Functional Scala: Toronto edition

day-3.scala

// Copyright(C) 2018 - John A. De Goes. All rights reserved.

package net.degoes.effects

import scalaz.zio._
import scalaz.zio.console._

import scala.annotation.tailrec
import scala.concurrent.duration._

object notepad {
  sealed trait Program[A] { self =>
    import Program._

    // derived combinator of flatMap
    final def map[B](f: A => B): Program[B] =
      flatMap(a => point(f(a)))

    // derived combinator of flatMap
    final def zip[B](that: Program[B]): Program[(A, B)] =
      for {
        a <- self
        b <- that
      } yield (a, b)

    final def flatMap[B](f: A => Program[B]): Program[B] =
      self match {
        case Return(thunk) => f(thunk())
        case WriteLine(line, next) => WriteLine(line, next.flatMap(f))
        case ReadLine(inputToNext) => ReadLine(input => inputToNext(input).flatMap(f))
      }
  }

  object Program {
    // returns a value (lazily)
    final case class Return[A](value: () => A) extends Program[A]
    final case class WriteLine[A](line: String, next: Program[A]) extends Program[A]
    // get line of input from Console and feed it to next part of the program
    final case class ReadLine[A](inputToNext: String => Program[A]) extends Program[A]

    // easy to use language that make use of the above data structures
    // Program is an immutable Data structure

    // make the user supplied value lazy
    def point[A](a: => A): Program[A] = Return(() => a)
    def writeLine(line: String): Program[Unit] = WriteLine(line, point(()))
    def readLine: Program[String] = ReadLine(input => point(input))
  }

  import Program._
  val helloWorld: Program[Unit] = writeLine("Hello World!")
  val getInput: Program[String] = readLine

  val sequentialProgram: Program[Unit] =
    for {
      _ <- helloWorld
      i <- getInput
      _ <- writeLine(s"Hello $i")
    } yield ()
}

object zio_background {
  sealed trait Program[A] { self =>
    // zip programs together and throws away the left side (<*) but keeps the right side (*>)
    // both program effects are evaluated but we only care about the return value of one of them
    final def *> [B](that: Program[B]): Program[B] = self.flatMap(_ => that)

    final def <* [B](that: Program[B]): Program[A] = self.flatMap(a => that.map(_ => a))

    final def map[B](f: A => B): Program[B] =
      self match {
        case Program.ReadLine(next) => Program.ReadLine(input => next(input).map(f))
        case Program.WriteLine(line, next) => Program.WriteLine(line, next.map(f))
        case Program.Return(value) => Program.Return(() => f(value()))
      }

    final def flatMap[B](f: A => Program[B]): Program[B] =
      self match {
        case Program.ReadLine(next) => Program.ReadLine(input => next(input).flatMap(f))
        case Program.WriteLine(line, next) => Program.WriteLine(line, next.flatMap(f))
        case Program.Return(value) => f(value())
      }
  }
  object Program {
    final case class ReadLine[A](next: String => Program[A]) extends Program[A]
    final case class WriteLine[A](line: String, next: Program[A]) extends Program[A]
    final case class Return[A](value: () => A) extends Program[A]

    val readLine: Program[String] = ReadLine(point[String](_))
    def writeLine(line: String): Program[Unit] = WriteLine(line, point(()))
    def point[A](a: => A): Program[A] = Return(() => a)
  }

  import Program.{readLine, writeLine, point}

  val yourName1: Program[Unit] =
    writeLine("What is your name?").flatMap(_ =>
      readLine.flatMap(name =>
        writeLine("Hello, " + name + ", good to meet you!").flatMap(_ =>
          point(())
        )
      )
    )

  //
  // EXERCISE 1
  //
  // Rewrite `program1` to use a for comprehension.
  //
  val yourName2: Program[Unit] = for {
    _     <- writeLine("What is your name?")
    name  <- readLine
    _     <- writeLine(s"Hello $name, good to meet you")
    u     <- point(())
  } yield u

  //
  // EXERCISE 2
  //
  // Rewrite `yourName2` using the helper function `getName`, which shows how
  // to create larger programs from smaller programs.
  //
  def yourName3: Program[Unit] = for {
    name  <- getName
    _     <- writeLine(s"Hello $name, good to meet you")
    u     <- point(())
  } yield u

  val getName: Program[String] =
    writeLine("What is your name?").flatMap(_ => readLine)

  //
  // EXERCISE 3
  //
  // Implement the following effectful procedure, which interprets
  // `Program[A]` into `A`. You can use this procedure to "run" programs.
  //
  @tailrec
  def interpret[A](program: Program[A]): A = {
    import scala.io.StdIn
    program match {
      case Program.ReadLine(nextInstructionFn: (String => Program[A])) =>
        val input = StdIn.readLine()  // side-effect
        interpret(nextInstructionFn(input))

      case Program.WriteLine(line, nextInstruction: Program[A]) =>
        println(line) // side-effect
        interpret(nextInstruction)

      case Program.Return(value: (() => A)) =>
        value(): A
    }
  }

  //
  // EXERCISE 4
  //
  // Implement the following function, which shows how to write a combinator
  // that operates on programs.
  //
  def sequence[A](programs: List[Program[A]]): Program[List[A]] = {
    // this is essentially traverse
    @tailrec
    def inner(acc: Program[List[A]], rem: List[Program[A]]): Program[List[A]] = rem match {
      case Nil => acc
      case headProgram :: rest =>
        // execute the headProgram and append it to the existing list of results
        // NOTE: the order in which you flatMap matters so pull the existing list then the new program
        // when you flatMap, you execute the program
        val newAcc: Program[List[A]] = for {
          existing  <- acc
          a         <- headProgram
        } yield existing :+ a

        inner(newAcc, rest)
    }
    inner(Program.point(List.empty[A]), programs)
  }

  //
  // EXERCISE 5
  //
  // Translate the following procedural program into a purely functional program
  // using `Program` and a for comprehension.
  //
  def ageExplainer1(): Unit = {
    println("What is your age?")
    scala.util.Try(scala.io.StdIn.readLine().toInt).toOption match {
      case Some(age) =>
        if (age < 12) println("You are a kid")
        else if (age < 20) println("You are a teenager")
        else if (age < 30) println("You are a grownup")
        else if (age < 50) println("You are an adult")
        else if (age < 80) println("You are a mature adult")
        else if (age < 100) println("You are elderly")
        else println("You are probably lying.")
      case None =>
        println("That's not an age, try again")

        ageExplainer1()
    }

  }

  def ageExplainer2: Program[Unit] = {
    def askForAge: Program[Int] = for {
      _               <- writeLine("What is your age?")
      potentialAge    <- readLine
      optAge          =  scala.util.Try(potentialAge.toInt).toOption
      age             <- optAge.fold(
                            ifEmpty = writeLine("That's not an age, try again") *> askForAge)(
                            f = age => Program.point(age)
                         )
    } yield age

    def printAge(age: Int): Program[Unit] =
      if (age < 12)       writeLine("You are a kid")
      else if (age < 20)  writeLine("You are a teenager")
      else if (age < 30)  writeLine("You are a grownup")
      else if (age < 50)  writeLine("You are an adult")
      else if (age < 80)  writeLine("You are a mature adult")
      else if (age < 100) writeLine("You are elderly")
      else                writeLine("You are probably lying.")

    for {
      age <- askForAge
      _   <- printAge(age)
    } yield ()
  }


  // Calvin
  def main(args: Array[String]): Unit = {
    println {
      interpret {
        sequence {
          List(
            // we purposely make each program return a value
            writeLine("Hello, welcome to Calvin's program!") *> Program.point(1),
            ageExplainer2 *> Program.point(2),
            writeLine("Once again!") *> Program.point(3),
            ageExplainer2 *> Program.point(4)
          )
        }
      }
    }
  }
}

object zio_type {
  type ??? = Nothing

  //
  // EXERCISE 1
  //
  // Write the type of `IO` values that can fail with an `Exception`, or
  // may produce an `A`.
  //
  type Exceptional[A] = IO[Exception, A]

  //
  // EXERCISE 2
  //
  // Write the type of `IO` values that can fail with a `Throwable`, or
  // may produce an `A`.
  //
  type Task[A] = IO[Throwable, A]

  //
  // EXERCISE 3
  //
  // Write the type of `IO` values that cannot fail, but may produce an `A.`
  //
  type Infallible[A] = IO[Nothing, A]

  //
  // EXERCISE 4
  //
  // Write the type of `IO` values that cannot produce a value, but may fail
  // with an `E`.
  //
  type Unproductive[E] = IO[E, Nothing]

  //
  // EXERCISE 5
  //
  // Write the type of `IO` values that cannot fail or produce a value.
  //
  type Unending = IO[Nothing, Nothing]
}

object zio_values {
  //
  // EXERCISE 1
  //
  // Using the `IO.now` method, lift the integer `2` into a strictly-evaluated
  // `IO`.
  //
  val ioInteger: IO[Nothing, Int] = IO.now(2)

  //
  // EXERCISE 2
  //
  // Using the `IO.point` method, lift the string "Functional Scala" into a
  // lazily-evaluated `IO`.
  //
  val ioString: IO[Nothing, String] = IO.point("Functional Scala")

  //
  // EXERCISE 3
  //
  // Using the `IO.fail` method to lift the string "Bad Input" into a failed
  // `IO`.
  //
  val failedInput: IO[String, Nothing] = IO.fail("Bad Input")
}

object zio_composition {
  // a sign that something is missing
  implicit class FixMe[A](a: A) {
    def ?[B] = ???
  }

  //
  // EXERCISE 1
  //
  // Map the `IO[Nothing, Int]` into an `IO[Nothing, String]` by converting the
  // integer into its string rendering using the `map` method of the `IO`
  // object.
  //
  (IO.point(42) ? : IO[Nothing, String])

  //
  // EXERCISE 2
  //
  // Map the `IO[Int, Nothing]` into an `IO[String, Nothing]` by converting the
  // integer error into its string rendering using the `leftMap` method of the
  // `IO` object.
  //
  (IO.fail(42) ? : IO[String, Nothing])

  //
  // EXERCISE 3
  //
  // Using the `flatMap` and `map` methods of `IO`, add `ioX` and `ioY`
  // together.
  //
  val ioX: IO[Nothing, Int] = IO.point(42)
  val ioY: IO[Nothing, Int] = IO.point(58)
  val ioXPlusY: IO[Nothing, Int] = ioX.flatMap(???)

  //
  // EXERCISE 4
  //
  // Using the `flatMap` method of `IO`, implement `ifThenElse`.
  //
  def ifThenElse[E, A](bool: IO[E, Boolean])(
    ifTrue: IO[E, A], ifFalse: IO[E, A]): IO[E, A] =
    for {
      b <- bool
      res <- if (b) ifTrue else ifFalse
    } yield res

  val exampleIf = ifThenElse(IO.point(true))(IO.point("It's true!"), IO.point("It's false!"))

  //
  // EXERCISE 5
  //
  // Translate the following program, which uses for-comprehensions, to its
  // equivalent chain of `flatMap`'s, followed by a final `map`.
  //
  for {
    v1 <- IO.point(42)
    v2 <- IO.point(58)
  } yield "The total is: " + (v1 + v2).toString

  IO.point(42).flatMap { v1 =>
    IO.point(58).map { v2 =>
      "The total is: " + (v1 + v2).toString
    }
  }

  //
  // EXERCISE 6
  //
  // Rewrite the following procedural program, which uses conditionals, into its
  // ZIO equivalent.
  //
  def decode1(read: () => Byte): Either[Byte, Int] = {
    val b = read()
    if (b < 0) Left(b)
    else {
      Right(b.toInt +
        (read().toInt << 8) +
        (read().toInt << 16) +
        (read().toInt << 24))
    }
  }
  def decode2[E](read: IO[E, Byte]): IO[E, Either[Byte, Int]] =
    for {
      b   <-  read
      res <-  if (b < 0) IO.point(Left(b))
              else for {
                shift8  <- read.map(_.toInt << 8)
                shift16 <- read.map(_.toInt << 16)
                shift24 <- read.map(_.toInt << 24)
              } yield Right(b.toInt + shift8 + shift16 + shift24)
    } yield res

  //
  // EXERCISE 7
  //
  // Rewrite the following procedural program, which uses conditionals, into its
  // ZIO equivalent.
  //
  def getName1(print: String => Unit, read: () => String): Option[String] = {
    print("Do you want to enter your name?")
    read().toLowerCase.take(1) match {
      case "y" => Some(read())
      case _ => None
    }
  }

  def getName2[E](print: String => IO[E, String], read: IO[E, String]): IO[E, Option[String]] =
    for {
      _     <- print("Do you want to enter your name?")
      input <- read
      res   <- input match {
        case "y"  => read.map(userInput => Some(userInput))
        case _    => IO.point(None)
      }
    } yield res

  //
  // EXERCISE 8
  //
  // Translate the following loop into its ZIO equivalent.
  //
  def forever1(action: () => Unit): Unit = while (true) action()

  def forever2[A](action: IO[Nothing, A]): IO[Nothing, Nothing] =
  // execute the action, discard the result and repeat
    action.flatMap(_ => forever2(action))

  //
  // EXERCISE 9
  //
  // Translate the following loop into its ZIO equivalent.
  //
  def repeatN1(n: Int, action: () => Unit): Unit =
    if (n <= 0) ()
    else {
      action()
      repeatN1(n - 1, action)
    }

  def repeatN2[E](n: Int, action: IO[E, Unit]): IO[E, Unit] =
    if (n <= 0) IO.now(())
    else action.flatMap(_ => repeatN2(n - 1, action))

  //
  // EXERCISE 10
  //
  // Translate the following expression into its `flatMap` equivalent.
  //
  IO.point(42) *> IO.point(19)

  IO.point(42).flatMap(_ => IO.point(19))

  //
  // EXERCISE 11
  //
  // Translate the following expression into its `flatMap` equivalent.
  //
  IO.point(42) <* IO.point(19)
  IO.point(42).flatMap { fortyTwo =>
    IO.point(19).map { nineteen =>
      fortyTwo
    }
  }

  //
  // EXERCISE 12
  //
  // Translate the following expression into an equivalent expression using
  // the `map` and `flatMap` methods of the `IO` object.
  //
  (IO.point(42) <* IO.point(19)) *> IO.point(1)

  val p1: IO[Nothing, Int] =
    IO.point(42).flatMap { fortyTwo =>
      IO.point(19).map { nineteen =>
        fortyTwo
      }
    }

  p1.flatMap(_ => IO.point(1))
}

object zio_failure {
  // indicates that something is missing
  implicit class FixMe[A](a: A) {
    def ?[B] = ???
  }

  //
  // EXERCISE 1
  //
  // Using the `IO.fail` method, create an `IO[String, Int]` value that
  // represents a failure with a string error message, containing a user-
  // readable description of the failure.
  //
  val stringFailure1: IO[String, Int] =
  IO.fail[String]("oh noes")

  //
  // EXERCISE 2
  //
  // Using the `IO.fail` method, create an `IO[Int, String]` value that
  // represents a failure with an integer error code.
  //
  val intFailure: IO[Int, String] =
  IO.fail(1)

  //
  // EXERCISE 3
  //
  // Transform the error of `intFailure` into its string representation using
  // the `leftMap` method of `IO`.
  //
  val stringFailure2: IO[String, String] = intFailure.leftMap(i => i.toString)

  //
  // EXERCISE 4
  //
  // Translate the following exception-throwing program into its ZIO equivalent.
  //
  def accessArr1[A](i: Int, a: Array[A]): A =
    if (i < 0 || i >= a.length) throw new IndexOutOfBoundsException("The index " + i + " is out of bounds [0, " + a.length + ")")
    else a(i)
  def accessArr2[A](i: Int, a: Array[A]): IO[IndexOutOfBoundsException, A] =
    if (i < 0 || i >= a.length) IO.fail(new IndexOutOfBoundsException("The index " + i + " is out of bounds [0, " + a.length + ")"))
    else IO.point(a(i))

  //
  // EXERCISE 5
  //
  // Translate the following ZIO program into its exception-throwing equivalent.
  //
  trait DenomIsZero
  object DenomIsZero extends DenomIsZero {}
  def divide1(n: Int, d: Int): IO[DenomIsZero, Int] =
    if (d == 0) IO.fail(DenomIsZero)
    else IO.now(n / d)
  def divide2(n: Int, d: Int): Int =
    if (d == 0) throw new Exception("Denominator is zero")
    else n / d

  //
  // EXERCISE 6
  //
  // Recover from a division by zero error by returning `-1`.
  //
  val recovered1: IO[Nothing, Int] =
  divide1(100, 0).attempt.map {
    case Left(error) => -1
    case Right(value) => value
  }

  //
  // EXERCISE 7
  //
  // Recover from a division by zero error by using `redeem`.
  //
  val recovered2: IO[Nothing, Int] =
  divide1(100, 0).redeem(
    err = error => IO.now(-1),
    succ = value => IO.now(value)
  )

  //
  // EXERCISE 8
  //
  // Use the `orElse` method of `IO` to try `firstChoice`, and fallback to
  // `secondChoice` only if `firstChoice` fails.
  //
  val firstChoice: IO[DenomIsZero, Int] = divide1(100, 0)
  val secondChoice: IO[Nothing, Int] = IO.now(400)
  val combined: IO[Nothing, Int] = firstChoice orElse secondChoice
}

object zio_effects {
  import scala.io.StdIn.readLine
  import scala.io.Source
  import java.io.File
  import java.util.concurrent.{Executors, TimeUnit}

  type ??? = Nothing
  // this is just done to indicate that something is missing
  implicit class FixMe[A](a: A) {
    def ?[B] = ???
  }

  //
  // EXERCISE 1
  //
  // Using the `IO.sync` method, wrap Scala's `println` method to import it into
  // the world of pure functional programming.
  //
  def putStrLn(line: String): IO[Nothing, Unit] = IO.sync(println(line))

  //
  // EXERCISE 2
  //
  // Using the `IO.sync` method, wrap Scala's `readLine` method to import it
  // into the world of pure functional programming.
  //
  val getStrLn: IO[Nothing, String] = IO.sync(readLine())

  //
  // EXERCISE 3
  //
  // Using the `IO.syncException` method, wrap Scala's `getLines` method to
  // import it into the world of pure functional programming.
  //
  def readFile(file: File): IO[Exception, List[String]] =
    IO.syncException(Source.fromFile(file).getLines.toList)

  // Exercise 3.5
  // Use IO.syncThrowable (more for Scala code) to wrap Scala's getLines
  def readFileT(file: File): IO[Throwable, List[String]] =
    IO.syncThrowable(Source.fromFile(file).getLines.toList)

  // Exercise 3.75
  def readFileIOE(file: File): IO[java.io.IOException, List[String]] =
    // look at a subset of errors, catch those and turn those Throwables into specific E's in IO[E, A]
    IO.syncCatch(Source.fromFile(file).getLines.toList) {
      case e: java.io.IOException => e
    }

  //
  // EXERCISE 4
  //
  // Identify the correct method and error type to import `System.nanoTime`
  // safely into the world of pure functional programming.
  // Calvin: nanoTime cannot fail
  def nanoTime: IO[Nothing, Long] = IO.sync(System.nanoTime())

  //
  // EXERCISE 5
  //
  // Identify the correct method, error, and value type to import `System.exit`
  // safely into the world of pure functional programming.
  //
  def sysExit(code: Int): IO[SecurityException, Nothing] = IO.syncCatch(System.exit(code)) {
    case e : SecurityException => e
  }.flatMap(_ => IO.terminate)

  //
  // EXERCISE 6
  //
  // Identify the correct method, error, and value type to import
  // `Array.update` safely into the world of pure functional programming.
  //
  def arrayUpdate[A](a: Array[A], i: Int, f: A => A): IO[ArrayIndexOutOfBoundsException, Unit] =
    IO.syncCatch(a.update(i, f(a(i)))) {
      case e: ArrayIndexOutOfBoundsException => e
    }

  //
  // EXERCISE 7
  //
  // Use the `IO.async` method to implement the following `sleep` method, and
  // choose the correct error type.
  //
  val scheduledExecutor = Executors.newScheduledThreadPool(1)
  def sleep(l: Long, u: TimeUnit): IO[Nothing, Unit] = IO.async { callback: Callback[Nothing, Unit] =>
    scheduledExecutor.schedule(
      new Runnable {
        def run(): Unit = callback(ExitResult.Completed(()))
      }, l, u
    )
  }

  //
  // EXERCISE 8
  //
  // Translate the following procedural program into ZIO.
  //
  def playGame1(): Unit = {
    val number = scala.util.Random.nextInt(5)
    println("Enter a number between 0 - 5: ")
    scala.util.Try(scala.io.StdIn.readLine().toInt).toOption match {
      case None =>
        println("You didn't enter an integer!")
        playGame1

      case Some(guess) if (guess == number) =>
        println("You guessed right! The number was " + number)

      case _ =>
        println("You guessed wrong! The number was " + number)
    }
  }
  def playGame2: IO[Exception, Unit] = {
    def guess(providedGuess: Int, randomNumber: Int): IO[Nothing, Unit] =
      if (providedGuess == randomNumber) IO.sync(s"You guessed right! The number was $randomNumber")
      else IO.sync(s"You guessed wrong! The number was $randomNumber")

    for {
      randomNumber <- IO.sync(scala.util.Random.nextInt(5))
      _            <- IO.sync(println("Enter a number between 0 - 5: "))
      stringInt    <- IO.sync(readLine())
      optInt       =  scala.util.Try(stringInt.toInt).toOption
      _            <- optInt.fold(
                        IO.sync(println("You didn't enter an integer!")) *> playGame2)(
                        validInt => guess(validInt, randomNumber)
                      )
    } yield ()
  }
}

object zio_concurrency {
  type ??? = Nothing

  // indicates that something is missing
  implicit class FixMe[A](a: A) {
    def ?[B] = ???
  }

  //
  // EXERCISE 1
  //
  // Race `leftContestent1` and `rightContestent1` using the `race` method of
  // `IO` to see which one finishes first with a successful value.
  //
  val leftContestent1 = IO.never
  val rightContestent1 = putStrLn("Hello World")
  val raced1 = leftContestent1 race rightContestent1
  // rightContestent1 wins

  //
  // EXERCISE 2
  //
  // Race `leftContestent2` and `rightContestent2` using the `race` method of
  // `IO` to see which one finishes first with a successful value.
  //
  val leftContestent2: IO[Exception, Nothing] = IO.fail(new Exception("Uh oh!"))
  val rightContestent2: IO[Exception, Unit] = IO.sleep(10.milliseconds) *> putStrLn("Hello World")
  val raced2: IO[Exception, Unit] = leftContestent2 race rightContestent2
  // right wins because left fails

  //
  // EXERCISE 3
  //
  // Compute `leftWork1` and `rightWork1` in parallel using the `par` method of
  // `IO`.
  //
  val leftWork1: IO[Nothing, Int] = fibonacci(10)
  val rightWork1: IO[Nothing, Int] = fibonacci(10)
  val par1: IO[Nothing, (Int, Int)] = leftWork1.par(rightWork1)

  //
  // EXERCISE 4
  //
  // Compute all values `workers` in parallel using `IO.parAll`.
  //
  val workers: List[IO[Nothing, Int]] =
  (1 to 10).toList.map(fibonacci(_))
  val workersInParallel: IO[Nothing, List[Int]] = IO.parAll(workers)

  //
  // EXERCISE 5
  //
  // Implement `myPar` by forking `left` and `right`, and then joining them
  // and yielding a tuple of their results.
  //
  def myPar[E, A, B](left: IO[E, A], right: IO[E, B]): IO[E, (A, B)] =
    for {
      lFiber <- left.fork
      rFiber <- right.fork
      l      <- lFiber.join
      r      <- rFiber.join
    } yield (l, r)

  //
  // EXERCISE 6
  //
  // Use the `IO.supervise` method to ensure that when the main fiber exits,
  // all fibers forked within it will be terminated cleanly.
  //
  val supervisedExample: IO[Nothing, Unit] =
  (for {
    fiber <- fibonacci(10000).fork
  } yield ())
  IO.supervise(supervisedExample)

  //
  // EXERCISE 7
  //
  // Use the `interrupt` method of the `Fiber` object to cancel the long-running
  // `fiber`.
  //
  val interrupted1: IO[Nothing, Unit] =
  for {
    fiber <- fibonacci(10000).fork
    _     <- fiber.interrupt
  } yield ()
  interrupted1

  //
  // EXERCISE 8
  //
  // Use the `zipWith` method of the `Fiber` object to combine `fiber1` and
  // `fiber2` into a single fiber (by summing the results), so they can be
  // interrupted together.
  //
  val interrupted2: IO[Nothing, Unit] =
  for {
    fiber1 <- fibonacci(10).fork
    fiber2 <- fibonacci(20).fork
    both   <- (IO.now(fiber1.zipWith(fiber2)((res1, res2) => res1 + res2)) : IO[Nothing, Fiber[Nothing, Int]])
    _      <- both.interrupt
  } yield ()

  //
  // EXERCISE 9
  //
  // io.timeout[Option[A]](None)(Some(_))(60.seconds)

  def fibonacci(n: Int): IO[Nothing, Int] =
    if (n <= 1) IO.now(n)
    else fibonacci(n - 1).seqWith[Nothing, Int, Int](fibonacci(n - 2))(_ + _)
                         .timeout[Int](z = -1)(f = identity)(duration = 60.seconds)

  // Exercise 10, use IO.parTraverse to compute fibonacci numbers of the list of integers in parallel
  val fibsToCompute = List(1, 2, 3, 4, 5, 6, 7)
  val inParallel: IO[Nothing, List[Int]] = IO.parTraverse(fibsToCompute) { eachElement =>
    fibonacci(eachElement)
  }
}

object zio_resources {
  import java.io.{File, FileInputStream}
  class InputStream private (is: FileInputStream) {
    def read: IO[Exception, Option[Byte]] =
      IO.syncException(is.read).map(i => if (i < 0) None else Some(i.toByte))
    def close: IO[Exception, Unit] =
      IO.syncException(is.close())
  }
  object InputStream {
    def openFile(file: File): IO[Exception, InputStream] =
      IO.syncException(new InputStream(new FileInputStream(file)))
  }

  //
  // EXERCISE 1
  //
  // Rewrite the following procedural program to ZIO, using `IO.fail` and the
  // `bracket` method of the `IO` object.
  //
  def tryCatch1(): Unit =
    try throw new Exception("Uh oh")
    finally println("On the way out...")
  val tryCatch2: IO[Exception, Unit] =
    IO.fail(new Exception("Uh oh"))
      .bracket(_ => IO.sync(println("On the way out...")))(use = _ => IO.now(()))

  //
  // EXERCISE 2
  //
  // Rewrite the `readFile1` function to use `bracket` so resources can be
  // safely cleaned up in the event of errors, defects, or interruption.
  //
  def readFile1(file: File): IO[Exception, List[Byte]] = {
    def readAll(is: InputStream, acc: List[Byte]): IO[Exception, List[Byte]] =
      is.read.flatMap {
        case None => IO.now(acc.reverse)
        case Some(byte) => readAll(is, byte :: acc)
      }

    for {
      stream <- InputStream.openFile(file)
      bytes  <- readAll(stream, Nil)
    } yield bytes
  }
  def readFile2(file: File): IO[Exception, List[Byte]] = {
    def readAll(is: InputStream, acc: List[Byte]): IO[Exception, List[Byte]] =
      is.read.flatMap {
        case None => IO.now(acc.reverse)
        case Some(byte) => readAll(is, byte :: acc)
      }
    IO.bracket(
      acquire = InputStream.openFile(file))(
      release = handle => handle.close.attempt.void)(
      use = (inputStream: InputStream) => readAll(inputStream, Nil)
    )
  }

  //
  // EXERCISE 3
  //
  // Implement the `tryCatchFinally` method using `bracket`.
  //
  def tryCatchFinally[E, A]
  (try0: IO[E, A])
  (catch0: PartialFunction[E, IO[E, A]])
  (finally0: IO[Nothing, Unit]): IO[E, A] =
    try0
      .catchSome(catch0)
      .bracket(release = _ => finally0)(use = IO.now)

  //
  // EXERCISE 4
  //
  // Use the `bracket` method to rewrite the following snippet to ZIO.
  //
  def readFileTCF1(file: File): List[Byte] = {
    var fis : FileInputStream = null

    try {
      fis = new FileInputStream(file)
      val array: Array[Byte] = Array.ofDim[Byte](file.length.toInt)
      fis.read(array)
      array.toList
    } catch {
      case e : java.io.IOException => Nil
    } finally if (fis != null) fis.close()
  }
  def readFileTCF2(file: File): IO[Exception, List[Byte]] = {
    def readBytes(is: InputStream, acc: List[Byte]): IO[Exception, List[Byte]] =
      for {
        optByte <- is.read
        res     <- optByte.fold(
                    ifEmpty = IO.syncException(acc))(
                    byte => readBytes(is, acc :+ byte)
                   )
      } yield res

    InputStream.openFile(file).bracket(
      release = inputStream => inputStream.close.attempt.void)(
      use = is => readBytes(is, Nil)
    )
  }
}

object zio_schedule {
  implicit class FixMe[A](a: A) {
    def ? = ???
  }

  //
  // EXERCISE 1
  //
  // Using `Schedule.recurs`, create a schedule that recurs 5 times.
  //
  val fiveTimes: Schedule[Any, Int] = Schedule.recurs(5)

  //
  // EXERCISE 2
  //
  // Using the `repeat` method of the `IO` object, repeat printing "Hello World"
  // five times to the console.
  //
  val repeated1 = putStrLn("Hello World").repeat(fiveTimes)

  //
  // EXERCISE 3
  //
  // Using `Schedule.spaced`, create a schedule that recurs forever every 1
  // second.
  //
  val everySecond: Schedule[Any, Int] = Schedule.spaced(1.second)

  //
  // EXERCISE 4
  //
  // Using the `&&` method of the `Schedule` object, the `fiveTimes` schedule,
  // and the `everySecond` schedule, create a schedule that repeats fives times,
  // every second.
  //
  val fiveTimesEverySecond = fiveTimes && everySecond

  //
  // EXERCISE 5
  //
  // Using the `repeat` method of the `IO` object, repeat the action
  // putStrLn("Hi hi") using `fiveTimesEverySecond`.
  //
  val repeated2 = putStrLn("Hi hi").repeat(fiveTimesEverySecond)

  //
  // EXERCISE 6
  //
  // Using the `andThen` method of the `Schedule` object, the `fiveTimes`
  // schedule, and the `everySecond` schedule, create a schedule that repeats
  // fives times rapidly, and then repeats every second forever.
  //
  val fiveTimesThenEverySecond = fiveTimes andThen everySecond

  //
  // EXERCISE 7
  //
  // Using the `retry` method of the `IO` object, retry the following error
  // a total of five times.
  //
  val error1 = IO.fail("Uh oh!")
  val retried5 = error1.retry(Schedule.recurs(5))

  //
  // EXERCISE 8
  //
  // Using the `&&` method of the `Schedule` object, the `fiveTimes` schedule,
  // and the `everySecond` schedule, create a schedule that repeats the minimum
  // of five times and every second.
  //
  val fiveTimesOrEverySecond = fiveTimes || everySecond

  //
  // EXERCISE 9
  //
  // Produce a jittered schedule that first does exponential spacing (starting
  // from 10 milliseconds), but then after the spacing reaches 60 seconds,
  // switches over to fixed spacing of 60 seconds between recurrences, but will
  // only do that for up to 100 times, and produce a list of the results.
  //
  def mySchedule[A]: Schedule[A, List[A]] = {
    // FYI: Schedule has an Applicative typeclass
    // Produce a jittered schedule that first does exponential spacing (starting
    // from 10 milliseconds), but then after the spacing reaches 60 seconds,
    val part1 = Schedule.exponential(10.milliseconds).whileValue(currentDuration => currentDuration < 60.seconds)

    // fixed spacing of 60 seconds between recurrences,
    // but will only do that for up to 100 times,
    val part2 = Schedule.fixed(60.seconds) && Schedule.recurs(100)

    // Produce a jittered schedule that first does exponential spacing (starting
    // from 10 milliseconds), but then after the spacing reaches 60 seconds,
    // switches over to fixed spacing of 60 seconds between recurrences, but will
    // only do that for up to 100 times
    val combined = (part1 andThen part2).jittered

    // collect values from the input
    val identitySchedule: Schedule[A, List[A]] = Schedule.identity[A].collect

    combined *> identitySchedule
  }
}

object zio_interop {
  implicit class FixMe[A](a: A) {
    def ? = ???
  }

  import scala.concurrent.Future
  import scalaz.zio.interop.future._
  import scala.concurrent.ExecutionContext.Implicits.global

  //
  // EXERCISE 1
  //
  // Use `IO.fromFuture` method to convert the following `Future` into an `IO`.
  //
  val future1 = () => Future.successful("Hello World")
  val io1 = IO.fromFuture(future1)(global)

  //
  // EXERCISE 2
  //
  // Use the `toFuture` method on `IO` to convert the following `io` to `Future`.
  //
  val io2: IO[Throwable, Int] = IO.point(42)
  val future2: IO[Nothing, Future[Int]] = io2.toFuture

  //
  // EXERCISE 3
  //
  // Use the Fiber.fromFuture` method to convert the following `Future` into
  // an `IO`.
  //
  val future3 = () => Future.failed[Int](new Error("Uh ohs!"))
  val fiber1: Fiber[Throwable, Int] = Fiber.fromFuture(future3())(global)

  // Use the following to make ScalaZ ZIO work with Cats Effect typeclases
  import scalaz.zio.interop.Task
  import scalaz.zio.interop.catz._
  import cats.effect.concurrent.Ref

  //
  // EXERCISE 4
  //
  // The following example uses the `Ref` from `cats-effect`, demonstrating how
  // `cats-effect` structures work with ZIO.
  //
  class Worker(number: Int, ref: Ref[Task, Int]) {
    def work: Task[Unit] =
      for {
        c1 <- ref.get
        _  <- putStrLn(s"#$number >> $c1")
        c2 <- ref.modify(x => (x + 1, x))
        _  <- putStrLn(s"#$number >> $c2")
      } yield ()
  }

  // Note the Ref is from Cats Effect not from ZIO here
  val program: Task[Unit] =
    for {
      ref <- Ref.of[Task, Int](0)
      w1  = new Worker(1, ref)
      w2  = new Worker(2, ref)
      w3  = new Worker(3, ref)
      f   <- IO.forkAll(List(w1.work, w2.work, w3.work))
      _   <- f.join
    } yield ()
}

object zio_ref {
  // EXERCISE 1
  // Use Ref.apply to create a Ref that is initially 0
  // Ref can be thought of as a var but is nicer
  val makeZero: IO[Nothing, Ref[Int]] = Ref(0)  // haven't done anything yet, description only

  // EXERCISE 2
  // use get and set to change the value to be 10 greater than the initial value
  val incrementedBy10: IO[Nothing, Int] = for {
    ref       <- makeZero
    value     <- ref.get: IO[Nothing, Int]
    newValue  = value + 10
    _         <- ref.set(newValue): IO[Nothing, Unit]
  } yield newValue

  // EXERCISE 3
  // Use update instead of get and set
  val atomicallyIncrementedBy10: IO[Nothing, Int] = for {
    ref       <- makeZero
    newValue  <- ref.update(existing => existing + 10): IO[Nothing, Int]
  } yield newValue

  // EXERCISE 4
  // Use modify to atomically increment the value but return the old value
  val modifyIncrementedBy10: IO[Nothing, Int] = for {
    ref       <- makeZero
    newValue  <- ref.modify(existing => (existing /*old*/, existing + 10 /*new*/)): IO[Nothing, Int]
  } yield newValue
}

object zio_promise {
  // EXERCISE 1
  // Using `make` of `Promise`, construct a promise that cannot fail but can be completed with an integer
  val makeIntPromise: IO[Nothing, Promise[Nothing, Int]] = Promise.make[Nothing, Int] // description only

  // EXERCISE 2 use the `complete` method of `Promise` to complete a Promise constructed with `makeIntPromise`
  val completed1: IO[Nothing, Boolean] = for {
    promise               <- makeIntPromise
    completedSuccessfully <- promise.complete(42): IO[Nothing, Boolean]
  } yield completedSuccessfully

  // EXERCISE 3
  // use the error of Promise, try to complete a Promise with makeIntPromise. Explain your findings.
//  val errored1: IO[Nothing, Boolean] =
//    for {
//      promise <- makeIntPromise
//      completed <- promise.error(/* you don't have values of type Nothing so you cannot construct this program */)
//    } yield completed

  // EXERCISE 4
  // Use the error of Promise to complete a new Promise that can fail for any Error
  val errored2: IO[Nothing, Boolean] =
   for {
      promise <- Promise.make[Error, String]
      completed <- promise.error(new Error("boom!")): IO[Nothing, Boolean]  // note we are explicitly typing this to be
                                                                            // IO[Nothing, Boolean], it's actually
                                                                            // IO[Error, Boolean]
    } yield completed

  // EXERCISE 5
  // Using interrupt of Promise, complete a new promise you construct with Promise.make which can fail for any Error or produce a String
  val interrupted: IO[Nothing, Boolean] = for {
    promise <- Promise.make[Error, String]
    completed <- promise.interrupt: IO[Nothing, Boolean]    // doesn't actually complete the promise so completed = false
  } yield completed

  // EXERCISE 6
  // Using get of a Promise, retrieve a cvaclue computed from inside another Fiber
  val handoff1: IO[Nothing, Int] =
    for {
      promise <- Promise.make[Nothing, Int]
      _       <- promise.complete(42).delay(10.milliseconds).fork   // complete the promise in 10 seconds with value 42 on another fiber
      value   <- promise.get
    } yield value

  // EXERCISE 7
  // similar to 6
  val handoff2: IO[Error, Int] =
    for {
      promise <- Promise.make[Error, Int]
      _       <- promise.error(new Error("Uh oh!")).delay(10.milliseconds).fork
      value   <- promise.get  // this will fail using the E in IO[E=Error, A=Int], get suspends until a value/error is present
    } yield value

  // EXERCISE 8
  // retrieve a value from a promise that was interrupted in another fiber
  val handoff3: IO[Error, Int] =
    for {
      promise <- Promise.make[Error, Int]
      _       <- promise.interrupt.delay(10.milliseconds).fork
      value   <- promise.get: IO[Error, Int]
    } yield value
}

object zio_queue {
  // EXERCISE 1
  // Use Queue.bounded to create a queue with capacity 10
  val makeQueue: IO[Nothing, Queue[Int]] = Queue.bounded(10)

  // EXERCISE 2
  // Use offer to place a value 42 into the queue
  val offered1: IO[Nothing, Unit] =
    for {
      queue <- makeQueue
      _     <- queue.offer(42)
    } yield ()

  // EXERCISE 3
  // Use take to take a value from the queue
  val taken1: IO[Nothing, Int] =
  for {
    queue <- makeQueue
    _     <- queue.offer(42)
    value <- queue.take
  } yield value

  // EXERCISE 4
  // In one fiber, place 2 values into a queue and in the main fiber, read 2 values from the queue
  val offeredTaken1: IO[Nothing, (Int, Int)] =
    for {
      queue <- makeQueue
      _     <- (queue.offer(42) *> queue.offer(42)).fork
      v1    <- queue.take: IO[Nothing, Int]
      v2    <- queue.take: IO[Nothing, Int]
    } yield (v1, v2)

  // EXERCISE 5
  // In one fiber, read infinitely from the queue in a child fiber and in the main fiber, write 100 values into the queue
  val infiniteReader1: IO[Nothing, List[Int]] = {
    def repeatedlyOffer[A](element: A, times: Int, queue: Queue[A]): IO[Nothing, List[A]] =
      if (times == 0) IO.point(Nil)
      else for {
        _       <- queue.offer(element)
        listInt <- repeatedlyOffer(element, times - 1, queue)
      } yield listInt :+ element

    for {
      queue <- makeQueue
      _     <- (queue.take.flatMap(element => putStrLn(s"Taking $element").attempt.void).forever: IO[Nothing, Nothing]).fork
      vs    <- repeatedlyOffer(element = 42, times = 100, queue = queue): IO[Nothing, List[Int]]
    } yield vs
  }

  // EXERCISE 6
  // Using Queue, Ref and Promise, implement an Actor like construct that can atomically update values of a counter
  val makeCounter: IO[Nothing, Int => IO[Nothing, Int]] =
    for {
      counter <- Ref(0)
      queue   <- Queue.bounded[(Int, Promise[Nothing, Int])](100)
                 // consume from the queue and update the counter and then complete the promise to indicate
                 // we have finished with the element we have pulled from the queue
      _       <- queue.take
                      .flatMap{
                        case (completeWith: Int, promise: Promise[Nothing, Int]) =>
                          counter.update(existing => existing + completeWith) *> promise.complete(completeWith)
                      }
                      .forever
                      .fork: IO[Nothing, Fiber[Nothing, Nothing]]
    } yield {
      // we yield a function for a way to send messages to the actor, let it do the work and get the updated value
      // only the actor deals with the Ref and communicates back with us (this piece of code below) the updated Ref value
      // using the Promise
      amount: Int =>
        for {
          promise   <- Promise.make[Nothing, Int]
          _         <- queue.offer((amount, promise))
          newValue  <- promise.get      // get the new value of the counter (this work is done by the "actor" fiber)
        } yield newValue
    }

  // create a producer
  val counterExample: IO[Nothing, Int] =
    for {
      counterFn <- makeCounter
      _         <- IO.parAll(List.fill(100)(IO.traverse(0 to 100)(counterFn)))
      value     <- counterFn(0) // call the counterFn with 0, this will happen 100 times in parallel thanks to parAll
    } yield value
}

object zio_rts {
  // Exercise 1
  // Create a new runtime system that extends scalaz.zio.RTS
  val MyRTS: RTS = new RTS {}

  // Exercise 2
  // Run the following IO using MyRTS
  MyRTS.unsafeRun(putStrLn("Hello world")): Unit

  // Exercise 3
  // Use unsafeRunSync so it doesn't throw exceptions
  MyRTS.unsafeRunSync(putStrLn("Hello world")): ExitResult[java.io.IOException, Unit]

  // Exercise 4
  // Implement a purely functional application using ZIO's App
  object MyApp extends scalaz.zio.App {
    override def run(args: List[String]): IO[Nothing, MyApp.ExitStatus] = {
      val core = for {
        _     <- putStrLn("Hello there!")
        name  <- getStrLn
        _     <- putStrLn(s"Hello $name")
      } yield ()

      core.redeemPure(
        err = _ => ExitStatus.ExitNow(1),
        succ = _ => ExitStatus.ExitNow(0)
      )
    }
  }
}

object zio_advanced {
  // change thread pool of RTS

  // make it so that Fibers will yield after every operation
}

Please don't hesitate to point out mistakes in the implementation and post your solution here 😄

Revisiting Schedule

import scalaz._
import Scalaz._
import scalaz.zio._
import scalaz.zio.console._
import scala.concurrent.duration._

// https://static.javadoc.io/org.scalaz/scalaz-zio_2.12/0.2.9/scalaz/zio/Schedule$.html
object ScheduleMePlease extends App {
  override def run(args: List[String]): IO[Nothing, ScheduleMePlease.ExitStatus] = {
    def `recur 10 times with a 500ms delay between recurrences`[A]: Schedule[A, Int] =
      Schedule.recurs(10).delayed(inputDuration => 500.milliseconds)

    // && returns a new schedule that continues only as long as both schedules continue,
    // using the maximum of the delays of the two schedules.
    def `recur 10 times with a 1s delay between recurrences`[A]: Schedule[A, (Int, Int)] =
      (`recur 10 times with a 500ms delay between recurrences`: Schedule[A, Int]) && (Schedule.spaced(1.second): Schedule[A, Int])

    def customSchedule[A](start: Duration = 10.milliseconds, max: Duration = 60.seconds, nrOfTriesAfterMaxHit: Int = 100): Schedule[A, List[A]] =
      (Schedule.exponential(start).whileValue(duration => duration < max) /* apply the exponential schedule as long as the duration is less than max, if it's more then this schedule will stop */
        andThen (Schedule.spaced(max) && Schedule.recurs(nrOfTriesAfterMaxHit)) /* then apply a fixed spacing schedule of max duration for 100 times */).jittered /* jitter all */ *>
        Schedule.identity[A].collect /* collect the input from the preceding IO as a list for every invocation of this schedule */

    putStrLn("Hello world!")
      .map(_ => 1) // this is the preceding input IO that our custom schedule will collect the data, the IO outputs Int so that is the input to our Schedule
      .repeat(customSchedule(start = 1.milli, max = 2.seconds, nrOfTriesAfterMaxHit = 10): Schedule[Int, List[Int]])
      .flatMap((i: List[Int]) => putStrLn(s"data after schedule: $i"))
      .attempt
      .map(_ => ExitStatus.ExitNow(1))
  }
}

Creating our own Schedule from scratch

    // this custom schedule accepts preceding IO[E, A] where A=Int (input), it will repeat the computation if the input is < 65 and will record the input that came through in the String
    // if this Schedule does not get run to completion (i.e. we don't hit Decision.done) (because you combined it with a shorter schedule then it will emit "premature finish")
    def superCustomSchedule: Schedule[Int /* Input */, String /* Emit */] = Schedule.apply[String /* Internal State */, Int /* Input */, String /* Emit */](
      IO.point("Initial State: "),
      (incoming: Int, acc: String) =>
        if (incoming > 65) IO.point(
          // do not want another execution to happen
          Decision.done(
            d = 0.milliseconds /* delay */,
            a = "" /* internal state */,
            b = acc /* value to emit */)
        )
        else IO.point(
          // want another execution to happen
          Decision.cont(
            d = 500.milliseconds /* delay 500ms before the next schedule */,
            a = acc ++ "," ++ incoming.toString /* internal state */,
            b = "premature-finish" /* value to emit in case this schedule is not run to completion */
          )
        )
    )

    putStrLn("Hello world!")
      .flatMap(_ => IO.sync(Random.nextInt(70)))
      .repeat(superCustomSchedule <* Schedule.recurs(3)) // <* is like && but takes the output of the left scheduler (superCustomSchedule)
      .flatMap((i: String) => putStrLn(s"data after schedule: $i"))
      .attempt
      .map(_ => ExitStatus.ExitNow(1))

Regarding https://gist.github.com/calvinlfer/197d3f4a7aa1365293946da93a24d484#file-day-3-scala-L1283, the work i'm referencing is the completion of the Promise with the new counter value