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Create Empty from EmptyK #4714

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Create Empty from EmptyK #4714

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8 changes: 5 additions & 3 deletions alleycats-core/src/main/scala/alleycats/Empty.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -38,8 +38,6 @@ object Empty extends EmptyInstances0 {
def apply[A](a: => A): Empty[A] =
new Empty[A] { lazy val empty: A = a }

def fromEmptyK[F[_], T](implicit ekf: EmptyK[F]): Empty[F[T]] = ekf.synthesize[T]

/**
* Summon an instance of [[Empty]] for `A`.
*/
Expand Down Expand Up @@ -81,8 +79,12 @@ object Empty extends EmptyInstances0 {

private[alleycats] trait EmptyInstances0 extends compat.IterableEmptyInstance with EmptyInstances1

private[alleycats] trait EmptyInstances1 {
private[alleycats] trait EmptyInstances1 extends EmptyInstances2 {
// If Monoid extended Empty then this could be an exported subclass instance provided by Monoid
implicit def monoidIsEmpty[A: Monoid]: Empty[A] =
Empty(Monoid[A].empty)
}

private[alleycats] trait EmptyInstances2 {
implicit def fromEmptyK[F[_], T](implicit ekf: EmptyK[F]): Empty[F[T]] = ekf.synthesize[T]
}
12 changes: 12 additions & 0 deletions alleycats-core/src/test/scala/alleycats/SyntaxSuite.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -54,6 +54,18 @@ object SyntaxSuite {
val a1: Boolean = x.nonEmpty
}

def testOptionEmpty: Unit = {
case class A[T](x: T)
case class B()

import alleycats.std.option.*
implicit def emptyA[T: Empty]: Empty[A[T]] = new Empty[A[T]] {
def empty: A[T] = A(Empty[T].empty)
}

Empty[A[Option[B]]].empty
}

def testFoldable[F[_]: Foldable, A]: Unit = {
val x = mock[F[A]]
val y = mock[A => Unit]
Expand Down