Scala example source code file (Monads.scala)

This example Scala source code file (Monads.scala) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Scala tags/keywords

application, monads, monads, monadtc, monadtc, none, none, option, option, optioninstofmonad, optionmonad, some

The Scala Monads.scala source code

package examples.tcpoly.monad;

trait Monads {
  /**
   * class Monad m where
   *   (>>=)  :: m a -> (a -> m b) -> m b
   *   return :: a -> m a
   *
   * MonadTC encodes the above Haskell type class, 
   * an instance of MonadTC corresponds to a method dictionary.
   * (see http://lampwww.epfl.ch/~odersky/talks/wg2.8-boston06.pdf)
   *
   * Note that the identity (`this') of the method dictionary does not really correspond
   * to the instance of m[x] (`self') that is `wrapped': e.g., unit does not use `self' (which 
   * corresponds to the argument of the implicit conversion that encodes an instance of this type class)
   */
   // Option =:= [x] => Option[x] <: [x] => Any
//  trait MonadTC[m <: [x] => Any, a] {  
    // MonadTC[m[x], a] x is a type parameter too -- should not write e.g., m[Int] here
  trait MonadTC[m[x], a] {      
    def unit[a](orig: a): m[a]

    // >>='s first argument comes from the implicit definition constructing this "method dictionary"
    def >>=[b](fun: a => m[b]): m[b]
  }
}

/**
 * instance Monad Maybe where
 *   (Just x) >>= k = k x
 *   Nothing  >>= _ = Nothing
 */
trait OptionMonad extends Monads {
  // this implicit method encodes the Monad type class instance for Option
  implicit def OptionInstOfMonad[a](self: Option[a]): MonadTC[Option, a] 
    = new MonadTC[Option, a] {
        def unit[a](orig: a) = Some(orig)
        def >>=[b](fun: a => Option[b]): Option[b] = self match {
          case Some(x) => fun(x)
          case None => None
        }
      }
}

object main extends OptionMonad with Application {
  Console.println(Some("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") >>= (x => Some(x.length)))
}


/* 
trait MonadTC[m[x], a] requires m[x] {      
  def unit[a](orig: a): m[a]

  // >>='s first argument comes from the implicit definition constructing this "method dictionary"
  def >>=[b](fun: a => m[b]): m[b]
}

abstract class OptionIsMonad[t[x] <: Option[x], a] implicit extends MonadTC[t, a] {
  def unit[a](orig: a) = Some(orig) // TODO: problematic.. is a meta-member: not invoked on this
}

class SomeIsMonad[a] extends OptionIsMonad[Some, a] {
  def >>=[b](fun: a => Option[b]): Option[b] = fun(x)
}

class NoneIsMonad[a] extends OptionIsMonad[None, a] {
  def >>=[b](fun: a => Option[b]): Option[b] = None
}
*/