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Add Either.unit, use in Applicative[Either[A, ?]] #3346

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Mar 10, 2020
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Add Either.unit, use in Applicative[Either[A, ?]] #3346

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2 changes: 2 additions & 0 deletions core/src/main/scala/cats/instances/either.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -37,6 +37,8 @@ trait EitherInstances extends cats.kernel.instances.EitherInstances {
implicit def catsStdInstancesForEither[A]
: MonadError[Either[A, *], A] with Traverse[Either[A, *]] with Align[Either[A, *]] =
new MonadError[Either[A, *], A] with Traverse[Either[A, *]] with Align[Either[A, *]] {
override def unit: Either[A, Unit] = Either.unit

def pure[B](b: B): Either[A, B] = Right(b)

def flatMap[B, C](fa: Either[A, B])(f: B => Either[A, C]): Either[A, C] =
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5 changes: 5 additions & 0 deletions core/src/main/scala/cats/syntax/either.scala
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Expand Up @@ -357,6 +357,9 @@ final class EitherObjectOps(private val either: Either.type) extends AnyVal { //
case None => left[A, B](ifNone)
case Some(a) => right(a)
}

/** Cached value of `Right(())` to avoid allocations for a common case. */
def unit[A]: Either[A, Unit] = EitherUtil.unit
}

final class LeftOps[A, B](private val left: Left[A, B]) extends AnyVal {
Expand Down Expand Up @@ -456,4 +459,6 @@ private[cats] object EitherUtil {
right.asInstanceOf[Either[C, B]]
def rightCast[A, B, C](left: Left[A, B]): Either[A, C] =
left.asInstanceOf[Either[A, C]]

private[cats] val unit = Right(())
}