Expose Left/Right cast syntax

typelevel · Aug 17, 2016 · e38ec8d · e38ec8d
1 parent eedd057
commit e38ec8d
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Showing 3 changed files with 26 additions and 6 deletions.
diff --git a/core/src/main/scala/cats/data/EitherT.scala b/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
     })
 

diff --git a/core/src/main/scala/cats/syntax/either.scala 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 {
@@ -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](l: Left[A, B]): Either[A, C] = new LeftOps(l).rightCast[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](r: Right[A, B]): Either[C, B] = new RightOps(r).leftCast[C]
 }
diff --git a/tests/src/test/scala/cats/tests/EitherTests.scala b/tests/src/test/scala/cats/tests/EitherTests.scala
@@ -44,6 +44,14 @@ class EitherTests extends CatsSuite {
   checkAll("Either[Int, String]", orderLaws.partialOrder(partialOrder))
   checkAll("Either[Int, String]", orderLaws.order(order))
 
+  test("Left/Right syntax") {
+    forAll { (e: Either[Int, String]) =>
+      e match {
+        case l @ Left(_)  => l.rightCast[Double].isLeft should === (true)
+        case r @ Right(_) => r.leftCast[Int].isRight should === (true)
+      }
+    }
+  }
 
   test("implicit instances resolve specifically") {
     val eq = catsStdEqForEither[Int, String]