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Backport #3103 traverseFilter Queue instance to scala_2.11 #3292

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Mar 11, 2020
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Backport #3103 traverseFilter Queue instance to scala_2.11 #3292

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c541f77
backported #3103 Add traverseFilter instances for Queue
gagandeepkalra Feb 11, 2020
8335f18
Merge branch 'scala_2.11' into backport/queueInstances/traverseFilter
gagandeepkalra Mar 11, 2020
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1 change: 1 addition & 0 deletions core/src/main/scala/cats/instances/all.scala
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Expand Up @@ -69,3 +69,4 @@ trait AllInstancesBinCompat7
with VectorInstancesBinCompat1
with EitherInstancesBinCompat0
with StreamInstancesBinCompat1
with QueueInstancesBinCompat0
29 changes: 29 additions & 0 deletions core/src/main/scala/cats/instances/queue.scala
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Expand Up @@ -2,6 +2,7 @@ package cats
package instances

import cats.syntax.show._

import scala.annotation.tailrec
import scala.collection.immutable.Queue
import scala.util.Try
Expand Down Expand Up @@ -153,3 +154,31 @@ trait QueueInstances extends cats.kernel.instances.QueueInstances {
fa.iterator.map(_.show).mkString("Queue(", ", ", ")")
}
}

private[instances] trait QueueInstancesBinCompat0 {
implicit val catsStdTraverseFilterForQueue: TraverseFilter[Queue] = new TraverseFilter[Queue] {
val traverse: Traverse[Queue] = cats.instances.queue.catsStdInstancesForQueue

override def mapFilter[A, B](fa: Queue[A])(f: (A) => Option[B]): Queue[B] =
fa.collect(Function.unlift(f))

override def filter[A](fa: Queue[A])(f: (A) => Boolean): Queue[A] = fa.filter(f)

override def collect[A, B](fa: Queue[A])(f: PartialFunction[A, B]): Queue[B] = fa.collect(f)

override def flattenOption[A](fa: Queue[Option[A]]): Queue[A] = fa.flatten

def traverseFilter[G[_], A, B](fa: Queue[A])(f: (A) => G[Option[B]])(implicit G: Applicative[G]): G[Queue[B]] =
fa.foldRight(Eval.now(G.pure(Queue.empty[B])))(
(x, xse) => G.map2Eval(f(x), xse)((i, o) => i.fold(o)(_ +: o))
)
.value
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@gagandeepkalra gagandeepkalra Feb 11, 2020 •
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We can have an implementation without using Eval, might even perform a little better (for a bigger queue); without all those extra Eval wrappers.

fa.foldRight(G.pure(Queue.empty[B]))( (x, xse) => G.map2(f(x), xse)((i, o) => i.fold(o)(_ +: o)))

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That won't short-circuit, though, right?

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@travisbrown yes agreed it won't, but for the same reason the present implementation wouldn't.

If by short circuit you mean f wouldn't have to run on the remaining Queue elements (given we are folding from the right).

This is my understanding and derived reasoning-
map2Eval short-circuits: given the underlying G structure allows it, then the Eval argument skips running.

In this case however, the Eval description is computed by running f on each element of the Queue already. The first argument to map2Eval is not Lazy. In every situation f will run on each element.

Please correct me If I understood this wrongly.

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@gagandeepkalra gagandeepkalra Feb 23, 2020 •
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I see it now, we are using the wrong foldRight here, queue.foldRight ends up with a while loop (standard library). This should have been-

traverse.foldRight[A, G[Queue[B]]](fa, Always(G.pure(Queue.empty))) { (a, lglb) => G.map2Eval(f(a), lglb)((i, o) => i.fold(o)(_ +: o)) } .value

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I see the same problem with other traverseFilter implementations, should I open a new issue?


override def filterA[G[_], A](fa: Queue[A])(f: (A) => G[Boolean])(implicit G: Applicative[G]): G[Queue[A]] =
fa.foldRight(Eval.now(G.pure(Queue.empty[A])))(
(x, xse) => G.map2Eval(f(x), xse)((b, vec) => if (b) x +: vec else vec)
)
.value
}

}
5 changes: 4 additions & 1 deletion tests/src/test/scala/cats/tests/QueueSuite.scala
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Expand Up @@ -2,13 +2,13 @@ package cats
package tests

import scala.collection.immutable.Queue

import cats.laws.discipline.{
AlternativeTests,
CoflatMapTests,
MonadTests,
SemigroupalTests,
SerializableTests,
TraverseFilterTests,
TraverseTests
}

Expand All @@ -28,6 +28,9 @@ class QueueSuite extends CatsSuite {
checkAll("Queue[Int] with Option", TraverseTests[Queue].traverse[Int, Int, Int, Set[Int], Option, Option])
checkAll("Traverse[Queue]", SerializableTests.serializable(Traverse[Queue]))

checkAll("Queue[Int]", TraverseFilterTests[Queue].traverseFilter[Int, Int, Int])
checkAll("TraverseFilter[Queue]", SerializableTests.serializable(TraverseFilter[Queue]))

test("show") {
Queue(1, 2, 3).show should ===("Queue(1, 2, 3)")
Queue.empty[Int].show should ===("Queue()")
Expand Down