Rx.scala

package btcboard.ui

import cats.Monad
import monocle.Lens

import scala.languageFeature.higherKinds

class Rx[A[_], T, O] private(t: T, lens: Lens[T, O], observers: T => A[Unit], monad: Monad[A]) {

  def getT: T = t

  def getO: O = lens.get(t)

  def set(newValue: O): A[Rx[A, T, O]] = updateValue(lens.set(newValue)(t))

  def bind(newObserver: O => A[Unit]): Rx[A, T, O] =
    new Rx[A, T, O](t, lens, composeObservers(observers, newObserver, lens)(_), monad)

  def zoom[Z](implicit newLens: monocle.Lens[O, Z]): Rx[A, T, Z] =
    new Rx[A, T, Z](t, lens.composeLens(newLens), observers, monad)

  private def updateValue(newValue: T): A[Rx[A, T, O]] =
    monad.map[Unit, Rx[A, T, O]](observers(newValue))(_ => new Rx[A, T, O](newValue, lens, observers, monad))

  private def composeObservers(observers: T => A[Unit], newObserver: O => A[Unit], lens: Lens[T, O])(t: T): A[Unit] =
    monad.flatMap(observers(t)) { _ =>
      monad.map(newObserver(lens.get(t)))(_ => ())
    }

}

object Rx {

  def apply[A[_], T](t: T)(implicit monad: Monad[A]): Rx[A, T, T] =
    new Rx(t, Lens[T, T](identity)(t => _ => t), _ => monad.pure(()), monad)

  def apply[A[_], T, O](t: T, lens: Lens[T, O])(implicit monad: Monad[A]): Rx[A, T, O] =
    new Rx(t, lens, _ => monad.pure(()), monad)

  def apply[A[_], T, O](t: T, lens: Lens[T, O], callbacks: T => A[Unit])(implicit monad: Monad[A]): Rx[A, T, O] =
    new Rx(t, lens, callbacks, monad)

}