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Add EitherT.liftAttemptK #3374

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Mar 31, 2020
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Add EitherT.liftAttemptK #3374

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19 changes: 19 additions & 0 deletions core/src/main/scala/cats/data/EitherT.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -742,6 +742,25 @@ object EitherT extends EitherTInstances {
final def liftK[F[_], A](implicit F: Functor[F]): F ~> EitherT[F, A, *] =
new (F ~> EitherT[F, A, *]) { def apply[B](fb: F[B]): EitherT[F, A, B] = right(fb) }

/**
* Lifts an effect into EitherT, catching all errors from the effect and lifting them into EitherT's error channel.
*
* {{{
* scala> import cats._, data._, implicits._
* scala> val a: Option[Int] = None
* scala> val b: EitherT[Option, Unit, Int] = EitherT.liftAttemptK[Option, Unit].apply(a)
* scala> b.value
* res0: Option[Either[Unit, Int]] = Some(Left(()))
*
* scala> val a2: Option[Int] = Some(42)
* scala> val b2: EitherT[Option, Unit, Int] = EitherT.liftAttemptK[Option, Unit].apply(a2)
* scala> b2.value
* res1: Option[Either[Unit, Int]] = Some(Right(42))
* }}}
*/
final def liftAttemptK[F[_], E](implicit F: ApplicativeError[F, E]): F ~> EitherT[F, E, *] =
λ[F ~> EitherT[F, E, *]](fa => EitherT(F.attempt(fa)))
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We've been removing the value-level kind-projector syntax for polymorphic functions, since there's no real possibility of it ever cross-building on Dotty, and since writing this out explicitly as:

new (F ~> EitherT[F, E, *]) { def apply[A](fa: F[A]): EitherT[F, E, A] = EitherT(F.attempt(fa)) }

…is only marginally less readable. I don't think that needs to be changed in this PR, though, since we're not actually cross-building on Dotty yet.

(Note that the type-level λ is fine on Dotty with -Ykind-projector.)

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oh, I was under the impression this was also part of the -Ykind-projector feature. But it makes sense, I'll keep it in mind.

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@kubukoz No, I hate kind-projector's value-level lambda syntax and couldn't be bothered to implement it in -Ykind-projector. 😄 (Also it would have required much more disruptive changes than anything else that made it into -Ykind-projector.)


@deprecated("Use EitherT.liftF.", "1.0.0-RC1")
final def liftT[F[_], A, B](fb: F[B])(implicit F: Functor[F]): EitherT[F, A, B] = right(fb)

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