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Signal.scala
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Signal.scala
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/*
* Copyright (c) 2013 Functional Streams for Scala
*
* Permission is hereby granted, free of charge, to any person obtaining a copy of
* this software and associated documentation files (the "Software"), to deal in
* the Software without restriction, including without limitation the rights to
* use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
* the Software, and to permit persons to whom the Software is furnished to do so,
* subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in all
* copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
* FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
* COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
* IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/
package fs2
package concurrent
import cats.data.OptionT
import cats.kernel.Eq
import cats.effect.kernel.{Concurrent, Deferred, Ref, Resource}
import cats.effect.std.MapRef
import cats.effect.syntax.all._
import cats.syntax.all._
import cats.{Applicative, Functor, Invariant, Monad}
import scala.collection.immutable.LongMap
/** Pure holder of a single value of type `A` that can be read in the effect `F`. */
trait Signal[F[_], A] { outer =>
/** Returns a stream of the current value and subsequent updates to this signal.
*
* Even if you are pulling as fast as possible, updates that are very close together may
* result in only the last update appearing in the stream. In general, when you pull
* from this stream you may be notified of only the latest update since your last pull.
* If you want to be notified about every single update, use a `Queue` or `Channel` instead.
*/
def discrete: Stream[F, A]
/** Returns a stream of the current value of the signal. An element is always
* available -- on each pull, the current value is supplied.
*/
def continuous: Stream[F, A]
/** Gets the current value of this `Signal`.
*/
def get: F[A]
/** Returns the current value of this `Signal` and a `Stream` to subscribe to
* subsequent updates, with the same semantics as [[discrete]]. The updates
* stream should be compiled at most once.
*/
def getAndDiscreteUpdates(implicit F: Concurrent[F]): Resource[F, (A, Stream[F, A])] =
discrete.pull.uncons1
.flatMap(Pull.outputOption1(_))
.streamNoScope
.compile
.resource
.onlyOrError
/** Returns a signal derived from this one, that drops update events that did not change the value.
*/
def changes(implicit eqA: Eq[A]): Signal[F, A] =
new Signal[F, A] {
def discrete = outer.discrete.changes
def continuous = outer.continuous
def get = outer.get
override def getAndDiscreteUpdates(implicit F: Concurrent[F]) =
outer.getAndDiscreteUpdates.map { case (got, updates) =>
(got, updates.dropWhile(_ === got).changes)
}
}
/** Returns when the condition becomes true, semantically blocking
* in the meantime.
*
* This method is particularly useful to transform naive, recursive
* polling algorithms on the content of a `Signal`/ `SignallingRef`
* into semantically blocking ones. For example, here's how to
* encode a very simple cache with expiry, pay attention to the
* definition of `view`:
*
* {{{
* trait Refresh[F[_], A] {
* def get: F[A]
* }
* object Refresh {
* def create[F[_]: Temporal, A](
* action: F[A],
* refreshAfter: A => FiniteDuration,
* defaultExpiry: FiniteDuration
* ): Resource[F, Refresh[F, A]] =
* Resource
* .eval(SignallingRef[F, Option[Either[Throwable, A]]](None))
* .flatMap { state =>
* def refresh: F[Unit] =
* state.set(None) >> action.attempt.flatMap { res =>
* val t = res.map(refreshAfter).getOrElse(defaultExpiry)
* state.set(res.some) >> Temporal[F].sleep(t) >> refresh
* }
*
* def view = new Refresh[F, A] {
* def get: F[A] = state.get.flatMap {
* case Some(res) => Temporal[F].fromEither(res)
* case None => state.waitUntil(_.isDefined) >> get
* }
* }
*
* refresh.background.as(view)
* }
* }
* }}}
*
* Note that because `Signal` prioritizes the latest update when
* its state is updating very quickly, completion of the `F[Unit]`
* might not trigger if the condition becomes true and then false
* immediately after.
*
* Therefore, natural use cases of `waitUntil` tend to fall into
* two categories:
* - Scenarios where conditions don't change instantly, such as
* periodic timed processes updating the `Signal`/`SignallingRef`.
* - Scenarios where conditions might change instantly, but the `p`
* predicate is monotonic, i.e. if it tests true for an event, it
* will test true for the following events as well.
* Examples include waiting for a unique ID stored in a `Signal`
* to change, or waiting for the value of the `Signal` of an
* ordered `Stream[IO, Int]` to be greater than a certain number.
*/
def waitUntil(p: A => Boolean)(implicit F: Concurrent[F]): F[Unit] =
discrete.forall(a => !p(a)).compile.drain
}
object Signal extends SignalInstances {
def constant[F[_], A](a: A)(implicit F: Concurrent[F]): Signal[F, A] =
new Signal[F, A] {
def get: F[A] = F.pure(a)
def continuous: Stream[Pure, A] = Stream.constant(a)
override def getAndDiscreteUpdates(implicit
ev: Concurrent[F]
): Resource[F, (A, Stream[F, A])] =
Resource.pure((a, Stream.never(F)))
def discrete: Stream[F, A] = Stream(a) ++ Stream.never
}
def mapped[F[_]: Functor, A, B](fa: Signal[F, A])(f: A => B): Signal[F, B] =
new Signal[F, B] {
def continuous: Stream[F, B] = fa.continuous.map(f)
def discrete: Stream[F, B] = fa.discrete.map(f)
override def getAndDiscreteUpdates(implicit
ev: Concurrent[F]
): Resource[F, (B, Stream[F, B])] =
fa.getAndDiscreteUpdates(ev).map { case (a, updates) =>
(f(a), updates.map(f))
}
def get: F[B] = Functor[F].map(fa.get)(f)
}
implicit class SignalOps[F[_], A](val self: Signal[F, A]) extends AnyVal {
/** Converts this signal to signal of `B` by applying `f`.
*/
def map[B](f: A => B)(implicit F: Functor[F]): Signal[F, B] =
Signal.mapped(self)(f)
}
implicit class BooleanSignalOps[F[_]](val self: Signal[F, Boolean]) extends AnyVal {
/** Interrupts the supplied `Stream` when this `Signal` is `true`.
*/
def interrupt[A](s: Stream[F, A])(implicit F: Concurrent[F]): Stream[F, A] =
s.interruptWhen(self)
/** Predicates the supplied effect `f` on this `Signal` being `true`.
*/
def predicate[A](f: F[A])(implicit F: Monad[F]): F[Unit] =
self.get.flatMap(f.whenA)
}
}
/** Pure holder of a single value of type `A` that can be both read
* and updated in the effect `F`.
*
* The update methods have the same semantics as Ref, as well as
* propagating changes to `discrete` (with a last-update-wins policy
* in case of very fast updates).
*
* The `access` method differs slightly from `Ref` in that the update
* 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]
object SignallingRef {
private[fs2] final class PartiallyApplied[F[_]](
private val dummy: Boolean = true
) extends AnyVal {
/** @see [[SignallingRef.of]]
*/
def of[A](initial: A)(implicit F: Concurrent[F]): F[SignallingRef[F, A]] =
SignallingRef.of(initial)
}
/** Builds a `SignallingRef` value for data types that are `Concurrent`.
*
* This builder uses the
* [[https://typelevel.org/cats/guidelines.html#partially-applied-type-params Partially-Applied Type]]
* technique.
*
* {{{
* SignallingRef[IO].of(10L) <-> SignallingRef.of[IO, Long](10L)
* }}}
*
* @see [[of]]
*/
def apply[F[_]]: PartiallyApplied[F] = new PartiallyApplied[F]
/** Alias for `of`. */
def apply[F[_]: Concurrent, A](initial: A): F[SignallingRef[F, A]] =
of(initial)
/** Builds a `SignallingRef` for for effect `F`, initialized to the supplied value.
*/
def of[F[_], A](initial: A)(implicit F: Concurrent[F]): F[SignallingRef[F, A]] = {
case class State(
value: A,
lastUpdate: Long,
listeners: LongMap[Deferred[F, (A, Long)]]
)
F.ref(State(initial, 0L, LongMap.empty))
.product(F.ref(1L))
.map { case (state, ids) =>
def newId = ids.getAndUpdate(_ + 1)
def updateAndNotify[B](state: State, f: A => (A, B)): (State, F[B]) = {
val (newValue, result) = f(state.value)
val lastUpdate = state.lastUpdate + 1
val newState = State(newValue, lastUpdate, LongMap.empty)
val notifyListeners = state.listeners.values.toVector.traverse_ { listener =>
listener.complete(newValue -> lastUpdate)
}
newState -> notifyListeners.as(result)
}
new SignallingRef[F, A] {
def get: F[A] = state.get.map(_.value)
def continuous: Stream[F, A] = Stream.repeatEval(get)
def discrete: Stream[F, A] =
Stream.resource(getAndDiscreteUpdates).flatMap { case (a, updates) =>
Stream.emit(a) ++ updates
}
override def getAndDiscreteUpdates(implicit
ev: Concurrent[F]
): Resource[F, (A, Stream[F, A])] =
getAndDiscreteUpdatesImpl
private[this] def getAndDiscreteUpdatesImpl = {
def go(id: Long, lastSeen: Long): Stream[F, A] = {
def getNext: F[(A, Long)] =
F.deferred[(A, Long)]
.flatMap { wait =>
state.modify { case state @ State(value, lastUpdate, listeners) =>
if (lastUpdate != lastSeen)
state -> (value -> lastUpdate).pure[F]
else
state.copy(listeners = listeners + (id -> wait)) -> wait.get
}
}
.flatten // cancelable
Stream.eval(getNext).flatMap { case (a, lastUpdate) =>
Stream.emit(a) ++ go(id, lastSeen = lastUpdate)
}
}
def cleanup(id: Long): F[Unit] =
state.update(s => s.copy(listeners = s.listeners - id))
Resource.eval(state.get.map { s =>
(s.value, Stream.bracket(newId)(cleanup).flatMap(go(_, s.lastUpdate)))
})
}
def set(a: A): F[Unit] = update(_ => a)
def update(f: A => A): F[Unit] = modify(a => (f(a), ()))
def modify[B](f: A => (A, B)): F[B] =
state.flatModify(updateAndNotify(_, f))
def tryModify[B](f: A => (A, B)): F[Option[B]] =
state.tryModify(updateAndNotify(_, f)).flatMap(_.sequence).uncancelable
def tryUpdate(f: A => A): F[Boolean] =
tryModify(a => (f(a), ())).map(_.isDefined)
def access: F[(A, A => F[Boolean])] =
state.access.map { case (state, set) =>
val setter = { (newValue: A) =>
val (newState, notifyListeners) =
updateAndNotify(state, _ => (newValue, ()))
set(newState).flatTap { succeeded =>
notifyListeners.whenA(succeeded)
}
}
(state.value, setter)
}
def tryModifyState[B](state: cats.data.State[A, B]): F[Option[B]] = {
val f = state.runF.value
tryModify(a => f(a).value)
}
def modifyState[B](state: cats.data.State[A, B]): F[B] = {
val f = state.runF.value
modify(a => f(a).value)
}
}
}
}
/** Creates an instance focused on a component of another SignallingRef's value. Delegates every get and
* modification to underlying SignallingRef, so both instances are always in sync.
*/
def lens[F[_], A, B](
ref: SignallingRef[F, A]
)(get: A => B, set: A => B => A)(implicit F: Functor[F]): SignallingRef[F, B] =
new LensSignallingRef(ref)(get, set)
private final class LensSignallingRef[F[_], A, B](underlying: SignallingRef[F, A])(
lensGet: A => B,
lensSet: A => B => A
)(implicit F: Functor[F])
extends SignallingRef[F, B] {
def discrete: Stream[F, B] = underlying.discrete.map(lensGet)
def continuous: Stream[F, B] = underlying.continuous.map(lensGet)
def get: F[B] = F.map(underlying.get)(a => lensGet(a))
override def getAndDiscreteUpdates(implicit ev: Concurrent[F]): Resource[F, (B, Stream[F, B])] =
underlying.getAndDiscreteUpdates.map { case (a, updates) =>
(lensGet(a), updates.map(lensGet))
}
def set(b: B): F[Unit] = underlying.update(a => lensModify(a)(_ => b))
override def getAndSet(b: B): F[B] =
underlying.modify(a => (lensModify(a)(_ => b), lensGet(a)))
def update(f: B => B): F[Unit] =
underlying.update(a => lensModify(a)(f))
def modify[C](f: B => (B, C)): F[C] =
underlying.modify { a =>
val oldB = lensGet(a)
val (b, c) = f(oldB)
(lensSet(a)(b), c)
}
def tryUpdate(f: B => B): F[Boolean] =
F.map(tryModify(a => (f(a), ())))(_.isDefined)
def tryModify[C](f: B => (B, C)): F[Option[C]] =
underlying.tryModify { a =>
val oldB = lensGet(a)
val (b, result) = f(oldB)
(lensSet(a)(b), result)
}
def tryModifyState[C](state: cats.data.State[B, C]): F[Option[C]] = {
val f = state.runF.value
tryModify(a => f(a).value)
}
def modifyState[C](state: cats.data.State[B, C]): F[C] = {
val f = state.runF.value
modify(a => f(a).value)
}
val access: F[(B, B => F[Boolean])] =
F.map(underlying.access) { case (a, update) =>
(lensGet(a), b => update(lensSet(a)(b)))
}
private def lensModify(s: A)(f: B => B): A = lensSet(s)(f(lensGet(s)))
}
implicit def invariantInstance[F[_]: Functor]: Invariant[SignallingRef[F, *]] =
new Invariant[SignallingRef[F, *]] {
override def imap[A, B](fa: SignallingRef[F, A])(f: A => B)(g: B => A): SignallingRef[F, B] =
new SignallingRef[F, B] {
def get: F[B] = fa.get.map(f)
def discrete: Stream[F, B] = fa.discrete.map(f)
def continuous: Stream[F, B] = fa.continuous.map(f)
override def getAndDiscreteUpdates(implicit
ev: Concurrent[F]
): Resource[F, (B, Stream[F, B])] =
fa.getAndDiscreteUpdates(ev).map { case (a, updates) =>
(f(a), updates.map(f))
}
def set(b: B): F[Unit] = fa.set(g(b))
def access: F[(B, B => F[Boolean])] =
fa.access.map { case (getter, setter) =>
(f(getter), b => setter(g(b)))
}
def tryUpdate(h: B => B): F[Boolean] = fa.tryUpdate(a => g(h(f(a))))
def tryModify[B2](h: B => (B, B2)): F[Option[B2]] =
fa.tryModify(a => h(f(a)).leftMap(g))
def update(bb: B => B): F[Unit] =
modify(b => (bb(b), ()))
def modify[B2](bb: B => (B, B2)): F[B2] =
fa.modify { a =>
val (a2, b2) = bb(f(a))
g(a2) -> b2
}
def tryModifyState[C](state: cats.data.State[B, C]): F[Option[C]] =
fa.tryModifyState(state.dimap(f)(g))
def modifyState[C](state: cats.data.State[B, C]): F[C] =
fa.modifyState(state.dimap(f)(g))
}
}
}
/** A [[MapRef]] with a [[SignallingRef]] for each key. */
trait SignallingMapRef[F[_], K, V] extends MapRef[F, K, V] {
override def apply(k: K): SignallingRef[F, V]
}
object SignallingMapRef {
/** Builds a `SignallingMapRef` for effect `F`, initialized to the supplied value.
*
* Update semantics for `discrete` are the same as `SignallingRef`, with one exception:
* it cannot distinguish updates that remove a key (by setting its value to `None`).
*
* More specifically: if you remove a key, this will only notify once per listener
* i.e. setting it to `None` again will not trigger another update.
* Furthermore, if a listener's last pull returned `None`, and by the time it pulls again the
* current value is `None`, then it will not be notified regardless of any non-`None` updates
* that may have happened between the pulls. This special semantic for `None` is necessary to
* prevent memory leaks at keys with no values and no listeners.
*/
def ofSingleImmutableMap[F[_], K, V](
initial: Map[K, V] = Map.empty[K, V]
)(implicit F: Concurrent[F]): F[SignallingMapRef[F, K, Option[V]]] = {
case class State(
lastUpdate: Long,
keys: Map[K, KeyState]
)
type Listener = Deferred[F, (Option[V], Long)]
// fix lastUpdate at -1 when the value is not present
// so we don't have to keep an update counter when the key has neither a value nor any listeners
case class KeyState(
value: Option[V],
lastUpdate: Long,
listeners: LongMap[Listener]
)
F.ref(State(0L, initial.map { case (k, v) => k -> KeyState(Some(v), 0L, LongMap.empty) }))
.product(F.ref(1L))
.map { case (state, ids) =>
def newId = ids.getAndUpdate(_ + 1)
def updateAndNotify[U](state: State, k: K, f: Option[V] => (Option[V], U))
: (State, F[U]) = {
val keyState = state.keys.get(k)
val (newValue, result) = f(keyState.flatMap(_.value))
val lastUpdate = {
val lu = state.lastUpdate + 1
// skip -1 b/c of its special semantic
if (lu == -1L) 0L else lu
}
val lastKeyUpdate = if (newValue.isDefined) lastUpdate else -1L
val newKeys =
if (newValue.isDefined)
state.keys.updated(k, KeyState(newValue, lastKeyUpdate, LongMap.empty))
else
state.keys - k // prevent memory leak
val newState = State(lastUpdate, newKeys)
val notifyListeners = keyState.fold(F.unit) { keyState =>
keyState.listeners.values.toVector.traverse_ { listener =>
listener.complete(newValue -> lastKeyUpdate)
}
}
newState -> notifyListeners.as(result)
}
k =>
new SignallingRef[F, Option[V]] {
def get: F[Option[V]] = state.get.map(_.keys.get(k).flatMap(_.value))
def continuous: Stream[F, Option[V]] = Stream.repeatEval(get)
def discrete: Stream[F, Option[V]] =
Stream.resource(getAndDiscreteUpdates).flatMap { case (a, updates) =>
Stream.emit(a) ++ updates
}
override def getAndDiscreteUpdates(implicit
ev: Concurrent[F]
): Resource[F, (Option[V], Stream[F, Option[V]])] =
getAndDiscreteUpdatesImpl
private[this] def getAndDiscreteUpdatesImpl = {
def go(id: Long, lastSeen: Long): Stream[F, Option[V]] = {
def getNext: F[(Option[V], Long)] =
F.deferred[(Option[V], Long)]
.flatMap { wait =>
state.modify { state =>
val keyState = state.keys.get(k)
val value = keyState.flatMap(_.value)
val lastUpdate = keyState.fold(-1L)(_.lastUpdate)
val listeners = keyState.fold(LongMap.empty[Listener])(_.listeners)
if (lastUpdate != lastSeen)
state -> (value -> lastUpdate).pure[F]
else {
val newKeys =
state.keys
.updated(k, KeyState(value, lastUpdate, listeners.updated(id, wait)))
state.copy(keys = newKeys) -> wait.get
}
}
}
.flatten // cancelable
Stream.eval(getNext).flatMap { case (v, lastUpdate) =>
Stream.emit(v) ++ go(id, lastSeen = lastUpdate)
}
}
def cleanup(id: Long): F[Unit] =
state.update { state =>
state.keys.get(k).fold(state) { case KeyState(value, lastUpdate, listeners) =>
val newListeners = listeners - id
val newKeys =
if (value.isEmpty && newListeners.isEmpty)
state.keys - k // prevent memory leak
else
state.keys.updated(k, KeyState(value, lastUpdate, newListeners))
state.copy(keys = newKeys)
}
}
Resource.eval(state.get.map { state =>
(
state.keys.get(k).flatMap(_.value),
Stream
.bracket(newId)(cleanup)
.flatMap(go(_, state.keys.get(k).fold(-1L)(_.lastUpdate)))
)
})
}
def set(v: Option[V]): F[Unit] = update(_ => v)
def update(f: Option[V] => Option[V]): F[Unit] = modify(v => (f(v), ()))
def modify[U](f: Option[V] => (Option[V], U)): F[U] =
state.flatModify(updateAndNotify(_, k, f))
def tryModify[U](f: Option[V] => (Option[V], U)): F[Option[U]] =
state.tryModify(updateAndNotify(_, k, f)).flatMap(_.sequence).uncancelable
def tryUpdate(f: Option[V] => Option[V]): F[Boolean] =
tryModify(a => (f(a), ())).map(_.isDefined)
def access: F[(Option[V], Option[V] => F[Boolean])] =
state.access.map { case (state, set) =>
val setter = { (newValue: Option[V]) =>
val (newState, notifyListeners) =
updateAndNotify(state, k, _ => (newValue, ()))
set(newState).flatTap { succeeded =>
notifyListeners.whenA(succeeded)
}
}
(state.keys.get(k).flatMap(_.value), setter)
}
def tryModifyState[U](state: cats.data.State[Option[V], U]): F[Option[U]] = {
val f = state.runF.value
tryModify(v => f(v).value)
}
def modifyState[U](state: cats.data.State[Option[V], U]): F[U] = {
val f = state.runF.value
modify(v => f(v).value)
}
}
}
}
}
private[concurrent] trait SignalInstances extends SignalLowPriorityInstances {
implicit def applicativeInstance[F[_]: Concurrent]: Applicative[Signal[F, *]] = {
def nondeterministicZip[A0, A1](xs: Stream[F, A0], ys: Stream[F, A1]): Stream[F, (A0, A1)] = {
type PullOutput = (A0, A1, Stream[F, A0], Stream[F, A1])
val firstPull: OptionT[Pull[F, PullOutput, *], Unit] = for {
firstXAndRestOfXs <- OptionT(xs.pull.uncons1.covaryOutput[PullOutput])
(x, restOfXs) = firstXAndRestOfXs
firstYAndRestOfYs <- OptionT(ys.pull.uncons1.covaryOutput[PullOutput])
(y, restOfYs) = firstYAndRestOfYs
_ <- OptionT.liftF {
Pull.output1[F, PullOutput]((x, y, restOfXs, restOfYs)): Pull[F, PullOutput, Unit]
}
} yield ()
firstPull.value.void.stream
.flatMap { case (x, y, restOfXs, restOfYs) =>
restOfXs.either(restOfYs).scan((x, y)) {
case ((_, rightElem), Left(newElem)) => (newElem, rightElem)
case ((leftElem, _), Right(newElem)) => (leftElem, newElem)
}
}
}
new Applicative[Signal[F, *]] {
override def map[A, B](fa: Signal[F, A])(f: A => B): Signal[F, B] = Signal.mapped(fa)(f)
def pure[A](x: A): Signal[F, A] = Signal.constant(x)
def ap[A, B](ff: Signal[F, A => B])(fa: Signal[F, A]): Signal[F, B] =
new Signal[F, B] {
def discrete: Stream[F, B] =
nondeterministicZip(ff.discrete, fa.discrete).map { case (f, a) => f(a) }
def continuous: Stream[F, B] = Stream.repeatEval(get)
def get: F[B] = ff.get.ap(fa.get)
override def getAndDiscreteUpdates(implicit
ev: Concurrent[F]
): Resource[F, (B, Stream[F, B])] = getAndDiscreteUpdatesImpl
private[this] def getAndDiscreteUpdatesImpl =
(ff.getAndDiscreteUpdates, fa.getAndDiscreteUpdates).mapN { case ((f, fs), (a, as)) =>
(f(a), nondeterministicZip(fs, as).map { case (f, a) => f(a) })
}
}
}
}
}
private[concurrent] trait SignalLowPriorityInstances {
/** Note that this is not subsumed by [[Signal.applicativeInstance]] because
* [[Signal.applicativeInstance]] requires a `Concurrent[F]`
* since it non-deterministically zips elements together while our
* `Functor` instance has no other constraints.
*
* Separating the two instances allows us to make the `Functor` instance
* more general.
*
* We put this in a `SignalLowPriorityImplicits` trait to resolve ambiguous
* implicits if the [[Signal.applicativeInstance]] is applicable, allowing
* the `Applicative` instance to be chosen.
*/
implicit def functorInstance[F[_]: Functor]: Functor[Signal[F, *]] =
new Functor[Signal[F, *]] {
def map[A, B](fa: Signal[F, A])(f: A => B): Signal[F, B] =
Signal.mapped(fa)(f)
}
}