Add Either#toEitherNel extension and friends (#2475)

* Add Either#toEitherNel extension

* Add EitherObject.{leftNel, rightNel}, Ior#toIorNel

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

@@ -49,6 +49,7 @@ sealed abstract class Ior[+A, +B] extends Product with Serializable {
   final def pad: (Option[A], Option[B]) = fold(a => (Some(a), None), b => (None, Some(b)), (a, b) => (Some(a), Some(b)))
   final def unwrap: Either[Either[A, B], (A, B)] = fold(a => Left(Left(a)), b => Left(Right(b)), (a, b) => Right((a, b)))
 
+  final def toIorNel[AA >: A]: IorNel[AA, B] = leftMap(NonEmptyList.one)
   final def toEither: Either[A, B] = fold(Left(_), Right(_), (_, b) => Right(b))
   final def toValidated: Validated[A, B] = fold(Invalid(_), Valid(_), (_, b) => Valid(b))
   final def toOption: Option[B] = right

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

@@ -267,6 +267,8 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
    */
   def toEitherT[F[_]: Applicative]: EitherT[F, A, B] = EitherT.fromEither(eab)
 
+  def toEitherNel[AA >: A]: EitherNel[AA, B] = leftMap(NonEmptyList.one)
+
   def raiseOrPure[F[_]](implicit ev: ApplicativeError[F, A]): F[B] =
     ev.fromEither(eab)
 
@@ -290,6 +292,10 @@ final class EitherObjectOps(val either: Either.type) extends AnyVal { // scalast
 
   def right[A, B](b: B): Either[A, B] = Right(b)
 
+  def leftNel[A, B](a: A): EitherNel[A, B] = Left(NonEmptyList.one(a))
+
+  def rightNel[A, B](b: B): EitherNel[A, B] = Right(b)
+
   /**
    * Evaluates the specified block, catching exceptions of the specified type and returning them on the left side of
    * the resulting `Either`. Uncaught exceptions are propagated.

tests/src/test/scala/cats/tests/EitherSuite.scala

@@ -1,7 +1,7 @@
 package cats
 package tests
 
-import cats.data.{ EitherT, Validated }
+import cats.data.{ EitherT, Validated, NonEmptyList }
 import cats.laws.discipline._
 import cats.kernel.laws.discipline.{MonoidTests, SemigroupTests, OrderTests, PartialOrderTests, EqTests}
 import scala.util.Try
@@ -109,6 +109,18 @@ class EitherSuite extends CatsSuite {
     }
   }
 
+  test("leftNel is consistent with left(NEL)") {
+    forAll { s: String =>
+      Either.leftNel[String, Int](s) should === (Either.left[NonEmptyList[String], Int](NonEmptyList.one(s)))
+    }
+  }
+
+  test("rightNel is consistent with right") {
+    forAll { i: Int =>
+      Either.rightNel[String, Int](i) should === (Either.right[NonEmptyList[String], Int](i))
+    }
+  }
+
   test("double swap is identity") {
     forAll { (x: Either[Int, String]) =>
       x.swap.swap should === (x)
@@ -268,6 +280,16 @@ class EitherSuite extends CatsSuite {
     either.show should === ("Left(string)")
   }
 
+  test("toEitherNel Left") {
+    val either = Either.left[String, Int]("oops")
+    either.toEitherNel should === (Either.left[NonEmptyList[String], Int](NonEmptyList.one("oops")))
+  }
+
+  test("toEitherNel Right") {
+    val either = Either.right[String, Int](42)
+    either.toEitherNel should === (Either.right[NonEmptyList[String], Int](42))
+  }
+
   test("ap consistent with Applicative") {
     val fab = implicitly[Applicative[Either[String, ?]]]
     forAll { (fa: Either[String, Int],

tests/src/test/scala/cats/tests/IorSuite.scala

@@ -215,6 +215,21 @@ class IorSuite extends CatsSuite {
     }
   }
 
+  test("toIorNel Left") {
+    val ior = Ior.left[String, Int]("oops")
+    ior.toIorNel should === (Ior.left[NonEmptyList[String], Int](NonEmptyList.one("oops")))
+  }
+
+  test("toIorNel Right") {
+    val ior = Ior.right[String, Int](42)
+    ior.toIorNel should === (Ior.right[NonEmptyList[String], Int](42))
+  }
+
+  test("toIorNel Both") {
+    val ior = Ior.both[String, Int]("oops", 42)
+    ior.toIorNel should === (Ior.both[NonEmptyList[String], Int](NonEmptyList.one("oops"), 42))
+  }
+
   test("leftNel") {
     forAll { (x: String) =>
       Ior.leftNel(x).left should === (Some(NonEmptyList.one(x)))