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Make Id covariant #3264

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Make Id covariant #3264

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f5913dd
Make Id covariant
travisbrown Jan 21, 2020
07ea952
Use non-kind-projector type lambda syntax to get variance right for D…
travisbrown Jan 21, 2020
c68351f
Simplify
travisbrown Jan 21, 2020
540ea50
Add link to Scala PR with variance fix for Id
travisbrown Jan 23, 2020
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36 changes: 25 additions & 11 deletions core/src/main/scala/cats/package.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -31,11 +31,23 @@ package object cats {
* type `A` to get a pure value of type `B`. That is, the instance
* encodes pure unary function application.
*/
type Id[A] = A
type Id[+A] = A

// Workaround for a compiler bug that should be fixed soon.
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now that the PR on scala is in, can we refer to it here?
scala/scala#8651

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@kailuowang Yes, good idea (I linked it in the PR description but the comment would be more helpful).

// See https://github.com/scala/scala/pull/8651 for details.
private type IdWrapper = { type L[+A] = A }

type Endo[A] = A => A
implicit val catsInstancesForId
: Bimonad[Id] with CommutativeMonad[Id] with Comonad[Id] with NonEmptyTraverse[Id] with Distributive[Id] =
new Bimonad[Id] with CommutativeMonad[Id] with Comonad[Id] with NonEmptyTraverse[Id] with Distributive[Id] {
implicit val catsInstancesForId: Bimonad[IdWrapper#L]
with CommutativeMonad[IdWrapper#L]
with Comonad[IdWrapper#L]
with NonEmptyTraverse[IdWrapper#L]
with Distributive[IdWrapper#L] =
new Bimonad[IdWrapper#L]
with CommutativeMonad[IdWrapper#L]
with Comonad[IdWrapper#L]
with NonEmptyTraverse[IdWrapper#L]
with Distributive[IdWrapper#L] {
def pure[A](a: A): A = a
def extract[A](a: A): A = a
def flatMap[A, B](a: A)(f: A => B): B = f(a)
Expand Down Expand Up @@ -81,16 +93,18 @@ package object cats {
/**
* Witness for: Id[A] <-> Unit => A
*/
implicit val catsRepresentableForId: Representable.Aux[Id, Unit] = new Representable[Id] {
override type Representation = Unit
override val F: Functor[Id] = Functor[Id]
implicit val catsRepresentableForId: Representable.Aux[IdWrapper#L, Unit] =
new Representable[IdWrapper#L] {
override type Representation = Unit
override val F: Functor[Id] = Functor[Id]

override def tabulate[A](f: Unit => A): Id[A] = f(())
override def tabulate[A](f: Unit => A): Id[A] = f(())

override def index[A](f: Id[A]): Unit => A = (_: Unit) => f
}
override def index[A](f: Id[A]): Unit => A = (_: Unit) => f
}

implicit val catsParallelForId: Parallel.Aux[Id, Id] = Parallel.identity
implicit val catsParallelForId: Parallel.Aux[IdWrapper#L, IdWrapper#L] =
Parallel.identity[Id]

type Eq[A] = cats.kernel.Eq[A]
type PartialOrder[A] = cats.kernel.PartialOrder[A]
Expand Down