izumi.functional.bio.DivergenceHelper.scala Maven / Gradle / Ivy
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package izumi.functional.bio
import izumi.functional.bio.DivergenceHelper.Nondivergent
import scala.annotation.nowarn
trait DivergenceHelper {
type Divergence = Nondivergent
}
object DivergenceHelper {
type Nondivergent
object Divergent {
/**
* Upcast `Divergence` parameter instead of setting
* a contradictory value as in `{ type Divergence = Divergent }`
*
* This allows a Divergent implicit to be _less_ specific
* than the non-divergent instances, while at the same time
* not matching its own requirement of `{ type Divergence = Nondivergent }`
*
* Note: Upcasting Divergence also depends on returning a type
* variable instead of a known type. `Divergent.Of[SyncSafe3[F]]`
* will not fail the Nondivergent requirement, only `Divergent.Of[C[F]]` will.
*/
type Of[A] = A { type Divergence }
@inline def apply[A <: DivergenceHelper, A1 >: A](a: A): Divergent.Of[A1] = a.asInstanceOf[Divergent.Of[A1]]
}
/**
* Nondivergent helper does not work. To make use of
* divergence protection, conversion implicits should use type variables
* for summoning, instead of the direct type itself, as in:
*
* {{{
* implicit def convert1To2[C[f[_]] <: SyncSafe1[f], F[_, _], E](implicit F: C[F[Nothing, _]] { type Divergence = Nondivergent }): Divergent.Of[C[F[E, _]]]
* }}}
*
* Where using `Nondivergent.Of[C[F[Nothing, _]]]` or `SyncSafe1[F[Nothing, _]] { type Divergence = Nondivergent }`
* both will not work.
*
* The structural refinement must be placed directly on
* the `C` type variable, not through an alias to work.
*/
@nowarn("msg=never used") private object Nondivergent {
@nowarn("msg=never used") private type Of[A] = A { type Divergence = Nondivergent }
}
}