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

typelevel · Mar 10, 2020 · 222bd5a · 222bd5a
1 parent fc23079
commit 222bd5a
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diff --git a/core/src/main/scala/cats/instances/either.scala b/core/src/main/scala/cats/instances/either.scala
@@ -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] =

diff --git a/core/src/main/scala/cats/syntax/either.scala b/core/src/main/scala/cats/syntax/either.scala
@@ -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 {
@@ -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(())
 }