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FP to the Max — Code Examples
package fpmax
import scala.util.Try
import scala.io.StdIn.readLine
object App0 {
def main: Unit = {
println("What is your name?")
val name = readLine()
println("Hello, " + name + ", welcome to the game!")
var exec = true
while (exec) {
val num = scala.util.Random.nextInt(5) + 1
println("Dear " + name + ", please guess a number from 1 to 5:")
val guess = readLine().toInt
if (guess == num) println("You guessed right, " + name + "!")
else println("You guessed wrong, " + name + "! The number was: " + num)
println("Do you want to continue, " + name + "?")
readLine() match {
case "y" => exec = true
case "n" => exec = false
}
}
}
}
object App1 {
def parseInt(s: String): Option[Int] = Try(s.toInt).toOption
trait Program[F[_]] {
def finish[A](a: => A): F[A]
def chain[A, B](fa: F[A], afb: A => F[B]): F[B]
def map[A, B](fa: F[A], ab: A => B): F[B]
}
object Program {
def apply[F[_]](implicit F: Program[F]): Program[F] = F
}
implicit class ProgramSyntax[F[_], A](fa: F[A]) {
def map[B](f: A => B)(implicit F: Program[F]): F[B] = F.map(fa, f)
def flatMap[B](afb: A => F[B])(implicit F: Program[F]): F[B] = F.chain(fa, afb)
}
def finish[F[_], A](a: => A)(implicit F: Program[F]): F[A] = F.finish(a)
trait Console[F[_]] {
def putStrLn(line: String): F[Unit]
def getStrLn: F[String]
}
object Console {
def apply[F[_]](implicit F: Console[F]): Console[F] = F
}
def putStrLn[F[_]: Console](line: String): F[Unit] = Console[F].putStrLn(line)
def getStrLn[F[_]: Console]: F[String] = Console[F].getStrLn
trait Random[F[_]] {
def nextInt(upper: Int): F[Int]
}
object Random {
def apply[F[_]](implicit F: Random[F]): Random[F] = F
}
def nextInt[F[_]](upper: Int)(implicit F: Random[F]): F[Int] = Random[F].nextInt(upper)
case class IO[A](unsafeRun: () => A) { self =>
def map[B](f: A => B): IO[B] = IO(() => f(self.unsafeRun()))
def flatMap[B](f: A => IO[B]): IO[B] = IO(() => f(self.unsafeRun()).unsafeRun())
}
object IO {
def point[A](a: => A): IO[A] = IO(() => a)
implicit val ProgramIO = new Program[IO] {
def finish[A](a: => A): IO[A] = IO.point(a)
def chain[A, B](fa: IO[A], afb: A => IO[B]): IO[B] = fa.flatMap(afb)
def map[A, B](fa: IO[A], ab: A => B): IO[B] = fa.map(ab)
}
implicit val ConsoleIO = new Console[IO] {
def putStrLn(line: String): IO[Unit] = IO(() => println(line))
def getStrLn: IO[String] = IO(() => readLine())
}
implicit val RandomIO = new Random[IO] {
def nextInt(upper: Int): IO[Int] = IO(() => scala.util.Random.nextInt(upper))
}
}
case class TestData(input: List[String], output: List[String], nums: List[Int]) {
def putStrLn(line: String): (TestData, Unit) =
(copy(output = line :: output), ())
def getStrLn: (TestData, String) =
(copy(input = input.drop(1)), input.head)
def nextInt(upper: Int): (TestData, Int) =
(copy(nums = nums.drop(1)), nums.head)
def showResults = output.reverse.mkString("\n")
}
case class TestIO[A](run: TestData => (TestData, A)) { self =>
def map[B](ab: A => B): TestIO[B] =
TestIO(t => self.run(t) match { case (t, a) => (t, ab(a)) })
def flatMap[B](afb: A => TestIO[B]): TestIO[B] =
TestIO(t => self.run(t) match { case (t, a) => afb(a).run(t) })
def eval(t: TestData): TestData = run(t)._1
}
object TestIO {
def point[A](a: => A): TestIO[A] = TestIO(t => (t, a))
implicit val ProgramTestIO = new Program[TestIO] {
def finish[A](a: => A): TestIO[A] = TestIO.point(a)
def chain[A, B](fa: TestIO[A], afb: A => TestIO[B]): TestIO[B] = fa.flatMap(afb)
def map[A, B](fa: TestIO[A], ab: A => B): TestIO[B] = fa.map(ab)
}
implicit val ConsoleTestIO = new Console[TestIO] {
def putStrLn(line: String): TestIO[Unit] = TestIO(t => t.putStrLn(line))
def getStrLn: TestIO[String] = TestIO(t => t.getStrLn)
}
implicit val RandomTestIO = new Random[TestIO] {
def nextInt(upper: Int): TestIO[Int] = TestIO(t => t.nextInt(upper))
}
}
def checkContinue[F[_]: Program: Console](name: String): F[Boolean] =
for {
_ <- putStrLn("Do you want to continue, " + name + "?")
input <- getStrLn.map(_.toLowerCase)
cont <- input match {
case "y" => finish(true)
case "n" => finish(false)
case _ => checkContinue(name)
}
} yield cont
def printResults[F[_]: Console](input: String, num: Int, name: String): F[Unit] =
parseInt(input).fold(
putStrLn("You did not enter a number")
)(guess =>
if (guess == num) putStrLn("You guessed right, " + name + "!")
else putStrLn("You guessed wrong, " + name + "! The number was: " + num)
)
def gameLoop[F[_]: Program: Random: Console](name: String): F[Unit] =
for {
num <- nextInt(5).map(_ + 1)
_ <- putStrLn("Dear " + name + ", please guess a number from 1 to 5:")
input <- getStrLn
_ <- printResults(input, num, name)
cont <- checkContinue(name)
_ <- if (cont) gameLoop(name) else finish(())
} yield ()
def main[F[_]: Program: Random: Console]: F[Unit] =
for {
_ <- putStrLn("What is your name?")
name <- getStrLn
_ <- putStrLn("Hello, " + name + ", welcome to the game!")
_ <- gameLoop(name)
} yield ()
def mainIO: IO[Unit] = main[IO]
def mainTestIO: TestIO[Unit] = main[TestIO]
val TestExample =
TestData(
input = "John" :: "1" :: "n" :: Nil,
output = Nil,
nums = 0 :: Nil
)
def runTest = mainTestIO.eval(TestExample).showResults
/*
What is your name?
Hello, John, welcome to the game!
Dear John, please guess a number from 1 to 5:
You guessed right, John!
Do you want to continue, John?
*/
}
object App2 {
object stdlib {
trait Program[F[_]] {
def finish[A](a: A): F[A]
def chain[A, B](fa: F[A], afb: A => F[B]): F[B]
def map[A, B](fa: F[A], ab: A => B): F[B]
}
object Program {
def apply[F[_]](implicit F: Program[F]): Program[F] = F
}
implicit class ProgramSyntax[F[_], A](fa: F[A]) {
def map[B](ab: A => B)(implicit F: Program[F]): F[B] = F.map(fa, ab)
def flatMap[B](afb: A => F[B])(implicit F: Program[F]): F[B] = F.chain(fa, afb)
}
def finish[F[_], A](a: A)(implicit F: Program[F]): F[A] = F.finish(a)
final case class IO[A](unsafeRun: () => A) { self =>
final def map[B](f: A => B): IO[B] = IO(() => f(self.unsafeRun()))
final def flatMap[B](f: A => IO[B]): IO[B] =
IO(() => f(self.unsafeRun()).unsafeRun())
}
object IO {
def point[A](a: => A): IO[A] = IO(() => a)
implicit val ProgramIO = new Program[IO] {
def finish[A](a: A): IO[A] = IO.point(a)
def chain[A, B](fa: IO[A], afb: A => IO[B]): IO[B] = fa.flatMap(afb)
def map[A, B](fa: IO[A], ab: A => B): IO[B] = fa.map(ab)
}
}
trait Console[F[_]] {
def putStrLn(line: String): F[Unit]
def getStrLn: F[String]
}
object Console {
def apply[F[_]](implicit F: Console[F]): Console[F] = F
implicit val ConsoleIO = new Console[IO] {
def putStrLn(line: String): IO[Unit] = IO(() => println(line))
def getStrLn: IO[String] = IO(() => readLine())
}
}
def putStrLn[F[_]: Console](line: String): F[Unit] = Console[F].putStrLn(line)
def getStrLn[F[_]: Console]: F[String] = Console[F].getStrLn
trait Random[F[_]] {
def nextInt(upper: Int): F[Int]
}
object Random {
def apply[F[_]](implicit F: Random[F]): Random[F] = F
implicit val RandomIO = new Random[IO] {
def nextInt(upper: Int): IO[Int] = IO(() => scala.util.Random.nextInt(upper))
}
}
def nextInt[F[_]: Random](upper: Int): F[Int] = Random[F].nextInt(upper)
}
import stdlib._
case class TestData(input: List[String], output: List[String], nums: List[Int]) {
def showResults = output.reverse.mkString("\n")
def nextInt: (TestData, Int) = (copy(nums = nums.drop(1)), nums.head)
def putStrLn(line: String): (TestData, Unit) = (copy(output = line :: output), ())
def getStrLn: (TestData, String) = (copy(input = input.drop(1)), input.head)
}
case class TestIO[A](run: TestData => (TestData, A)) { self =>
def map[B](f: A => B): TestIO[B] =
TestIO(t => self.run(t) match { case (t, a) => (t, f(a)) })
def flatMap[B](f: A => TestIO[B]): TestIO[B] =
TestIO(t => self.run(t) match { case (t, a) => f(a).run(t) })
def eval(t: TestData): TestData = self.run(t)._1
}
object TestIO {
def point[A](a: => A): TestIO[A] = TestIO(t => (t, a))
implicit val RandomTestIO = new Random[TestIO] {
def nextInt(upper: Int): TestIO[Int] =
TestIO(t => t.nextInt)
}
implicit val ProgramTestIO = new Program[TestIO] {
def finish[A](a: A): TestIO[A] = TestIO.point(a)
def chain[A, B](fa: TestIO[A], afb: A => TestIO[B]): TestIO[B] = fa.flatMap(afb)
def map[A, B](fa: TestIO[A], ab: A => B): TestIO[B] = fa.map(ab)
}
implicit val ConsoleTestIO = new Console[TestIO] {
def putStrLn(line: String): TestIO[Unit] =
TestIO(t => t.putStrLn(line))
def getStrLn: TestIO[String] =
TestIO(t => t.getStrLn)
}
}
def parseInt(s: String): Option[Int] = Try(s.toInt).toOption
def checkAnswer[F[_]: Console](name: String, num: Int, guess: Int): F[Unit] =
if (num == guess) putStrLn("You guessed right, " + name + "!")
else putStrLn("You guessed wrong, " + name + "! The number was: " + num)
def checkContinue[F[_]: Program: Console](name: String): F[Boolean] =
for {
_ <- putStrLn("Do you want to continue, " + name + "?")
choice <- getStrLn.map(_.toLowerCase)
cont <- if (choice == "y") finish(true)
else if (choice == "n") finish(false)
else checkContinue(name)
} yield cont
def gameLoop[F[_]: Program: Console: Random](name: String): F[Unit] =
for {
num <- nextInt(5).map(_ + 1)
_ <- putStrLn("Dear " + name + ", please guess a number from 1 to 5:")
guess <- getStrLn
_ <- parseInt(guess).fold(
putStrLn("That is not a valid selection, " + name + "!")
)((guess: Int) => checkAnswer(name, num, guess))
cont <- checkContinue(name)
_ <- if (cont) gameLoop(name) else finish(())
} yield ()
def main[F[_]: Program: Console: Random]: F[Unit] =
for {
_ <- putStrLn("What is your name?")
name <- getStrLn
_ <- putStrLn("Hello, " + name + ", welcome to the game!")
_ <- gameLoop(name)
} yield ()
def mainIO: IO[Unit] = main[IO]
def mainTestIO: TestIO[Unit] = main[TestIO]
val TestExample = TestData(
input = "john" :: "1" :: "n" :: Nil,
output = Nil,
nums = 0 :: Nil)
}
object App3 {
object stdlib {
trait Program[F[_]] {
def finish[A](a: A): F[A]
def chain[A, B](fa: F[A], afb: A => F[B]): F[B]
def map[A, B](fa: F[A], ab: A => B): F[B]
}
object Program {
def apply[F[_]](implicit F: Program[F]): Program[F] = F
}
implicit class ProgramSyntax[F[_], A](fa: F[A]) {
def map[B](ab: A => B)(implicit F: Program[F]): F[B] = F.map(fa, ab)
def flatMap[B](afb: A => F[B])(implicit F: Program[F]): F[B] = F.chain(fa, afb)
}
def finish[F[_], A](a: A)(implicit F: Program[F]): F[A] = F.finish(a)
final case class IO[A](unsafeRun: () => A) { self =>
final def map[B](f: A => B): IO[B] = IO(() => f(self.unsafeRun()))
final def flatMap[B](f: A => IO[B]): IO[B] =
IO(() => f(self.unsafeRun()).unsafeRun())
}
object IO {
def point[A](a: => A): IO[A] = IO(() => a)
implicit val ProgramIO = new Program[IO] {
def finish[A](a: A): IO[A] = IO.point(a)
def chain[A, B](fa: IO[A], afb: A => IO[B]): IO[B] = fa.flatMap(afb)
def map[A, B](fa: IO[A], ab: A => B): IO[B] = fa.map(ab)
}
}
sealed trait ConsoleOut {
def en: String
}
object ConsoleOut {
case class YouGuessedRight(name: String) extends ConsoleOut {
def en = "You guessed right, " + name + "!"
}
case class YouGuessedWrong(name: String, num: Int) extends ConsoleOut {
def en = "You guessed wrong, " + name + "! The number was: " + num
}
case class DoYouWantToContinue(name: String) extends ConsoleOut {
def en = "Do you want to continue, " + name + "?"
}
case class PleaseGuess(name: String) extends ConsoleOut {
def en = "Dear " + name + ", please guess a number from 1 to 5:"
}
case class ThatIsNotValid(name: String) extends ConsoleOut {
def en = "That is not a valid selection, " + name + "!"
}
case object WhatIsYourName extends ConsoleOut {
def en = "What is your name?"
}
case class WelcomeToGame(name: String) extends ConsoleOut {
def en = "Hello, " + name + ", welcome to the game!"
}
}
trait Console[F[_]] {
def putStrLn(line: ConsoleOut): F[Unit]
def getStrLn: F[String]
}
object Console {
def apply[F[_]](implicit F: Console[F]): Console[F] = F
implicit val ConsoleIO = new Console[IO] {
def putStrLn(line: ConsoleOut): IO[Unit] = IO(() => println(line.en))
def getStrLn: IO[String] = IO(() => readLine())
}
}
def putStrLn[F[_]: Console](line: ConsoleOut): F[Unit] = Console[F].putStrLn(line)
def getStrLn[F[_]: Console]: F[String] = Console[F].getStrLn
trait Random[F[_]] {
def nextInt(upper: Int): F[Int]
}
object Random {
def apply[F[_]](implicit F: Random[F]): Random[F] = F
implicit val RandomIO = new Random[IO] {
def nextInt(upper: Int): IO[Int] = IO(() => scala.util.Random.nextInt(upper))
}
}
def nextInt[F[_]: Random](upper: Int): F[Int] = Random[F].nextInt(upper)
}
import stdlib._
case class TestData(input: List[String], output: List[ConsoleOut], nums: List[Int]) {
def showResults = output.reverse.map(_.en).mkString("\n")
def nextInt: (TestData, Int) = (copy(nums = nums.drop(1)), nums.head)
def putStrLn(line: ConsoleOut): (TestData, Unit) = (copy(output = line :: output), ())
def getStrLn: (TestData, String) = (copy(input = input.drop(1)), input.head)
}
case class TestIO[A](run: TestData => (TestData, A)) { self =>
def map[B](f: A => B): TestIO[B] =
TestIO(t => self.run(t) match { case (t, a) => (t, f(a)) })
def flatMap[B](f: A => TestIO[B]): TestIO[B] =
TestIO(t => self.run(t) match { case (t, a) => f(a).run(t) })
def eval(t: TestData): TestData = self.run(t)._1
}
object TestIO {
def point[A](a: => A): TestIO[A] = TestIO(t => (t, a))
implicit val RandomTestIO = new Random[TestIO] {
def nextInt(upper: Int): TestIO[Int] =
TestIO(t => t.nextInt)
}
implicit val ProgramTestIO = new Program[TestIO] {
def finish[A](a: A): TestIO[A] = TestIO.point(a)
def chain[A, B](fa: TestIO[A], afb: A => TestIO[B]): TestIO[B] = fa.flatMap(afb)
def map[A, B](fa: TestIO[A], ab: A => B): TestIO[B] = fa.map(ab)
}
implicit val ConsoleTestIO = new Console[TestIO] {
def putStrLn(line: ConsoleOut): TestIO[Unit] =
TestIO(t => t.putStrLn(line))
def getStrLn: TestIO[String] =
TestIO(t => t.getStrLn)
}
}
def parseInt(s: String): Option[Int] = Try(s.toInt).toOption
def checkAnswer[F[_]: Console](name: String, num: Int, guess: Int): F[Unit] =
if (num == guess) putStrLn(ConsoleOut.YouGuessedRight(name))
else putStrLn(ConsoleOut.YouGuessedWrong(name, num))
def checkContinue[F[_]: Program: Console](name: String): F[Boolean] =
for {
_ <- putStrLn(ConsoleOut.DoYouWantToContinue(name))
choice <- getStrLn.map(_.toLowerCase)
cont <- if (choice == "y") finish(true)
else if (choice == "n") finish(false)
else checkContinue(name)
} yield cont
def gameLoop[F[_]: Program: Console: Random](name: String): F[Unit] =
for {
num <- nextInt(5).map(_ + 1)
_ <- putStrLn(ConsoleOut.PleaseGuess(name))
guess <- getStrLn
_ <- parseInt(guess).fold(
putStrLn(ConsoleOut.ThatIsNotValid(name))
)((guess: Int) => checkAnswer(name, num, guess))
cont <- checkContinue(name)
_ <- if (cont) gameLoop(name) else finish(())
} yield ()
def main[F[_]: Program: Console: Random]: F[Unit] =
for {
_ <- putStrLn(ConsoleOut.WhatIsYourName)
name <- getStrLn
_ <- putStrLn(ConsoleOut.WelcomeToGame(name))
_ <- gameLoop(name)
} yield ()
def mainIO: IO[Unit] = main[IO]
def mainTestIO: TestIO[Unit] = main[TestIO]
val TestExample = TestData(
input = "john" :: "1" :: "n" :: Nil,
output = Nil,
nums = 0 :: Nil)
}
@abdheshkumar

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abdheshkumar commented Jul 13, 2018

This is a really nice and well declarative programming. Thanks, @jdegoes

@pivovarit

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pivovarit commented Aug 4, 2018

You should start a Twitch channel ;)

@GMadorell

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GMadorell commented Sep 4, 2018

This has been one of the best explanations of FP I've ever seen, congratulations! Would be nice to add some more complex stuff here :P. Something like database access, how to compose different queries to make them transactional, etc.

@vasily802

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vasily802 commented Mar 18, 2019

Thanks for a great talk, really inspiring and educating!
Question: how to run the snippet correctly?
$ scalac fpmax.scala compiles everything to a new folder 'fpmax', but then when I run $ cd fpmax && scala App0, I get:
Exception in thread "main" java.lang.NoClassDefFoundError: App0 (wrong name: fpmax/App0)

@halyph

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halyph commented Apr 3, 2019

@vasily802
it should be like this

def main(args: Array[String]): Unit = {
    mainIO
  }

but there is no output.

@jdegoes, could you please provide as a hint how to run these apps (e.g. App1 or App2)?

@kell18

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kell18 commented Apr 7, 2019

@halyph good try, but to get actual results you need to evaluate an IO - via IO.unsafeRun() method.
It's so-called an "end of the world" when we're interpreting our pure IO values into real-world side effects.

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kell18 commented Apr 7, 2019

@vasily802 it's a JVM specific:
To run it like so you need to compile classes to separate inner folder with -d option for scalac. So it'd be:

  1. mkdir classes
  2. scalac -d classes fpmax.scala
  3. scala -cp classes fpmax.App0
    Or you can simply go one directory up after running scalac and provide it as classpath folder (-cp) instead of classes.

In general, it's simpler to use Intellij for all this stuff :)

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