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POhow to implement this future/state concept as a monad in scala
 

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    <p>I'm trying to implement a container for a match (like in sports) result so that I can create matches between the winners of other matches. This concept is close to what a future monads is as it contains a to be defined value, and also close to a state monad as it hides state change. Being mostly a begginer on the topic I have implemented an initial version in scala that is surely improvable. I added a get method that I'm not sure was a good idea, and so far the only way to create a value would be <code>Unknown(null)</code> which is not as elegant as I'd hoped. What do you think I could do to improve this design?</p> <pre><code>case class Unknown[T](t : T) { private var value : Option[T] = Option(t) private var applicatives: List[T =&gt; Unit] = Nil def set(t: T) { if (known) { value = Option(t) applicatives.foreach(f =&gt; f(t)) applicatives = Nil } else { throw new IllegalStateException } } def get : T = value.get def apply(f: T =&gt; Unit) = value match { case Some(x) =&gt; f(x); case None =&gt; applicatives ::= f } def known = value == None } </code></pre> <p><strong>UPDATE</strong>: a usage example of the current implementation follows</p> <pre><code>case class Match(val home: Unknown[Team], val visit: Unknown[Team], val result: Unknown[(Int, Int)]) { val winner: Unknown[Team] = Unknown(null) val loser: Unknown[Team] = Unknown(null) result.apply(result =&gt; { if (result._1 &gt; result._2) { home.apply(t =&gt; winner.set(t)) visit.apply(t =&gt; loser.set(t)) } else { home.apply(t =&gt; loser.set(t)) visit.apply(t =&gt; winner.set(t)) } }) } </code></pre> <p>And a test snippet:</p> <pre><code>val definedUnplayedMatch = Match(Unknown(Team("A")), Unknown(Team("B")), Unknown(null)); val definedPlayedMatch = Match(Unknown(Team("D")), Unknown(Team("E")), Unknown((1,0))); val undefinedUnplayedMatch = Match(Unknown(null), Unknown(null), Unknown(null)); definedUnplayedMatch.winner.apply(undefinedUnplayedMatch.home.set(_)) definedPlayedMatch.winner.apply(undefinedUnplayedMatch.visit.set(_)) undefinedUnplayedMatch.result.set((3,1)) definedUnplayedMatch.result.set((2,4)) undefinedUnplayedMatch.winner.get must be equalTo(Team("B")); undefinedUnplayedMatch.loser.get must be equalTo(Team("D")); </code></pre> <p><strong>UPDATE - CURRENT IDEA :</strong> I haven't had much time to work on this because my laptop broke down, but I though it would be useful to write the monad I have so far for those who are interested:</p> <pre><code>sealed abstract class Determine[+A] { def map[B](f: A =&gt; B): Determine[B] def flatMap[B](f: A =&gt; Determine[B]): Determine[B] def filter(p: A =&gt; Boolean): Determine[A] def foreach(b: A =&gt; Unit): Unit } final case class Known[+A](value: A) extends Determine[A] { def map[B](f: A =&gt; B): Determine[B] = Known(f(value)) def flatMap[B](f: A =&gt; Determine[B]): Determine[B] = f(value) def filter(p: A =&gt; Boolean): Determine[A] = if (p(value)) this else Unknown def foreach(b: A =&gt; Unit): Unit = b(value) } final case class TBD[A](definer: () =&gt; A) extends Determine[A] { private var value: A = _ def map[B](f: A =&gt; B): Determine[B] = { def newDefiner(): B = { f(cachedDefiner()) } TBD[B](newDefiner) } def flatMap[B](f: A =&gt; Determine[B]): Determine[B] = { f(cachedDefiner()) } def filter(p: A =&gt; Boolean): Determine[A] = { if (p(cachedDefiner())) this else Unknown } def foreach(b: A =&gt; Unit): Unit = { b(cachedDefiner()) } private def cachedDefiner(): A = { if (value == null) value = definer() value } } case object Unknown extends Determine[Nothing] { def map[B](f: Nothing =&gt; B): Determine[B] = this def flatMap[B](f: Nothing =&gt; Determine[B]): Determine[B] = this def filter(p: Nothing =&gt; Boolean): Determine[Nothing] = this def foreach(b: Nothing =&gt; Unit): Unit = {} } </code></pre> <p>I got rid of the set &amp; get and now the TBD class receives instead a function that will define provide the value or null if still undefined. This idea works great for the map method, but the rest of the methods have subtle bugs.</p>
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    1. CONot worrying about implementation, can you give some examples on how you would *use* such a monad using `flatMap` or `for` comprehensions? Your implementation is currently lacking the `flatMap` method which would define the semantic on your monad.
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    2. COI have updated the question with the test usage I'm giving to it right now. I recognize that my implementation is too clumsy to take advantage of for comprehensions at the moment, basically it is a port of an observer pattern in Java that is missing scala syntactic sugar.
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    3. COIt's not very clear from your examples what you're trying to accomplish in making the Unknown[A] monad. Can you tell us a little more about the application that will use this library?
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