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Add Endo type alias for (A =>A) #2076

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Dec 10, 2017
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Add Endo type alias for (A =>A) #2076

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b51cc0f
added reduceRightK and Endo
kailuowang Dec 6, 2017
9410809
added mima excpetion
kailuowang Dec 6, 2017
7796307
removed reduceRightK
kailuowang Dec 7, 2017
828be44
minor
kailuowang Dec 7, 2017
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added reduceRightK and Endo
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@kailuowang
kailuowang committed Dec 6, 2017
commit b51cc0fc1933bf15e53f72ad140b612dc349a4a7
18 changes: 18 additions & 0 deletions core/src/main/scala/cats/Foldable.scala
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Expand Up @@ -244,6 +244,24 @@ import Foldable.sentinel
A.combine(acc, a)
}

/**
* Like `foldK` but from the right side.
* One use case would be endo functions (`A => A`) chaining
* because the default MonoidK instance for endo functions
* is using `compose` rather than the `andThen`
* {{{
* scala> import cats.implicits._, cats.Endo
* scala> val l: List[Endo[Int]] = List(_ * 2, _ + 3)
* scala> val f = l.reduceRightK //using `compose` to chain the functions
* scala> f(1)
* res0: Int = 5
* }}}
*/
def reduceRightK[G[_], A](fga: F[G[A]])(implicit G: MonoidK[G]): G[A] =
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seems like combineAllK to me.

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@kailuowang kailuowang Dec 7, 2017 •
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combineAllK is foldK right? This one combines in the opposite direction

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I see. I misunderstood.

I am very -1 on this unless I am mistaken. reduceRight(Option) is right associative, but does not reverse the effective order that we iterate through. This method does by swapping.

I would call this reverseCombineAllK.

Since the MonoidK is associative, the result is the same if you right associate or left associate (though there may be efficiency reasons to pick one). This is something else: it is like making a new Monoid that has the property that combine(a, b) == original.combine(b, a)

This is actually still a monoid, but it behaves differently for non-commutative cases.

@kailuowang can you explain if I am wrong about this being different from reduceRight?

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You are right it's different from reduceRight(Option). reverseCombineAllK is probably a better name, but you also got to the bottom of it, we don't need it, we just need a different Monoid instance that does the original.combine(b, a). Maybe adding a reverse method to Monoid and MonoidK. But I am not sure are they worthwhile to be added?

What about the Endo type alias?

foldRight(fga, Eval.now(G.empty[A])) { (ga, ega) =>
ega.map(rga => G.combineK(rga, ga))
}.value

/**
* Alias for [[fold]].
*/
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1 change: 1 addition & 0 deletions core/src/main/scala/cats/package.scala
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Expand Up @@ -32,6 +32,7 @@ package object cats {
* encodes pure unary function application.
*/
type Id[A] = A
type Endo[A] = A => A
implicit val catsInstancesForId: Bimonad[Id] with CommutativeMonad[Id] with Comonad[Id] with NonEmptyTraverse[Id] =
new Bimonad[Id] with CommutativeMonad[Id] with Comonad[Id] with NonEmptyTraverse[Id] {
def pure[A](a: A): A = a
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1 change: 0 additions & 1 deletion tests/src/test/scala/cats/tests/CategorySuite.scala
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Expand Up @@ -9,7 +9,6 @@ import cats.laws.discipline.eq.catsLawsEqForFn1

class CategorySuite extends CatsSuite {
val functionCategory = Category[Function1]
type Endo[A] = Function1[A, A]

checkAll("Category[Function1].algebraK", MonoidKTests[Endo](functionCategory.algebraK).monoidK[Int])
checkAll("Category[Function1].algebraK", SerializableTests.serializable(functionCategory.algebraK))
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1 change: 0 additions & 1 deletion tests/src/test/scala/cats/tests/ComposeSuite.scala
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Expand Up @@ -8,7 +8,6 @@ import cats.laws.discipline.eq.catsLawsEqForFn1

class ComposeSuite extends CatsSuite {
val functionCompose = Compose[Function1]
type Endo[A] = Function1[A, A]

checkAll("Compose[Function1].algebraK", SemigroupKTests[Endo](functionCompose.algebraK).semigroupK[Int])
checkAll("Compose[Function1].algebraK", SerializableTests.serializable(functionCompose.algebraK))
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11 changes: 10 additions & 1 deletion tests/src/test/scala/cats/tests/FoldableSuite.scala
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Expand Up @@ -9,7 +9,10 @@ import cats.instances.all._
import cats.data._
import cats.laws.discipline.arbitrary._

abstract class FoldableSuite[F[_]: Foldable](name: String)(implicit ArbFInt: Arbitrary[F[Int]], ArbFString: Arbitrary[F[String]]) extends CatsSuite with PropertyChecks {
abstract class FoldableSuite[F[_]: Foldable](name: String)(
implicit ArbFInt: Arbitrary[F[Int]],
ArbFString: Arbitrary[F[String]],
ArbFListString: Arbitrary[F[List[String]]]) extends CatsSuite with PropertyChecks {

def iterator[T](fa: F[T]): Iterator[T]

Expand All @@ -33,6 +36,12 @@ abstract class FoldableSuite[F[_]: Foldable](name: String)(implicit ArbFInt: Arb
}
}

test("Foldable#reduceRightK") {
forAll { (fi: F[List[String]]) =>
fi.reduceRightK should === (fi.foldRight(Eval.now(List.empty[String]))((l, el) => el.map(_ ++ l)).value)
}
}

test("Foldable#partitionEither consistent with List#partition") {
forAll { (fi: F[Int], f: Int => Either[String, String]) =>
val list = Foldable[F].toList(fi)
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3 changes: 1 addition & 2 deletions tests/src/test/scala/cats/tests/ReducibleSuite.scala
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Expand Up @@ -2,7 +2,6 @@ package cats
package tests

import org.scalacheck.Arbitrary

import cats.data.NonEmptyList

class ReducibleSuiteAdditional extends CatsSuite {
Expand Down Expand Up @@ -71,7 +70,7 @@ class ReducibleSuiteAdditional extends CatsSuite {

}

abstract class ReducibleSuite[F[_]: Reducible](name: String)(implicit ArbFInt: Arbitrary[F[Int]], ArbFString: Arbitrary[F[String]]) extends FoldableSuite[F](name) {
abstract class ReducibleSuite[F[_]: Reducible](name: String)(implicit ArbFInt: Arbitrary[F[Int]], ArbFString: Arbitrary[F[String]], ArbFListString: Arbitrary[F[List[String]]]) extends FoldableSuite[F](name) {
def range(start: Long, endInclusive: Long): F[Long]

test(s"Reducible[$name].reduceLeftM stack safety") {
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