Merge pull request #533 from ceedubs/foldable-toStreaming

Add Foldable.toStreaming
typelevel · Sep 15, 2015 · e792851 · e792851
2 parents 14b5772 + 5cdeabb
commit e792851
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diff --git a/core/src/main/scala/cats/Foldable.scala b/core/src/main/scala/cats/Foldable.scala
@@ -1,5 +1,7 @@
 package cats
 
+import cats.data.Streaming
+
 import scala.collection.mutable
 import simulacrum.typeclass
 
@@ -206,6 +208,11 @@ import simulacrum.typeclass
       val F = self
       val G = ev
     }
+
+  def toStreaming[A](fa: F[A]): Streaming[A] =
+    foldRight(fa, Now(Streaming.empty[A])){ (a, ls) =>
+      Now(Streaming.cons(a, ls))
+    }.value
 }
 
 /**

diff --git a/core/src/main/scala/cats/data/Streaming.scala b/core/src/main/scala/cats/data/Streaming.scala
@@ -764,6 +764,9 @@ object Streaming extends StreamingInstances {
     loop(Empty(), as.reverse)
   }
 
+  def fromFoldable[F[_], A](fa: F[A])(implicit F: Foldable[F]): Streaming[A] =
+    F.toStreaming(fa)
+
   /**
    * Create a stream from an iterable.
    *
@@ -919,6 +922,9 @@ trait StreamingInstances extends StreamingInstances1 {
 
       override def isEmpty[A](fa: Streaming[A]): Boolean =
         fa.isEmpty
+
+      override def toStreaming[A](fa: Streaming[A]): Streaming[A] =
+        fa
     }
 
   implicit def streamOrder[A: Order]: Order[Streaming[A]] =

diff --git a/core/src/main/scala/cats/std/list.scala b/core/src/main/scala/cats/std/list.scala
@@ -4,6 +4,7 @@ package std
 import algebra.Eq
 import algebra.std.{ListMonoid, ListOrder}
 
+import cats.data.Streaming
 import cats.syntax.order._
 
 import scala.annotation.tailrec
@@ -62,6 +63,9 @@ trait ListInstances extends ListInstances1 {
         fa.forall(p)
 
       override def isEmpty[A](fa: List[A]): Boolean = fa.isEmpty
+
+      override def toStreaming[A](fa: List[A]): Streaming[A] =
+        Streaming.fromList(fa)
     }
 
   implicit def listAlgebra[A]: Monoid[List[A]] = new ListMonoid[A]

diff --git a/core/src/main/scala/cats/std/vector.scala b/core/src/main/scala/cats/std/vector.scala
@@ -1,6 +1,8 @@
 package cats
 package std
 
+import cats.data.Streaming
+
 trait VectorInstances {
   implicit val vectorInstance: Traverse[Vector] with MonadCombine[Vector] =
     new Traverse[Vector] with MonadCombine[Vector] {
@@ -36,6 +38,9 @@ trait VectorInstances {
         fa.exists(p)
 
       override def isEmpty[A](fa: Vector[A]): Boolean = fa.isEmpty
+
+      override def toStreaming[A](fa: Vector[A]): Streaming[A] =
+        Streaming.fromVector(fa)
     }
 
   // TODO: eventually use algebra's instances (which will deal with

diff --git a/tests/src/test/scala/cats/tests/FoldableTests.scala b/tests/src/test/scala/cats/tests/FoldableTests.scala
@@ -36,6 +36,7 @@ abstract class FoldableCheck[F[_]: ArbitraryK: Foldable](name: String) extends C
   test("toList/isEmpty/nonEmpty") {
     forAll { (fa: F[Int]) =>
       fa.toList shouldBe iterator(fa).toList
+      fa.toStreaming.toList shouldBe iterator(fa).toList
       fa.isEmpty shouldBe iterator(fa).isEmpty
       fa.nonEmpty shouldBe iterator(fa).nonEmpty
     }
@@ -94,5 +95,8 @@ class FoldableTestsAdditional extends CatsSuite {
       if (n == 2) Now(true) else lb
     }
     assert(result.value)
+
+    // toStreaming should be lazy
+    assert(dangerous.toStreaming.take(3).toList == List(0, 1, 2))
   }
 }