Merge pull request #4638 from danicheg/either-syntax

Add `EitherOps#leftMapOrKeep` and `EitherOps#leftFlatMapOrKeep`
typelevel · Jul 23, 2024 · aee3702 · aee3702
2 parents b7d6552 + 1ccfee3
commit aee3702
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diff --git a/core/src/main/scala/cats/syntax/either.scala b/core/src/main/scala/cats/syntax/either.scala
@@ -194,6 +194,12 @@ final class EitherOps[A, B](private val eab: Either[A, B]) extends AnyVal {
       case r @ Right(_) => EitherUtil.leftCast(r)
     }
 
+  def leftMapOrKeep[AA >: A](pf: PartialFunction[A, AA]): Either[AA, B] =
+    eab match {
+      case Left(a)        => Left(pf.applyOrElse(a, identity[AA]))
+      case r: Right[A, B] => r
+    }
+
   @deprecated("Included in the standard library", "2.1.0-RC1")
   private[syntax] def flatMap[AA >: A, D](f: B => Either[AA, D]): Either[AA, D] =
     eab match {
@@ -207,6 +213,12 @@ final class EitherOps[A, B](private val eab: Either[A, B]) extends AnyVal {
       case r @ Right(_) => EitherUtil.leftCast(r)
     }
 
+  def leftFlatMapOrKeep[AA >: A, BB >: B](pfa: PartialFunction[A, Either[AA, BB]]): Either[AA, BB] =
+    eab match {
+      case l @ Left(a)    => pfa.applyOrElse(a, (_: A) => l)
+      case r: Right[A, B] => r
+    }
+
   def compare[AA >: A, BB >: B](that: Either[AA, BB])(implicit AA: Order[AA], BB: Order[BB]): Int =
     eab match {
       case Left(a1) =>

diff --git a/tests/shared/src/test/scala/cats/tests/EitherSuite.scala b/tests/shared/src/test/scala/cats/tests/EitherSuite.scala
@@ -415,6 +415,25 @@ class EitherSuite extends CatsSuite {
     }
   }
 
+  test("leftFlatMapOrKeep consistent with leftMapOrKeep") {
+    forAll { (either: Either[String, Int], pf: PartialFunction[String, String]) =>
+      val liftedPF: PartialFunction[String, Either[String, Int]] = { case a =>
+        Either.left[String, Int](pf.applyOrElse(a, identity[String]))
+      }
+      assert(either.leftFlatMapOrKeep(liftedPF) === either.leftMapOrKeep(pf))
+    }
+  }
+
+  test("leftFlatMapOrKeep consistent with swap and then flatMapOrKeep") {
+    import cats.syntax.monad._
+
+    forAll { (either: Either[String, Int], pf: PartialFunction[String, Either[String, Int]]) =>
+      assert(either.leftFlatMapOrKeep(pf) === either.swap.flatMapOrKeep { case a =>
+        pf.applyOrElse(a, (_: String) => either).swap
+      }.swap)
+    }
+  }
+
   test("raiseWhen raises when true") {
     val result = Either.raiseWhen(true)("ok")
     assert(result === Left("ok"))