Functional reactive programming

• Reactive programming:

  Reacting to sequences of events that happen in time.

• Functional view:

  Aggregating an event sequence into a signal.

  Signal is a value that changes over time.

  Therefore, we can create a new signal based on existing one instead of changing a mutable state.

Basic signal operations

• obtain the value of the signal at the current time.

  mouseMove() // It returns the current position of the mouse.
  

• create a signal based on other signals.

  For example, we want to create a signal that indicates if the mouse is in the rectangle defined by LL and UR.

  def isInRectangle(LL: Position, UR: Position): Signal[Boolean] =
    Signal {
      val pos = mouseMove() // get mouse's current position
      LL <= pos && pos <= UR
    }
  

  To use it:

  val s = isInRectangle(L, R)
  s() // the current value
  s() // It may return a different value than the previous one.
  

Constant signal

val s3 = Signal(3) // It's a signal than always returns 3.

Signal variable

val x = Signal.Var[Int](0) // It's a signal than always returns 0.

x() = 3 // The signal is updated. Now, it always returns 3.