embracing-nondeterminism-code/src/main/scala/green/thisfieldwas/embracingnondeterminism/data/Either.scala

package green.thisfieldwas.embracingnondeterminism.data

/** The `Either` class encodes a dimension contextualized by "Either" the
  * "Right" term that you want, or you're "Left" with the term you don't want.
  * This context is frequently used to model success or failure, the specific
  * instance of which is unknown at runtime without specific knowledge of the
  * inputs to the operation producing the context.
  *
  * The presence of the "Right" term `A` is the desired case of this context. If
  * the "Left" term `X` is present, then the context is in the undesired case.
  *
  * @tparam X
  *   The "Left" term, or undesired case.
  * @tparam A
  *   The "Right" term, or desired case.
  */
sealed trait Either[+X, +A] {

  /** Gets the instance of the "Left" term if it exists.
    *
    * @return
    *   The "Left" instance.
    * @throws NoSuchElementException
    *   if the context is a "Right".
    */
  def left: X

  /** Gets the instance of the "Right" term if it exists.
    *
    * @return
    *   The "Right" instance.
    * @throws NoSuchElementException
    *   if the context is a "Left".
    */
  def right: A

  /** Gets whether this context is a "Left".
    *
    * @return
    *   True if left.
    */
  def isLeft: Boolean = false

  /** Gets whether this context is a "Right".
    *
    * @return
    *   True if right.
    */
  def isRight: Boolean = false
}

/** Encodes the "Left" or undesired case of the context.
  *
  * @param left
  *   The instance of the "Left" term.
  * @tparam X
  *   The "Left" term, or undesired case.
  * @tparam A
  *   The "Right" term, or desired case.
  */
case class Left[+X, +A](left: X) extends Either[X, A] {

  def right: A = throw new NoSuchElementException("Left.right")

  override def isLeft: Boolean = true
}

/** Encodes the "Right" or desired case of the context.
  *
  * @param right
  *   The instance of the "Right" case.
  * @tparam X
  *   The "Left" term, or undesired case.
  * @tparam A
  *   The "Right" term, or desired case.
  */
case class Right[+X, +A](right: A) extends Either[X, A] {

  def left: X = throw new NoSuchElementException("Right.left")

  override def isRight: Boolean = true
}

object Either {

  /** A smart constructor to create a "Left" typed specifically as an "Either".
    *
    * @param x
    *   The "Left" instance.
    * @tparam X
    *   The "Left" term, or undesired case.
    * @tparam A
    *   The "Right" term, or desired case.
    * @return
    *   An "Either" in the undesired case.
    */
  def left[X, A](x: X): Either[X, A] = Left(x)

  /** A smart constructor to create a "Right" typed specifically as an "Either".
    *
    * @param a
    *   The "Right" instance.
    * @tparam X
    *   The "Left" term, or undesired case.
    * @tparam A
    *   The "Right" term, or desired case.
    * @return
    *   An "Either" in the desired case.
    */
  def right[X, A](a: A): Either[X, A] = Right(a)

  implicit def eitherFunctor[X]: Functor[Either[X, *]] = new Functor[Either[X, *]] {

    /** Given a structure `F[A]` and function `f: A => B`, if the structure is
      * in the desired case, then apply the function such that `F[A] => F[B]`.
      * If the structure `F[A]` is in the undesired case, then propagate the
      * undesired case as `F[B]`.
      *
      * @param fa
      *   The instance of the structure to apply the function to.
      * @param f
      *   The function to apply, if the structure is in the desired case.
      * @tparam A
      *   The type of instances contained within the structure.
      * @tparam B
      *   The type to map the instances into.
      * @return
      *   The resulting structure.
      */
    override def map[A, B](fa: Either[X, A])(f: A => B): Either[X, B] = fa match {
      case Left(x)  => Left(x)
      case Right(a) => Right(f(a))
    }
  }
}