Functional APIs design pattern in action

functional-apis.scala

/* 1. APIs */
trait IO[P[_]] {
  def read: P[String]
  def write(msg: String): P[Unit]
}

object IO {
  object syntax {
    def read[P[_]](implicit IO: IO[P]) = IO.read
    def write[P[_]](msg: String)(implicit IO: IO[P]) = IO.write(msg)
  }
}

trait Monad[P[_]] {
  def flatMap[A, B](pa: P[A])(f: A => P[B]): P[B]
  def pure[A](a: A): P[A]
}

object Monad {
  object syntax {
    implicit class MonadOps[P[_], A](pa: P[A])(implicit M: Monad[P]) {
      def flatMap[B](f: A => P[B]) = M.flatMap(pa)(f)
      def map[B](f: A => B) = M.flatMap(pa)(f andThen M.pure)

      // aliases
      def >>=[B](f: A => P[B]) = M.flatMap(pa)(f)
      def >>[B](next: P[B]) = M.flatMap(pa)(_ => next)
    }
  }
}

/* 1.1 Possible interpreters */

// Console
type Id[A] = A
type SyncIO = IO[Id]

// Async
import scala.concurrent.Future
type AsyncIO = IO[Future]

// Pure state transformations
case class IOState(reads: List[String], writes: List[String])
type State[A] = IOState => (IOState, A)
type StateIO = IO[State]

/* 2. Programs */

/* 2.1 IO programs */
def hello[P[_]](IO: IO[P]): P[Unit] =
  IO.write("Hello, world!")

def sayWhat[P[_]](IO: IO[P]): P[String] =
  IO.read

/* 2.2 IO Monadic programs + syntax */
import IO.syntax._
import Monad.syntax._

def helloSayWhat[P[_]: Monad: IO]: P[String] =
  write("Hello, say something:") >>
  read

def echo[P[_]: Monad: IO]: P[Unit] =
  read >>= write[P]

def echo2[P[_]: Monad: IO]: P[String] = for {
  msg <- read
  _ <- write(msg)
} yield msg

/* 3. Instances */

implicit object ioTerminal extends IO[Id] {
  def write(msg: String) = println(msg)
  def read() = scala.io.StdIn.readLine
}

implicit object monadId extends Monad[Id] {
  def flatMap[A, B](pa: Id[A])(f: A => Id[B]): Id[B] = f(pa)
  def pure[A](a: A): Id[A] = a
}

/* 4. Composition */

def helloConsole(): Unit = hello[Id](ioTerminal)

def sayWhatConsole(): String = sayWhat(ioTerminal)

def helloSayWhatConsole() = helloSayWhat(monadId, ioTerminal)

def echoConsole() = echo[Id]

def echo2Console() = echo2[Id]