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Weaken some Alternative constraints in OneAnd #4739

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Merged
satorg merged 5 commits into from
Feb 27, 2026
+27 −6
Merged

Weaken some Alternative constraints in OneAnd #4739

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585e755
Weaken some `Alternative` constraints in `OneAnd`
kory33 Apr 20, 2025
c2d1444
Fix bincompat errors
kory33 Apr 20, 2025
af23366
Make `OneAndBinCompat0` sealed and make use of self-type annotation
kory33 May 8, 2025
175dd2a
Strip off deprecated annotation and make `OneAndInstancesBinCompat0` …
kory33 May 8, 2025
e059ed8
Let superseded methods invoke new implementation through `self`
kory33 May 8, 2025
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39 changes: 33 additions & 6 deletions core/src/main/scala/cats/data/OneAnd.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -34,12 +34,12 @@ import kernel.compat.scalaVersionSpecific.*
* type NonEmptyStream[A] = OneAnd[Stream, A]
* }}}
*/
final case class OneAnd[F[_], A](head: A, tail: F[A]) {
final case class OneAnd[F[_], A](head: A, tail: F[A]) extends OneAndBinCompat0[F, A] {

/**
* Combine the head and tail into a single `F[A]` value.
*/
def unwrap(implicit F: Alternative[F]): F[A] =
def unwrap(implicit F: NonEmptyAlternative[F]): F[A] =
F.prependK(head, tail)

/**
Expand All @@ -53,7 +53,7 @@ final case class OneAnd[F[_], A](head: A, tail: F[A]) {
/**
* Append another OneAnd to this
*/
def combine(other: OneAnd[F, A])(implicit F: Alternative[F]): OneAnd[F, A] =
def combine(other: OneAnd[F, A])(implicit F: NonEmptyAlternative[F]): OneAnd[F, A] =
OneAnd(head, F.combineK(tail, other.unwrap))

/**
Expand Down Expand Up @@ -123,8 +123,21 @@ final case class OneAnd[F[_], A](head: A, tail: F[A]) {
s"OneAnd(${A.show(head)}, ${FA.show(tail)})"
}

private[data] trait OneAndBinCompat0[F[_], A] {

@kory33 kory33 Apr 20, 2025

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[question] Is this the right way to pass the MiMa check for a case class modification?

I saw #3997 (comment) and thought that this was the (only) way, but I could not find any other place that defines a similar bincompat trait (other than ValidatedFunctionsBinCompat0, which patches the companion rather than the Validated class itself).

@johnynek johnynek May 7, 2025

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can we make this sealed? I think so.

@kory33 kory33 May 8, 2025

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Thanks, I made it sealed : af23366

val head: A
val tail: F[A]

@satorg satorg May 7, 2025 •
edited
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Abstract vals look suspicious a bit. And might not be necessary – since OneAndBinCompat0 is supposed to be extended by OneAnd only, a trick with self-type annotation might work here:

private[data] trait OneAndBinCompat0[F[_], A] {
  self: OneAnd[F, A] => // allow access to `head` and `tail` from withing this trait
    // no abstract `head` and `tail` should be necessary

@kory33 kory33 May 8, 2025

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For some reason I've never used a non-trait type for self-type annotation and couldn't come up with this!
Applied: af23366


@deprecated("Kept for binary compatibility", "2.14.0")

@kory33 kory33 Apr 20, 2025

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[question] Is since = "2.14.0" specification appropriate (is that the next cats version)? Or do I just not need @deprecated at all since it's private[data]?

@johnynek johnynek May 7, 2025

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personally I wouldn't add the deprecated and just add a comment.

@satorg satorg May 7, 2025

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If a method is superceded by some other method with the same name, we usually not only make it package local, but also remove all implicit keywords from its definition, e.g.:

private[data] def combine(other: OneAnd[F, A])(F: Alternative[F]): OneAnd[F, A] = ...

which makes it less likely to interfere with the new method while preserving binary compatibility.

I personally don't mind marking it @deprecated but don't have strong opinion on that – it is not supposed to be called anyway.

private[data] def unwrap(implicit F: Alternative[F]): F[A] =
F.prependK(head, tail)

@deprecated("Kept for binary compatibility", "2.14.0")
private[data] def combine(other: OneAnd[F, A])(implicit F: Alternative[F]): OneAnd[F, A] =
OneAnd(head, F.combineK(tail, other.unwrap))
}

@suppressUnusedImportWarningForScalaVersionSpecific
sealed abstract private[data] class OneAndInstances extends OneAndLowPriority0 {
sealed abstract private[data] class OneAndInstances extends OneAndLowPriority0 with OneAndInstancesBinCompat0 {

implicit def catsDataParallelForOneAnd[A, M[_]: Alternative, F0[_]: Alternative](implicit
P: Parallel.Aux[M, F0]
Expand Down Expand Up @@ -158,13 +171,13 @@ sealed abstract private[data] class OneAndInstances extends OneAndLowPriority0 {

implicit def catsDataShowForOneAnd[A, F[_]](implicit A: Show[A], FA: Show[F[A]]): Show[OneAnd[F, A]] = _.show

implicit def catsDataSemigroupKForOneAnd[F[_]: Alternative]: SemigroupK[OneAnd[F, *]] =
implicit def catsDataSemigroupKForOneAnd[F[_]: NonEmptyAlternative]: SemigroupK[OneAnd[F, *]] =
new SemigroupK[OneAnd[F, *]] {
def combineK[A](a: OneAnd[F, A], b: OneAnd[F, A]): OneAnd[F, A] =
a.combine(b)
}

implicit def catsDataSemigroupForOneAnd[F[_]: Alternative, A]: Semigroup[OneAnd[F, A]] =
implicit def catsDataSemigroupForOneAnd[F[_]: NonEmptyAlternative, A]: Semigroup[OneAnd[F, A]] =
catsDataSemigroupKForOneAnd[F].algebra

implicit def catsDataMonadForOneAnd[F[_]](implicit
Expand Down Expand Up @@ -331,4 +344,18 @@ sealed abstract private[data] class OneAndLowPriority0 extends OneAndLowPriority
}
}

private[data] trait OneAndInstancesBinCompat0 {

@deprecated("Kept for binary compatibility", "2.14.0")
def catsDataSemigroupKForOneAnd[F[_]: Alternative]: SemigroupK[OneAnd[F, *]] =
new SemigroupK[OneAnd[F, *]] {
def combineK[A](a: OneAnd[F, A], b: OneAnd[F, A]): OneAnd[F, A] =
a.combine(b)
}

@deprecated("Kept for binary compatibility", "2.14.0")
def catsDataSemigroupForOneAnd[F[_]: Alternative, A]: Semigroup[OneAnd[F, A]] =
catsDataSemigroupKForOneAnd[F].algebra
}

object OneAnd extends OneAndInstances