136 lines
3.7 KiB
Scala
136 lines
3.7 KiB
Scala
package green.thisfieldwas.embracingnondeterminism.data
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/** The `Either` class encodes a dimension contextualized by "Either" the
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* "Right" term that you want, or you're "Left" with the term you don't want.
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* This context is frequently used to model success or failure, the specific
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* instance of which is unknown at runtime without specific knowledge of the
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* inputs to the operation producing the context.
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*
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* The presence of the "Right" term `A` is the desired case of this context. If
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* the "Left" term `X` is present, then the context is in the undesired case.
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*
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* @tparam X
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* The "Left" term, or undesired case.
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* @tparam A
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* The "Right" term, or desired case.
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*/
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sealed trait Either[+X, +A] {
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/** Gets the instance of the "Left" term if it exists.
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*
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* @return
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* The "Left" instance.
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* @throws NoSuchElementException
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* if the context is a "Right".
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*/
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def left: X
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/** Gets the instance of the "Right" term if it exists.
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*
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* @return
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* The "Right" instance.
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* @throws NoSuchElementException
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* if the context is a "Left".
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*/
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def right: A
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/** Gets whether this context is a "Left".
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*
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* @return
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* True if left.
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*/
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def isLeft: Boolean = false
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/** Gets whether this context is a "Right".
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*
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* @return
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* True if right.
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*/
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def isRight: Boolean = false
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}
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/** Encodes the "Left" or undesired case of the context.
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*
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* @param left
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* The instance of the "Left" term.
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* @tparam X
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* The "Left" term, or undesired case.
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* @tparam A
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* The "Right" term, or desired case.
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*/
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case class Left[+X, +A](left: X) extends Either[X, A] {
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def right: A = throw new NoSuchElementException("Left.right")
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override def isLeft: Boolean = true
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}
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/** Encodes the "Right" or desired case of the context.
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*
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* @param right
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* The instance of the "Right" case.
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* @tparam X
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* The "Left" term, or undesired case.
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* @tparam A
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* The "Right" term, or desired case.
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*/
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case class Right[+X, +A](right: A) extends Either[X, A] {
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def left: X = throw new NoSuchElementException("Right.left")
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override def isRight: Boolean = true
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}
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object Either {
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/** A smart constructor to create a "Left" typed specifically as an "Either".
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*
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* @param x
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* The "Left" instance.
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* @tparam X
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* The "Left" term, or undesired case.
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* @tparam A
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* The "Right" term, or desired case.
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* @return
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* An "Either" in the undesired case.
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*/
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def left[X, A](x: X): Either[X, A] = Left(x)
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/** A smart constructor to create a "Right" typed specifically as an "Either".
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*
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* @param a
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* The "Right" instance.
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* @tparam X
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* The "Left" term, or undesired case.
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* @tparam A
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* The "Right" term, or desired case.
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* @return
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* An "Either" in the desired case.
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*/
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def right[X, A](a: A): Either[X, A] = Right(a)
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implicit def eitherFunctor[X]: Functor[Either[X, *]] = new Functor[Either[X, *]] {
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/** Given a structure `F[A]` and function `f: A => B`, if the structure is
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* in the desired case, then apply the function such that `F[A] => F[B]`.
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* If the structure `F[A]` is in the undesired case, then propagate the
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* undesired case as `F[B]`.
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*
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* @param fa
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* The instance of the structure to apply the function to.
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* @param f
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* The function to apply, if the structure is in the desired case.
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* @tparam A
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* The type of instances contained within the structure.
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* @tparam B
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* The type to map the instances into.
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* @return
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* The resulting structure.
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*/
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override def map[A, B](fa: Either[X, A])(f: A => B): Either[X, B] = fa match {
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case Left(x) => Left(x)
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case Right(a) => Right(f(a))
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}
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}
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}
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