Add Lift{Kind,Value} instances for Stream

Author: NthPortal

build.sbt

@@ -300,9 +300,11 @@ lazy val core = crossProject(JVMPlatform, JSPlatform, NativePlatform)
       "org.scodec" %%% "scodec-bits" % "1.2.4",
       "org.typelevel" %%% "cats-core" % "2.13.0",
       "org.typelevel" %%% "cats-effect" % "3.7.0-RC1",
+      "org.typelevel" %%% "cats-mtl" % "1.6.0",
       "org.typelevel" %%% "cats-effect-laws" % "3.7.0-RC1" % Test,
       "org.typelevel" %%% "cats-effect-testkit" % "3.7.0-RC1" % Test,
       "org.typelevel" %%% "cats-laws" % "2.13.0" % Test,
+      "org.typelevel" %%% "cats-mtl-laws" % "1.6.0" % Test,
       "org.typelevel" %%% "discipline-munit" % "2.0.0" % Test,
       "org.typelevel" %%% "munit-cats-effect" % "2.2.0-RC1" % Test,
       "org.typelevel" %%% "scalacheck-effect-munit" % "2.1.0-RC1" % Test

core/shared/src/main/scala/fs2/Stream.scala

@@ -39,8 +39,9 @@ import fs2.concurrent._
 import fs2.internal._
 import org.typelevel.scalaccompat.annotation._
 import Pull.StreamPullOps
+import cats.mtl.{LiftKind, LiftValue}
 
-import java.util.concurrent.Flow.{Publisher, Processor, Subscriber}
+import java.util.concurrent.Flow.{Processor, Publisher, Subscriber}
 
 /** A stream producing output of type `O` and which may evaluate `F` effects.
   *
@@ -5810,9 +5811,46 @@ object Stream extends StreamLowPriority {
     new Defer[Stream[F, *]] {
       override def defer[A](fa: => Stream[F, A]): Stream[F, A] = Stream.empty ++ fa
     }
+
+  implicit def liftKindInstance[F[_]](implicit F: Applicative[F]): LiftKind[F, Stream[F, *]] =
+    liftKindImpl(F)
+
+  implicit def liftValueFromResourceInstance[F[_]](implicit
+      F: MonadCancel[F, ?]
+  ): LiftValue[Resource[F, *], Stream[F, *]] =
+    liftValueFromResourceImpl(implicitly)
 }
 
 private[fs2] trait StreamLowPriority {
   implicit def monadInstance[F[_]]: Monad[Stream[F, *]] =
     new Stream.StreamMonad[F]
+
+  protected[this] def liftKindImpl[F[_]](F: Applicative[F]): LiftKind[F, Stream[F, *]] =
+    new LiftKind[F, Stream[F, *]] {
+      val applicativeF: Applicative[F] = F
+      val applicativeG: Applicative[Stream[F, *]] = monadInstance
+      def apply[A](fa: F[A]): Stream[F, A] = Stream.eval(fa)
+      def limitedMapK[A](ga: Stream[F, A])(scope: F ~> F): Stream[F, A] =
+        ga.translate(scope)
+    }
+
+  implicit def liftKindComposedInstance[F[_], G[_]](implicit
+      inner: LiftKind[F, G]
+  ): LiftKind[F, Stream[G, *]] =
+    inner.andThen(liftKindImpl(inner.applicativeG))
+
+  protected[this] def liftValueFromResourceImpl[F[_]](
+      applicativeResource: Applicative[Resource[F, *]]
+  )(implicit F: MonadCancel[F, ?]): LiftValue[Resource[F, *], Stream[F, *]] =
+    new LiftValue[Resource[F, *], Stream[F, *]] {
+      val applicativeF: Applicative[Resource[F, *]] = applicativeResource
+      val applicativeG: Applicative[Stream[F, *]] = monadInstance
+      def apply[A](fa: Resource[F, A]): Stream[F, A] = Stream.resource(fa)
+    }
+
+  implicit def liftValueFromResourceComposedInstance[F[_], G[_]](implicit
+      inner: LiftValue[F, Resource[G, *]],
+      G: MonadCancel[G, ?]
+  ): LiftValue[F, Stream[G, *]] =
+    inner.andThen(liftValueFromResourceImpl(inner.applicativeG))
 }

core/shared/src/test/scala/fs2/StreamLawsSuite.scala

@@ -21,20 +21,39 @@
 
 package fs2
 
-import cats.Eq
-import cats.effect.IO
+import cats.{Applicative, Eq, ~>}
+import cats.data.{IdT, OptionT}
+import cats.effect.{Concurrent, IO, Ref, Resource}
 import cats.effect.testkit.TestInstances
 import cats.laws.discipline._
 import cats.laws.discipline.arbitrary._
+import cats.mtl.LiftValue
+import cats.mtl.laws.discipline.{LiftKindTests, LiftValueTests}
+import org.scalacheck.{Arbitrary, Gen}
 
 class StreamLawsSuite extends Fs2Suite with TestInstances {
   implicit val ticker: Ticker = Ticker()
 
-  implicit def eqStream[O: Eq]: Eq[Stream[IO, O]] =
-    Eq.instance((x, y) =>
-      Eq[IO[Vector[Either[Throwable, O]]]]
-        .eqv(x.attempt.compile.toVector, y.attempt.compile.toVector)
-    )
+  implicit def eqStream[F[_], O](implicit
+      F: Concurrent[F],
+      eqFVecEitherThrowO: Eq[F[Vector[Either[Throwable, O]]]]
+  ): Eq[Stream[F, O]] =
+    Eq.by((_: Stream[F, O]).attempt.compile.toVector)
+
+  private[this] val counter: IO[Ref[IO, Int]] = IO.ref(0)
+
+  implicit val arbitraryScope: Arbitrary[IO ~> IO] =
+    Arbitrary {
+      Gen.const {
+        new (IO ~> IO) {
+          def apply[A](fa: IO[A]): IO[A] =
+            for {
+              ref <- counter
+              res <- ref.update(_ + 1) >> fa
+            } yield res
+        }
+      }
+    }
 
   checkAll(
     "MonadError[Stream[F, *], Throwable]",
@@ -50,4 +69,32 @@ class StreamLawsSuite extends Fs2Suite with TestInstances {
     "Align[Stream[F, *]]",
     AlignTests[Stream[IO, *]].align[Int, Int, Int, Int]
   )
+  checkAll(
+    "LiftKind[IO, Stream[IO, *]",
+    LiftKindTests[IO, Stream[IO, *]].liftKind[Int, Int]
+  )
+  checkAll(
+    "LiftKind[IO, Stream[OptionT[IO, *], *]",
+    LiftKindTests[IO, Stream[OptionT[IO, *], *]].liftKind[Int, Int]
+  )
+  checkAll(
+    "LiftValue[Resource[IO, *], Stream[IO, *]",
+    LiftValueTests[Resource[IO, *], Stream[IO, *]].liftValue[Int, Int]
+  )
+  locally {
+    // this is a somewhat silly instance, but we need a
+    // `LiftValue[X, Resource[IO, *]]` instance where `X` is not `IO` because
+    // that already has a higher priority implicit instance
+    implicit val liftIdTResource: LiftValue[IdT[IO, *], Resource[IO, *]] =
+      new LiftValue[IdT[IO, *], Resource[IO, *]] {
+        val applicativeF: Applicative[IdT[IO, *]] = implicitly
+        val applicativeG: Applicative[Resource[IO, *]] = implicitly
+        def apply[A](fa: IdT[IO, A]): Resource[IO, A] =
+          Resource.eval(fa.value)
+      }
+    checkAll(
+      "LiftValue[IdT[IO, *], Stream[IO, *]] via Resource[IO, *]",
+      LiftValueTests[IdT[IO, *], Stream[IO, *]].liftValue[Int, Int]
+    )
+  }
 }