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added a new foldRight lazy law, move forallLazy and existLazy laws #2817

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merged 8 commits into from
May 5, 2019
Merged

added a new foldRight lazy law, move forallLazy and existLazy laws #2817

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b9b8144
added a new foldRight lazy law, move forAll and exist laws
kailuowang Apr 28, 2019
70d513f
reformat
kailuowang Apr 28, 2019
d6b1584
fix foldRight on NonEmptyChain
kailuowang Apr 28, 2019
5811126
reformat
kailuowang Apr 28, 2019
1e9fcd7
reformat
kailuowang Apr 28, 2019
f75c18a
Update FoldableLaws.scala
kailuowang Apr 28, 2019
cc8f633
incorporated Ross and Oscar suggestion
kailuowang May 5, 2019
fbc7cf8
override default implementation in map and set
kailuowang May 5, 2019
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2 changes: 1 addition & 1 deletion core/src/main/scala/cats/data/NonEmptyChain.scala
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Expand Up @@ -452,7 +452,7 @@ sealed abstract private[data] class NonEmptyChainInstances extends NonEmptyChain
fa.foldLeft(b)(f)

override def foldRight[A, B](fa: NonEmptyChain[A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] =
fa.foldRight(lb)(f)
Foldable[Chain].foldRight(fa.toChain, lb)(f)

override def foldMap[A, B](fa: NonEmptyChain[A])(f: A => B)(implicit B: Monoid[B]): B =
B.combineAll(fa.toChain.iterator.map(f))
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29 changes: 29 additions & 0 deletions laws/src/main/scala/cats/laws/FoldableLaws.scala
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Expand Up @@ -8,6 +8,16 @@ import scala.collection.mutable
trait FoldableLaws[F[_]] extends UnorderedFoldableLaws[F] {
implicit def F: Foldable[F]

def foldRightLazy[A](fa: F[A]): Boolean = {
var i = 0
F.foldRight(fa, Eval.now("empty")) { (_, _) =>
i += 1
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We use += here but not below. Any reason for the difference?

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no real reason except this one is written by me and the other ones are from before. Just changed the other ones to be more consistent.

Eval.now("not empty")
}
.value
i == (if (F.isEmpty(fa)) 0 else 1)
}

def leftFoldConsistentWithFoldMap[A, B](
fa: F[A],
f: A => B
Expand Down Expand Up @@ -111,6 +121,25 @@ trait FoldableLaws[F[_]] extends UnorderedFoldableLaws[F] {
def orderedConsistency[A: Eq](x: F[A], y: F[A])(implicit ev: Eq[F[A]]): IsEq[List[A]] =
if (x === y) (F.toList(x) <-> F.toList(y))
else List.empty[A] <-> List.empty[A]

def existsLazy[A](fa: F[A]): Boolean = {
var i = 0
F.exists(fa) { _ =>
i = i + 1
true
}
i == (if (F.isEmpty(fa)) 0 else 1)
}

def forallLazy[A](fa: F[A]): Boolean = {
var i = 0
F.forall(fa) { _ =>
i = i + 1
false
}
i == (if (F.isEmpty(fa)) 0 else 1)
}

}

object FoldableLaws {
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18 changes: 0 additions & 18 deletions laws/src/main/scala/cats/laws/UnorderedFoldableLaws.scala
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Expand Up @@ -21,24 +21,6 @@ trait UnorderedFoldableLaws[F[_]] {
(F.isEmpty(fa) || F.exists(fa)(p))
} else true // can't test much in this case

def existsLazy[A](fa: F[A]): Boolean = {
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Foldable extends UnorderedFoldable. To me, the bug here is that we weren’t calling these in UnorderedFoldable tests and that FoldableLaws were not calling through to them as we should do any time we extend a typeclass.

That seems like the better fix to me than moving the law (since we should still want these laws on UnorderedFoldable).

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It does look that way but actually the default implementations of exist and forall in UnorderedFoldable do not obey these laws. These laws can only be supported in Foldable through foldRight, hence they should be in Foldable

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We can keep the laws in UnorderedFoldableTests if we implement the CommutativeMonoid[Eval[Boolean]]s directly in UnorderedFoldable. See rossabaker@4af246d.

var i = 0
F.exists(fa) { _ =>
i = i + 1
true
}
i == (if (F.isEmpty(fa)) 0 else 1)
}

def forallLazy[A](fa: F[A]): Boolean = {
var i = 0
F.forall(fa) { _ =>
i = i + 1
false
}
i == (if (F.isEmpty(fa)) 0 else 1)
}

/**
* If `F[A]` is empty, forall must return true.
*/
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1 change: 1 addition & 0 deletions laws/src/main/scala/cats/laws/discipline/FoldableTests.scala
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Expand Up @@ -27,6 +27,7 @@ trait FoldableTests[F[_]] extends UnorderedFoldableTests[F] {
parent = Some(unorderedFoldable[A, B]),
"foldLeft consistent with foldMap" -> forAll(laws.leftFoldConsistentWithFoldMap[A, B] _),
"foldRight consistent with foldMap" -> forAll(laws.rightFoldConsistentWithFoldMap[A, B] _),
"foldRight is lazy" -> forAll(laws.foldRightLazy[A] _),
"ordered constistency" -> forAll(laws.orderedConsistency[A] _),
"exists consistent with find" -> forAll(laws.existsConsistentWithFind[A] _),
"exists is lazy" -> forAll(laws.existsLazy[A] _),
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