1-CustomIO.scala

import scala.io.StdIn._

object CustomIO extends App {

  /**
    * Slides: https://docs.google.com/presentation/d/15O8fzIEa85a8S_vk6N3b_S8F4IYbrFkLOXmtwbcy0cQ
    */

  case class IO[A](unsafeRun: () => A) {
    def map[B](f: A => B): IO[B] =
      IO(() => f(this.unsafeRun()))

    def flatMap[B](f: A => IO[B]): IO[B] =
      IO(() => f(this.unsafeRun()).unsafeRun())
  }

  def print(line: String): IO[Unit] =
    IO(() => println(line))

  val read: IO[String] =
    IO(() => readLine)

  val io =
    for {
      _     <- print("Enter your name:")
      str   <- read
      upper = str.toUpperCase
      _     <- print(s"Your name in uppercase: $upper")
    } yield ()

  io.unsafeRun()
}

2-CatsIO.scala

import cats.implicits._
import cats.effect._

import scala.io.StdIn._
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent._

object CatsIO extends App {

  /**
    * Slides: https://docs.google.com/presentation/d/15O8fzIEa85a8S_vk6N3b_S8F4IYbrFkLOXmtwbcy0cQ
    */


  val io =
    for {
      _     <- IO(println("Enter your name:"))
      str   <- IO(readLine)
      upper = str.toUpperCase
      _     <- IO(println(s"Your name in uppercase: $upper"))
    } yield ()

  io.unsafeRunSync()




  // composing io actions sequentially
  val sayHello = IO(println("hello"))
  val sayWorld = IO(println("world"))
  val read = IO(readLine)

  for {
    _ <- sayHello
    _ <- sayWorld
  } yield ()

  sayHello *> sayWorld

  List(sayHello, sayWorld).sequence

  (read, read).mapN { _ + _ }




  // composing io actions in parallel
  List(sayHello, sayWorld).parSequence
  (read, read).parMapN { _ + _ }




  // failing, short-circuiting
  for {
    name <- IO(readLine("Enter your name:"))
    _ <- name match {
          case "exit" => IO.raiseError(new Exception("exiting"))
          case _      => IO.unit
        }
    _ <- IO(println(s"Your name in uppercase: ${name.toUpperCase}"))
  } yield ()




  // converting futures to io
  val readFromDb: Future[Int] = Future(3)

  val readFromDB2: IO[Int] = IO.fromFuture(IO(readFromDb))




  // manipulating IO as values
  def nTimes[A](io: IO[A], n: Int): IO[List[A]] =
    List.fill(n)(io).sequence

  def forever[A](io: IO[A]): IO[A] =
    io >> forever(io)

  def retry[A](io: IO[A]): IO[A] =
    io orElse retry(io)

  def retryN[A](io: IO[A], times: Int): IO[A] =
    if (times <= 1)
      io
    else
      io orElse retryN(io, times - 1)
}

build.sbt

lazy val root = (project in file(".")).
  settings(
    inThisBuild(List(
      organization := "com.example",
      scalaVersion := "2.12.3",
      version      := "0.1.0-SNAPSHOT"
    )),
    name := "Hello",
    libraryDependencies ++=
      List(
        scalaTest % Test,
        "org.typelevel" %% "cats-core" % "1.2.0",
        "org.typelevel" %% "cats-effect" % "1.0.0-RC2-93ac33d"
      )
  )

initialCommands in Compile in console :=
  """import notes._
    |import cats.implicits._
    |import cats.effect._
  """.stripMargin

scalacOptions ++= Seq(
  "-deprecation",                      // Emit warning and location for usages of deprecated APIs.
  "-encoding", "utf-8",                // Specify character encoding used by source files.
  "-explaintypes",                     // Explain type errors in more detail.
  "-feature",                          // Emit warning and location for usages of features that should be imported explicitly.
  "-language:existentials",            // Existential types (besides wildcard types) can be written and inferred
  "-language:experimental.macros",     // Allow macro definition (besides implementation and application)
  "-language:higherKinds",             // Allow higher-kinded types
  "-language:implicitConversions",     // Allow definition of implicit functions called views
  "-unchecked",                        // Enable additional warnings where generated code depends on assumptions.
  "-Xcheckinit",                       // Wrap field accessors to throw an exception on uninitialized access.
  "-Xfuture",                          // Turn on future language features.
  "-Xlint:adapted-args",               // Warn if an argument list is modified to match the receiver.
  "-Xlint:by-name-right-associative",  // By-name parameter of right associative operator.
  "-Xlint:constant",                   // Evaluation of a constant arithmetic expression results in an error.
  "-Xlint:delayedinit-select",         // Selecting member of DelayedInit.
  "-Xlint:doc-detached",               // A Scaladoc comment appears to be detached from its element.
  "-Xlint:inaccessible",               // Warn about inaccessible types in method signatures.
  "-Xlint:infer-any",                  // Warn when a type argument is inferred to be `Any`.
  "-Xlint:missing-interpolator",       // A string literal appears to be missing an interpolator id.
  "-Xlint:nullary-override",           // Warn when non-nullary `def f()' overrides nullary `def f'.
  "-Xlint:nullary-unit",               // Warn when nullary methods return Unit.
  "-Xlint:option-implicit",            // Option.apply used implicit view.
  "-Xlint:package-object-classes",     // Class or object defined in package object.
  "-Xlint:poly-implicit-overload",     // Parameterized overloaded implicit methods are not visible as view bounds.
  "-Xlint:private-shadow",             // A private field (or class parameter) shadows a superclass field.
  "-Xlint:stars-align",                // Pattern sequence wildcard must align with sequence component.
  "-Xlint:type-parameter-shadow",      // A local type parameter shadows a type already in scope.
  "-Xlint:unsound-match",              // Pattern match may not be typesafe.
  "-Yno-adapted-args",                 // Do not adapt an argument list (either by inserting () or creating a tuple) to match the receiver.
  "-Ypartial-unification",             // Enable partial unification in type constructor inference
  "-Ywarn-dead-code",                  // Warn when dead code is identified.
  "-Ywarn-extra-implicit",             // Warn when more than one implicit parameter section is defined.
  "-Ywarn-inaccessible",               // Warn about inaccessible types in method signatures.
  "-Ywarn-infer-any",                  // Warn when a type argument is inferred to be `Any`.
  "-Ywarn-nullary-override",           // Warn when non-nullary `def f()' overrides nullary `def f'.
  "-Ywarn-nullary-unit",               // Warn when nullary methods return Unit.
  "-Ywarn-numeric-widen",              // Warn when numerics are widened.
  "-Ywarn-unused:implicits",           // Warn if an implicit parameter is unused.
  "-Ywarn-unused:imports",             // Warn if an import selector is not referenced.
  "-Ywarn-unused:locals",              // Warn if a local definition is unused.
  "-Ywarn-unused:params",              // Warn if a value parameter is unused.
  "-Ywarn-unused:patvars",             // Warn if a variable bound in a pattern is unused.
  "-Ywarn-unused:privates",            // Warn if a private member is unused.
  "-Ywarn-value-discard"               // Warn when non-Unit expression results are unused.
)