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Don't require Order for the value on Cogen for SortedMap and NonEmptyMap #4296

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Oct 10, 2022
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Don't require Order for the value on Cogen for SortedMap and NonEmptyMap #4296

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f486ae2
Don't require Order for the value on Cogen for SortedMap and NonEmptyMap
cquiroz Sep 11, 2022
e4236f5
Preserve previous signatures for binary compatibility
cquiroz Sep 11, 2022
a704e1d
Delegate deprecated cogen methods to the new ones
cquiroz Sep 11, 2022
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5 changes: 2 additions & 3 deletions laws/src/main/scala/cats/laws/discipline/arbitrary.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -130,7 +130,7 @@ object arbitrary extends ArbitraryInstances0 with ScalaVersionSpecific.Arbitrary
a <- A.arbitrary
} yield NonEmptyMap((k, a), fa))

implicit def cogenNonEmptyMap[K: Order: Cogen, A: Order: Cogen]: Cogen[NonEmptyMap[K, A]] =
implicit def cogenNonEmptyMap[K: Order: Cogen, A: Cogen]: Cogen[NonEmptyMap[K, A]] =
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I'm confused why we even need the K: Order here. A SortedMap knows the Ordering[K].

Doesn't this also work only with K: Cogen, A: Cogen?

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@armanbilge armanbilge Sep 12, 2022 •
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I was confused about this as well :) Scalacheck implements the Cogen[Map] by transforming it to an ordered collection.

https://github.com/typelevel/scalacheck/blob/7bb1a47af2313f7e7935d34cce255f7b7c0b1b31/core/shared/src/main/scala/org/scalacheck/Cogen.scala#L129-L130

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Good catch! As a side note: in fact, there are quite a lot of places in the Cats codebase where a constraint is demanded but not actually used. So this is why I think it would be great to have "unused" warnings enforced eventually.

Cogen[SortedMap[K, A]].contramap(_.toSortedMap)
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For binary compatibility we have to keep the old signature as well, like this:

@deprecated("Preserved for bincompat", "2.9.0")
def cogenNonEmptyMap[K, A](kOrder: Order[K], kCogen: Cogen[K], aOrder: Order[A], aCogen: Coge[A]): Cogen[NonEmptyMap[K, A]]

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Do you mean copy the old signature and make it non implicit?

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yes, exactly.


implicit def catsLawsArbitraryForEitherT[F[_], A, B](implicit
Expand Down Expand Up @@ -286,9 +286,8 @@ object arbitrary extends ArbitraryInstances0 with ScalaVersionSpecific.Arbitrary
implicit def catsLawsArbitraryForSortedMap[K: Arbitrary: Order, V: Arbitrary]: Arbitrary[SortedMap[K, V]] =
Arbitrary(getArbitrary[Map[K, V]].map(s => SortedMap.empty[K, V](implicitly[Order[K]].toOrdering) ++ s))

implicit def catsLawsCogenForSortedMap[K: Order: Cogen, V: Order: Cogen]: Cogen[SortedMap[K, V]] = {
implicit def catsLawsCogenForSortedMap[K: Order: Cogen, V: Cogen]: Cogen[SortedMap[K, V]] = {
implicit val orderingK: Ordering[K] = Order[K].toOrdering
implicit val orderingV: Ordering[V] = Order[V].toOrdering

implicitly[Cogen[Map[K, V]]].contramap(_.toMap)
}
Expand Down