Question

In Scala I have the following trait:

trait Reactive {

  type Condition = () => Boolean
  type Reaction = (Condition) => Unit

  var reactions = Map[Condition, Reaction]()

  def addReaction(c: Condition, r: Reaction) { reactions += (c -> r) }

  def addReactions(rs: List[Tuple2[Condition, Reaction]]) {
    for(r <- rs) r match { case(condition, reaction) => addReaction(condition, reaction) }
  }


  def updateReactive() {
    for(reaction <- reactions) reaction match {
      case (c, r) => r(c)
    }
  } 
}

then when I am trying to call the addReactions() method:

addReactions(List(
(() => UserInput.lastInput.isKeyHit('g'), () => (is: Boolean) => activateDevice(0))
))

I get the following error message on the second argument of the tuple:

• type mismatch; found : () => Boolean => Unit required: () => Boolean => Unit

I do not understand why. What do I have to do in order for the Reactive trait to store a set of boolean-conditioned functions that should be executed later, if their condition function returns true. Maybe I am going a round way? Is there any simpler approach?

Answer 1

Try writing this instead:

addReactions(List(
  (() => UserInput.lastInput.isKeyHit('g'), condition => activateDevice(0))
))

() => (is: Boolean) => activateDevice(0) is a function with no parameters that returns a function from Boolean to Unit. condition => activateDevice(0) is a function with a single parameter called condition (whose type, () => Boolean, is inferred) and returns Unit.