bit about monadic IO but let’s recap. IO monad is a data structure that represents a tiny language which let’s you describe and compose IO actions without actually performing them - allowing you to keep your functions pure and compose/reuse better.

Monadic HelloWorld with scalaz

I’ll be using SBT to manage dependencies(scala and scalaz) so let’s create a project. We just need build.sbt file with this content(or use my template: g8 edofic/scalaz-empty)

scalaVersion := "2.9.2"

resolvers += "Scala Tools Snapshots" at ""

libraryDependencies += "org.scalaz" %% "scalaz-core" % "6.0.4"

and then run “sbt console” to get a REPL with scala 2.9.2 and scalaz 6.0.4. Pretty useful. Let’s get to code now.

import scalaz._
import Scalaz._
import scalaz.effects._

val greeter = println("hello world").pure[IO]

//scalaz provides a helper for that so we can write
val greeter2 = putStrLn("hello from scalaz")


First a bunch of imports to get scalaz magic and then the crucial line. Scalaz provides an implicit conversion which allows us to call .pure[A] on every value. This gets Monad[A] instance from implicit scope and lifts the value into the monad. It’s a type class way (think dependency injection) to invoke monad “constructor”, a simple sample

scala> "hi".pure[Option]
res0: Option[java.lang.String] = Some(hi)

Scalaz also provides a helper putStrLn(and many more, including readLn) for more succinct code. Note that lifting our println(by hand) does call by name(lazy evaluation) so println is not actually invoked yet! To perform the IO action you need to explicitly call unsafePerformIO. Intentionally verbose and scary name to make you think twice where you perform your side effects. But printline is not that interesting. Let’t take a look at input. Type of readLn is IO[String] and it’s perform method returns a String. Notice that IO is a monad so you can do map and flatMap on it. For example gives you and action that will read and parse an integer returning you something of type Int. FlatMap is more interesting because it gives you composition. FlatMapping another IO action will execute actions in sequence giving last action access to previous actions’ return values via closures. That’s referred to as monadic style. It’s basically a pure way of doing imperative programming. A bit more involved example

  _    <- putStrLn("who are you?")
  name <- readLn
  _    <- putStrLn("hello " + name)
} yield ()

And a tiny calculator showing off composition

val inputInt = for{
  _ <- putStrLn("enter an integer")
  raw <- readLn
} yield raw.toInt

val adderIO = for {
  a <- inputInt
  b <- inputInt
  _ <- putStrLn((a+b).toString)
} yield ()

//and to run it
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