Implement {Signal,SignallingRef}#mapK

core/shared/src/main/scala/fs2/concurrent/Signal.scala

@@ -28,8 +28,11 @@ import cats.effect.std.MapRef
 import cats.effect.syntax.all._
 import cats.syntax.all._
 import cats.{Applicative, Functor, Invariant, Monad}
-
+import cats.arrow.FunctionK
 import scala.collection.immutable.LongMap
+import fs2.concurrent.SignallingRef.TransformedSignallingRef
+import fs2.concurrent.Signal.TransformedSignal
+import cats.data.State
 
 /** Pure holder of a single value of type `A` that can be read in the effect `F`. */
 trait Signal[F[_], A] { outer =>
@@ -135,6 +138,11 @@ trait Signal[F[_], A] { outer =>
     */
   def waitUntil(p: A => Boolean)(implicit F: Concurrent[F]): F[Unit] =
     discrete.forall(a => !p(a)).compile.drain
+
+  def mapK[G[_]](
+      f: FunctionK[F, G]
+  ): Signal[G, A] =
+    new TransformedSignal(this, f)
 }
 
 object Signal extends SignalInstances {
@@ -162,6 +170,16 @@ object Signal extends SignalInstances {
       def get: F[B] = Functor[F].map(fa.get)(f)
     }
 
+  final private class TransformedSignal[F[_], G[_], A](
+      underlying: Signal[F, A],
+      trans: FunctionK[F, G]
+  ) extends Signal[G, A] {
+    override def get: G[A] = trans(underlying.get)
+    override def discrete: Stream[G, A] = underlying.discrete.translate(trans)
+    override def continuous: Stream[G, A] = underlying.continuous.translate(trans)
+    override def changes(implicit eqA: Eq[A]): Signal[G, A] = underlying.changes.mapK(trans)
+  }
+
   implicit class SignalOps[F[_], A](val self: Signal[F, A]) extends AnyVal {
 
     /** Converts this signal to signal of `B` by applying `f`.
@@ -196,7 +214,12 @@ object Signal extends SignalInstances {
   * function, in the presence of `discrete`, can return `false` and
   * need looping even without any other writers.
   */
-abstract class SignallingRef[F[_], A] extends Ref[F, A] with Signal[F, A]
+abstract class SignallingRef[F[_], A] extends Ref[F, A] with Signal[F, A] {
+  def mapK[G[_]](
+      f: FunctionK[F, G]
+  )(implicit G: Functor[G], dummy: DummyImplicit): SignallingRef[G, A] =
+    new TransformedSignallingRef(this, f)
+}
 
 object SignallingRef {
 
@@ -222,6 +245,7 @@ object SignallingRef {
     *
     * @see [[of]]
     */
+
   def apply[F[_]]: PartiallyApplied[F] = new PartiallyApplied[F]
 
   /** Alias for `of`. */
@@ -341,7 +365,31 @@ object SignallingRef {
       ref: SignallingRef[F, A]
   )(get: A => B, set: A => B => A)(implicit F: Functor[F]): SignallingRef[F, B] =
     new LensSignallingRef(ref)(get, set)
-
+  final private class TransformedSignallingRef[F[_], G[_], A](
+      underlying: SignallingRef[F, A],
+      trans: FunctionK[F, G]
+  )(implicit G: Functor[G])
+      extends SignallingRef[G, A] {
+
+    // --- Ref methods: these are lifted using trans, just like in TransformedRef2
+    override def get: G[A] = trans(underlying.get)
+    override def set(a: A): G[Unit] = trans(underlying.set(a))
+    override def getAndSet(a: A): G[A] = trans(underlying.getAndSet(a))
+    override def tryUpdate(f: A => A): G[Boolean] = trans(underlying.tryUpdate(f))
+    override def tryModify[B](f: A => (A, B)): G[Option[B]] = trans(underlying.tryModify(f))
+    override def update(f: A => A): G[Unit] = trans(underlying.update(f))
+    override def modify[B](f: A => (A, B)): G[B] = trans(underlying.modify(f))
+    override def tryModifyState[B](state: State[A, B]): G[Option[B]] =
+      trans(underlying.tryModifyState(state))
+    override def modifyState[B](state: State[A, B]): G[B] = trans(underlying.modifyState(state))
+    override def access: G[(A, A => G[Boolean])] =
+      G.compose[(A, *)].compose[A => *].map(trans(underlying.access))(trans(_))
+
+    // --- Signal-specific methods
+    override def discrete: Stream[G, A] = underlying.discrete.translate(trans)
+    override def continuous: Stream[G, A] = underlying.continuous.translate(trans)
+    override def changes(implicit eqA: Eq[A]): Signal[G, A] = underlying.changes.mapK(trans)
+  }
   private final class LensSignallingRef[F[_], A, B](underlying: SignallingRef[F, A])(
       lensGet: A => B,
       lensSet: A => B => A