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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Let superseded methods invoke new implementation through `self`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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36 changes: 30 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 +123,18 @@ final case class OneAnd[F[_], A](head: A, tail: F[A]) {
s"OneAnd(${A.show(head)}, ${FA.show(tail)})"
}

sealed private[data] trait OneAndBinCompat0[F[_], A] { self: OneAnd[F, A] =>
// Kept for binary compatibility
private[data] def unwrap(implicit F: Alternative[F]): F[A] =
F.prependK(head, tail)

// Kept for binary compatibility
private[data] def combine(other: OneAnd[F, A])(implicit F: Alternative[F]): OneAnd[F, A] =
OneAnd(head, F.combineK(tail, other.unwrap))
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@satorg satorg May 8, 2025

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While not a requirement, but since there's self: OneAnd in the scope and because Alternative extends NonEmptyAlternative, I wonder if it could be simplified even further:

Suggested change
private[data] def unwrap(implicit F: Alternative[F]): F[A] =
F.prependK(head, tail)
// Kept for binary compatibility
private[data] def combine(other: OneAnd[F, A])(implicit F: Alternative[F]): OneAnd[F, A] =
OneAnd(head, F.combineK(tail, other.unwrap))
private[data] def unwrap(implicit F: Alternative[F]): F[A] =
self.unwrap(F)
// Kept for binary compatibility
private[data] def combine(other: OneAnd[F, A])(F: Alternative[F]): OneAnd[F, A] =
self.combine(other)(F)

Also, I would still recommend to go without implicit parameters here because even though these methods are private[data] they are still visible to Cats' internals and may cause unexpected unexpecties in the future. Not a requirement either, though.

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@kory33 kory33 May 8, 2025

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That makes sense. I think the same applies to OneAndInstancesBinCompat0 as well, so I made a similar change to the instances trait too (e059ed8).

}

@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 +168,13 @@ 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 +341,18 @@ sealed abstract private[data] class OneAndLowPriority0 extends OneAndLowPriority
}
}

sealed private[data] trait OneAndInstancesBinCompat0 {

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

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

object OneAnd extends OneAndInstances