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Add empty to State and StateT objects #2533

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Oct 1, 2018
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Add empty to State and StateT objects #2533

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12 changes: 11 additions & 1 deletion core/src/main/scala/cats/data/IndexedStateT.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -203,7 +203,7 @@ private[data] trait CommonStateTConstructors {
IndexedStateT(s => F.pure((s, s)))
}

object IndexedStateT extends IndexedStateTInstances with CommonStateTConstructors {
object IndexedStateT extends IndexedStateTInstances with CommonStateTConstructors0 {
def apply[F[_], SA, SB, A](f: SA => F[(SB, A)])(implicit F: Applicative[F]): IndexedStateT[F, SA, SB, A] =
new IndexedStateT(F.pure(f))

Expand All @@ -223,6 +223,11 @@ object IndexedStateT extends IndexedStateTInstances with CommonStateTConstructor
IndexedStateT(_ => F.map(fsb)(s => (s, ())))
}

private[data] trait CommonStateTConstructors0 extends CommonStateTConstructors {
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Is there any particular reason to introduce this new trait as opposed to just putting this method directly into the object?

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Binary compatibility: #2533 (comment)

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To be precise, the function is not only provided on the IndexedStateT object, but also on StateT. This is the reason why it has to go into a trait in the first place.

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Oh okay that makes sense. Thanks for the explanation.

def empty[F[_], S, A](implicit A: Monoid[A], F: Applicative[F]): IndexedStateT[F, S, S, A] =
pure(A.empty)
}

private[data] abstract class StateTFunctions extends CommonStateTConstructors {
def apply[F[_], S, A](f: S => F[(S, A)])(implicit F: Applicative[F]): StateT[F, S, A] =
IndexedStateT(f)
Expand Down Expand Up @@ -299,6 +304,11 @@ private[data] abstract class StateFunctions {
*/
def pure[S, A](a: A): State[S, A] = State(s => (s, a))

/**
* Return `A`'s empty monoid value and maintain the input state.
*/
def empty[S, A](implicit A: Monoid[A]): State[S, A] = pure(A.empty)

/**
* Modify the input state and return Unit.
*/
Expand Down
2 changes: 1 addition & 1 deletion core/src/main/scala/cats/data/package.scala
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Expand Up @@ -49,7 +49,7 @@ package object data {
* context along with the `A` value.
*/
type StateT[F[_], S, A] = IndexedStateT[F, S, S, A]
object StateT extends StateTFunctions
object StateT extends StateTFunctions with CommonStateTConstructors0

type State[S, A] = StateT[Eval, S, A]
object State extends StateFunctions
Expand Down
11 changes: 11 additions & 0 deletions tests/src/test/scala/cats/tests/IndexedStateTSuite.scala
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Expand Up @@ -55,6 +55,17 @@ class IndexedStateTSuite extends CatsSuite {
}
}