Expose Left/Right cast syntax - casting naming indicates which type p…

…arameter to cast

core/src/main/scala/cats/data/EitherT.scala

@@ -3,7 +3,6 @@ package data
 
 import cats.functor.Bifunctor
 import cats.instances.either._
-import cats.syntax.EitherUtil
 import cats.syntax.either._
 
 /**
@@ -73,7 +72,7 @@ final case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
 
   def flatMap[D](f: B => EitherT[F, A, D])(implicit F: Monad[F]): EitherT[F, A, D] =
     EitherT(F.flatMap(value) {
-      case l @ Left(_) => F.pure(EitherUtil.leftCast(l))
+      case l @ Left(_) => F.pure(l.rightCast)
       case Right(b) => f(b).value
     })
 

core/src/main/scala/cats/instances/either.scala

@@ -55,7 +55,7 @@ trait EitherInstances extends EitherInstances1 {
 
       override def map2Eval[B, C, Z](fb: Either[A, B], fc: Eval[Either[A, C]])(f: (B, C) => Z): Eval[Either[A, Z]] =
         fb match {
-          case l @ Left(_) => Now(EitherUtil.leftCast(l))
+          case l @ Left(_) => Now(EitherUtil.rightCast(l))
           case Right(b) => fc.map(_.right.map(f(b, _)))
         }
 

core/src/main/scala/cats/syntax/either.scala

@@ -9,6 +9,10 @@ trait EitherSyntax {
   implicit def catsSyntaxEither[A, B](eab: Either[A, B]): EitherOps[A, B] = new EitherOps(eab)
 
   implicit def catsSyntaxEitherObject(either: Either.type): EitherObjectOps = new EitherObjectOps(either) // scalastyle:off ensure.single.space.after.token
+
+  implicit def catsSyntaxLeft[A, B](left: Left[A, B]): LeftOps[A, B] = new LeftOps(left)
+
+  implicit def catsSyntaxRight[A, B](right: Right[A, B]): RightOps[A, B] = new RightOps(right)
 }
 
 final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
@@ -24,7 +28,7 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
 
   def orElse[C](fallback: => Either[C, B]): Either[C, B] = eab match {
     case Left(_)      => fallback
-    case r @ Right(_) => EitherUtil.rightCast(r)
+    case r @ Right(_) => EitherUtil.leftCast(r)
   }
 
   def recover(pf: PartialFunction[A, B]): Either[A, B] = eab match {
@@ -103,23 +107,23 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
   }
 
   def map[C](f: B => C): Either[A, C] = eab match {
-    case l @ Left(_) => EitherUtil.leftCast(l)
+    case l @ Left(_) => EitherUtil.rightCast(l)
     case Right(b)    => Right(f(b))
   }
 
   def map2Eval[C, Z](fc: Eval[Either[A, C]])(f: (B, C) => Z): Eval[Either[A, Z]] =
     eab match {
-      case l @ Left(_) => Now(EitherUtil.leftCast(l))
+      case l @ Left(_) => Now(EitherUtil.rightCast(l))
       case Right(b)    => fc.map(either => new EitherOps(either).map(f(b, _)))
     }
 
   def leftMap[C](f: A => C): Either[C, B] = eab match {
     case Left(a)      => Left(f(a))
-    case r @ Right(_) => EitherUtil.rightCast(r)
+    case r @ Right(_) => EitherUtil.leftCast(r)
   }
 
   def flatMap[D](f: B => Either[A, D]): Either[A, D] = eab match {
-    case l @ Left(_) => EitherUtil.leftCast(l)
+    case l @ Left(_) => EitherUtil.rightCast(l)
     case Right(b)    => f(b)
   }
 
@@ -163,7 +167,7 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
   }
 
   def traverse[F[_], D](f: B => F[D])(implicit F: Applicative[F]): F[Either[A, D]] = eab match {
-    case l @ Left(_) => F.pure(EitherUtil.leftCast(l))
+    case l @ Left(_) => F.pure(EitherUtil.rightCast(l))
     case Right(b)    => F.map(f(b))(Right(_))
   }
 
@@ -311,10 +315,19 @@ final class CatchOnlyPartiallyApplied[T] private[syntax] {
     }
 }
 
+final class LeftOps[A, B](val left: Left[A, B]) extends AnyVal {
+  /** Cast the right type parameter of the `Left`. */
+  def rightCast[C]: Either[A, C] = left.asInstanceOf[Either[A, C]]
+}
+
+final class RightOps[A, B](val right: Right[A, B]) extends AnyVal {
+  /** Cast the left type parameter of the `Right`. */
+  def leftCast[C]: Either[C, B] = right.asInstanceOf[Either[C, B]]
+}
+
+/** Convenience methods to use `Either` syntax inside `Either` syntax definitions. */
 private[cats] object EitherUtil {
-  /** Cast the *right* type parameter of a `Left`. */
-  def leftCast[A, B, C](l: Left[A, B]): Either[A, C] = l.asInstanceOf[Left[A, C]]
+  def leftCast[A, B, C](r: Right[A, B]): Either[C, B] = new RightOps(r).leftCast[C]
 
-  /** Cast the *left* type parameter of a `Right` */
-  def rightCast[A, B, C](r: Right[A, B]): Either[C, B] = r.asInstanceOf[Right[C, B]]
+  def rightCast[A, B, C](l: Left[A, B]): Either[A, C] = new LeftOps(l).rightCast[C]
 }

tests/src/test/scala/cats/tests/EitherTests.scala

@@ -44,6 +44,18 @@ class EitherTests extends CatsSuite {
   checkAll("Either[Int, String]", orderLaws.partialOrder(partialOrder))
   checkAll("Either[Int, String]", orderLaws.order(order))
 
+  test("Left/Right cast syntax") {
+    forAll { (e: Either[Int, String]) =>
+      e match {
+        case l @ Left(_)  =>
+          l.rightCast[Double]: Either[Int, Double]
+          assert(true)
+        case r @ Right(_) =>
+          r.leftCast[List[Byte]]: Either[List[Byte], String]
+          assert(true)
+      }
+    }
+  }
 
   test("implicit instances resolve specifically") {
     val eq = catsStdEqForEither[Int, String]