Dotty ADTs

M.scala

import scala.io.StdIn

// Result[+T] is a Monoid with flatMap as the join fn
enum Result[+T] {
  case Success(value: T)
  case Failure(exception: Throwable)

  // bind hence a Monad
  def flatMap[U](f: T => Result[U]) =
    this match {
      case Success(value) => f(value)
      case _ => this
    }

   // makes this a Functor
  def map[U](f: T => U) =
    this match {
      // f(v) can safely blow up due to the construction of the unit
      case Success(value) => flatMap(v => unit(f(v)))
      case _ => this
    }
}

// The `=> T` is SUPER IMPORTANT here!!
def unit[T](f: => T) =
  try Result.Success(f) catch {
    case e: Throwable => Result.Failure(e)
  }

def readInt = {
  print("Enter a number: ")
  StdIn.readInt()
}

def safeReadAndAdd(prev: Int) =
  try Result.Success(prev + readInt) catch {
    case e: Throwable => Result.Failure(e)
  }

object M {
  def main(args: Array[String]) = {
    val r1 = safeReadAndAdd(0).flatMap(safeReadAndAdd)
      .flatMap(safeReadAndAdd)
      .map(_ / readInt)

    val r2 = for {
      n1 <- safeReadAndAdd(0)
      n2 <- safeReadAndAdd(n1)
      n3 <- safeReadAndAdd(n2)
    } yield n3 / readInt

    println(r1)
    println(r2)
  }
}

To compile and run:

• Install Dotty
• dotc M.scala
• dotr M