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Optimize Alternative (part 2): add prependK/appendK specializations for std containers #4052

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Merged
johnynek merged 1 commit into from
Nov 27, 2021
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Optimize Alternative (part 2): add prependK/appendK specializations for std containers #4052

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4 changes: 4 additions & 0 deletions alleycats-core/src/main/scala/alleycats/std/set.scala
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Expand Up @@ -73,6 +73,10 @@ trait SetInstances {
override def empty[A]: Set[A] = Set.empty

override def combineK[A](x: Set[A], y: Set[A]): Set[A] = x | y

override def prependK[A](a: A, fa: Set[A]): Set[A] = fa + a

override def appendK[A](fa: Set[A], a: A): Set[A] = fa + a
}

// Since iteration order is not guaranteed for sets, folds and other
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2 changes: 2 additions & 0 deletions core/src/main/scala-2.12/cats/instances/stream.scala
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Expand Up @@ -16,6 +16,8 @@ trait StreamInstances extends cats.kernel.instances.StreamInstances {

def combineK[A](x: Stream[A], y: Stream[A]): Stream[A] = x #::: y

override def prependK[A](a: A, fa: Stream[A]): Stream[A] = a #:: fa

@armanbilge armanbilge Nov 27, 2021

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For LazyList you overrode both prependK and appendK, why only prependK for Stream, are the implementations different?

@satorg satorg Nov 27, 2021

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Right, this is because appendK for LazyList utilizes LazyList#appended which is optimized for LazyList. But appended/prepended methods were first introduced in Scala 2.13 and Stream in Scala 2.12 does not have such methods yet. Therefore seems the best we can do here is to call

def appendK[A](fa: Stream[A], a: A) = fa #::: Stream(a)

but it is exactly what combineK (along with pure) already does.

@satorg satorg Nov 27, 2021 •
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On the second thought, Stream in Scala 2.13 does have the prepended/appended methods, but as I can see they're simply come from Seq and aren't optimized for Stream (unlike those in LazyList).

@armanbilge armanbilge Nov 27, 2021

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Makes sense, thanks for explaining!


def pure[A](x: A): Stream[A] = Stream(x)

override def map[A, B](fa: Stream[A])(f: A => B): Stream[B] =
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4 changes: 4 additions & 0 deletions core/src/main/scala-2.13+/cats/instances/arraySeq.scala
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Expand Up @@ -34,6 +34,10 @@ private[cats] object ArraySeqInstances {
def combineK[A](xs: ArraySeq[A], ys: ArraySeq[A]): ArraySeq[A] =
xs.concat(ys)

override def prependK[A](a: A, fa: ArraySeq[A]): ArraySeq[A] = fa.prepended(a)

override def appendK[A](fa: ArraySeq[A], a: A): ArraySeq[A] = fa.appended(a)

override def algebra[A]: Monoid[ArraySeq[A]] =
new cats.kernel.instances.ArraySeqInstances.ArraySeqMonoid

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4 changes: 4 additions & 0 deletions core/src/main/scala-2.13+/cats/instances/lazyList.scala
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Expand Up @@ -22,6 +22,10 @@ trait LazyListInstances extends cats.kernel.instances.LazyListInstances {

def combineK[A](x: LazyList[A], y: LazyList[A]): LazyList[A] = x.lazyAppendedAll(y)

override def prependK[A](a: A, fa: LazyList[A]): LazyList[A] = fa.prepended(a)

override def appendK[A](fa: LazyList[A], a: A): LazyList[A] = fa.appended(a)

def pure[A](x: A): LazyList[A] = LazyList(x)

override def map[A, B](fa: LazyList[A])(f: A => B): LazyList[B] =
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2 changes: 2 additions & 0 deletions core/src/main/scala-2.13+/cats/instances/stream.scala
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Expand Up @@ -17,6 +17,8 @@ trait StreamInstances extends cats.kernel.instances.StreamInstances {

def combineK[A](x: Stream[A], y: Stream[A]): Stream[A] = x #::: y

override def prependK[A](a: A, fa: Stream[A]): Stream[A] = a #:: fa

def pure[A](x: A): Stream[A] = Stream(x)

override def map[A, B](fa: Stream[A])(f: A => B): Stream[B] =
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6 changes: 5 additions & 1 deletion core/src/main/scala/cats/instances/list.scala
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Expand Up @@ -17,7 +17,11 @@ trait ListInstances extends cats.kernel.instances.ListInstances {
new Traverse[List] with Alternative[List] with Monad[List] with CoflatMap[List] with Align[List] {
def empty[A]: List[A] = Nil

def combineK[A](x: List[A], y: List[A]): List[A] = x ++ y
def combineK[A](x: List[A], y: List[A]): List[A] = x ::: y

override def prependK[A](a: A, fa: List[A]): List[A] = a :: fa

override def appendK[A](fa: List[A], a: A): List[A] = fa :+ a

def pure[A](x: A): List[A] = x :: Nil

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5 changes: 4 additions & 1 deletion core/src/main/scala/cats/instances/option.scala
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Expand Up @@ -23,7 +23,10 @@ trait OptionInstances extends cats.kernel.instances.OptionInstances {

def empty[A]: Option[A] = None

def combineK[A](x: Option[A], y: Option[A]): Option[A] = x.orElse(y)
def combineK[A](x: Option[A], y: Option[A]): Option[A] = if (x.isDefined) x else y

override def prependK[A](a: A, fa: Option[A]): Option[A] = Some(a)
override def appendK[A](fa: Option[A], a: A): Option[A] = if (fa.isDefined) fa else Some(a)

override def combineKEval[A](x: Option[A], y: Eval[Option[A]]): Eval[Option[A]] =
x match {
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4 changes: 4 additions & 0 deletions core/src/main/scala/cats/instances/queue.scala
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Expand Up @@ -17,6 +17,10 @@ trait QueueInstances extends cats.kernel.instances.QueueInstances {

def combineK[A](x: Queue[A], y: Queue[A]): Queue[A] = x ++ y

override def prependK[A](a: A, fa: Queue[A]): Queue[A] = a +: fa

override def appendK[A](fa: Queue[A], a: A): Queue[A] = fa.enqueue(a)

def pure[A](x: A): Queue[A] = Queue(x)

override def map[A, B](fa: Queue[A])(f: A => B): Queue[B] =
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4 changes: 4 additions & 0 deletions core/src/main/scala/cats/instances/seq.scala
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Expand Up @@ -17,6 +17,10 @@ trait SeqInstances extends cats.kernel.instances.SeqInstances {

def combineK[A](x: Seq[A], y: Seq[A]): Seq[A] = x ++ y

override def prependK[A](a: A, fa: Seq[A]): Seq[A] = a +: fa

override def appendK[A](fa: Seq[A], a: A): Seq[A] = fa :+ a

def pure[A](x: A): Seq[A] = Seq(x)

override def map[A, B](fa: Seq[A])(f: A => B): Seq[B] =
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4 changes: 4 additions & 0 deletions core/src/main/scala/cats/instances/vector.scala
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Expand Up @@ -18,6 +18,10 @@ trait VectorInstances extends cats.kernel.instances.VectorInstances {

def combineK[A](x: Vector[A], y: Vector[A]): Vector[A] = x ++ y

override def prependK[A](a: A, fa: Vector[A]): Vector[A] = a +: fa

override def appendK[A](fa: Vector[A], a: A): Vector[A] = fa :+ a

def pure[A](x: A): Vector[A] = Vector(x)

override def map[A, B](fa: Vector[A])(f: A => B): Vector[B] =
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