Add Cofree.ana (Cofree.unfold but with fused map)

typelevel · Jul 16, 2018 · 14b03e0 · 14b03e0
1 parent 186dbab
commit 14b03e0
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Showing 2 changed files with 11 additions and 2 deletions.
diff --git a/free/src/main/scala/cats/free/Cofree.scala b/free/src/main/scala/cats/free/Cofree.scala
@@ -61,7 +61,11 @@ object Cofree extends CofreeInstances {
 
   /** Cofree anamorphism, lazily evaluated. */
   def unfold[F[_], A](a: A)(f: A => F[A])(implicit F: Functor[F]): Cofree[F, A] =
-    Cofree[F, A](a, Eval.later(F.map(f(a))(unfold(_)(f))))
+    ana(a)(f, identity)
+
+  /** Cofree anamorphism with a fused map, lazily evaluated. */
+  def ana[F[_], A, C](a: A)(coalg: A => F[A], f: A => C)(implicit F: Functor[F]): Cofree[F, C] =
+    Cofree[F, C](f(a), Eval.later(F.map(coalg(a))(ana(_)(coalg, f))))
 
   /**
     * A stack-safe algebraic recursive fold out of the cofree comonad.
@@ -148,4 +152,3 @@ private trait CofreeTraverse[F[_]] extends Traverse[Cofree[F, ?]] with CofreeRed
     G.map2(f(fa.head), F.traverse(fa.tailForced)(traverse(_)(f)))((h, t) => Cofree[F, B](h, Eval.now(t)))
 
 }
-
diff --git a/free/src/test/scala/cats/free/CofreeSuite.scala b/free/src/test/scala/cats/free/CofreeSuite.scala
@@ -32,6 +32,12 @@ class CofreeSuite extends CatsSuite {
     cofNelToNel(unfoldedHundred) should ===(nelUnfoldedHundred)
   }
 
+  test("Cofree.ana") {
+    val unfoldedHundred: CofreeNel[String] = Cofree.ana[Option, Int, String](0)(i => if (i == 100) None else Some(i + 1), _.toString)
+    val nelUnfoldedHundred: NonEmptyList[String] = NonEmptyList.fromListUnsafe(List.tabulate(101)(_.toString))
+    cofNelToNel(unfoldedHundred) should ===(nelUnfoldedHundred)
+  }
+
   test("Cofree.tailForced") {
     val spooky = new Spooky
     val incrementor =