equational_reasoning.scala

object EquationalReasoningBreakdown extends SafeApp {
  import scalaz.effect._

  def parse(ioa: IO[String]): IO[Int] = 
    ioa
      .flatMap { s => 
        IO(s.toInt)
      }
      .except { case NonFatal(_) => 
        IO(-1) 
      }
  
  
  
  val runc: IO[Unit] = 
    parse(IO.readStrLn)
      .map(i => s"Thanks for $i")
      .flatMap(IO.putStrLn)
  
}

// Looking at the signature of parse alone, it'stempting to conclude that the program can be refactored as follows:

object EquationalReasoningBreakdown extends SafeApp {
  import scalaz.effect._

  def parse(a: String): IO[Int] = 
    IO(s.toInt)
      .except { case NonFatal(_) => 
        IO(-1) 
      }
  
  
  
  val runc: IO[Unit] = 
    IO.readStrLn
      .flatMap(parse)
      .map(i => s"Thanks for $i")
      .flatMap(IO.putStrLn)
  
}

//These programs are equivalent, aren't they? 
// Aren't they?
//  No!
//
// In the first program, a failure in `IO.readStrLn` was handled by the error-handling in parse
// In the second, it was not