backported typelevel#2742 Added separateFoldable

gagandeepkalra · Mar 3, 2020 · 1a70a19 · 1a70a19
1 parent 29efba5
commit 1a70a19
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diff --git a/core/src/main/scala/cats/syntax/alternative.scala b/core/src/main/scala/cats/syntax/alternative.scala
@@ -49,6 +49,30 @@ final class SeparateOps[F[_], G[_, _], A, B](private val fgab: F[G[A, B]]) exten
                F: Monad[F],
                A: Alternative[F],
                G: Bifoldable[G]): (F[A], F[B]) = A.separate[G, A, B](fgab)
+
+  /**
+   * Separate the inner foldable values into the "lefts" and "rights".
+   * A variant of [[separate]] that is specialized
+   * for Fs that have Foldable instances
+   * which allows for a single-pass implementation
+   * (as opposed to {{{separate}}} which is 2-pass).
+   *
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> val l: List[Either[String, Int]] = List(Right(1), Left("error"))
+   * scala> l.separateFoldable
+   * res0: (List[String], List[Int]) = (List(error),List(1))
+   * }}}
+   */
+  def separateFoldable(implicit AA: Alternative[F], G: Bifoldable[G], FF: Foldable[F]): (F[A], F[B]) =
+      FF.foldLeft(fgab, (AA.empty[A], AA.empty[B])) {
+        case (mamb, gab) =>
+          G.bifoldLeft(gab, mamb)(
+            (t, a) => (AA.combineK(t._1, AA.pure(a)), t._2),
+            (t, b) => (t._1, AA.combineK(t._2, AA.pure(b)))
+          )
+      }
 }
 
 final class GuardOps(private val condition: Boolean) extends AnyVal {

diff --git a/tests/src/test/scala/cats/tests/AlternativeSuite.scala b/tests/src/test/scala/cats/tests/AlternativeSuite.scala
@@ -20,6 +20,16 @@ class AlternativeSuite extends CatsSuite {
     }
   }
 
+  test("separateFoldable") {
+    forAll { (list: List[Either[Int, String]]) =>
+      val ints = list.collect { case Left(i)     => i }
+      val strings = list.collect { case Right(s) => s }
+      val expected = (ints, strings)
+
+      list.separateFoldable should ===(expected)
+    }
+  }
+
   test("guard") {
     assert(Alternative[Option].guard(true).isDefined)
     assert(Alternative[Option].guard(false).isEmpty)

diff --git a/tests/src/test/scala/cats/tests/SyntaxSuite.scala b/tests/src/test/scala/cats/tests/SyntaxSuite.scala
@@ -370,6 +370,11 @@ object SyntaxSuite
     val fafb = fhab.separate
   }
 
+  def testAlternativeFoldable[F[_]: Alternative: Foldable, G[_]: Foldable, H[_, _]: Bifoldable, A, B]: Unit = {
+    val fhab = mock[F[H[A, B]]]
+    val fafb = fhab.separateFoldable
+  }
+
   def testApplicative[F[_]: Applicative, A]: Unit = {
     val a = mock[A]
     val fa = a.pure[F]