Enhance EitherT and fix typelevel#2161: add MonadError.redeem/redeemWith

Author: alexandru

build.sbt

@@ -49,6 +49,13 @@ lazy val commonSettings = Seq(
   scalacOptions in (Compile, doc) := (scalacOptions in (Compile, doc)).value.filter(_ != "-Xfatal-warnings"),
   ivyConfigurations += CompileTime,
   unmanagedClasspath in Compile ++= update.value.select(configurationFilter(CompileTime.name)),
+  unmanagedSourceDirectories in Compile ++= {
+    val bd = baseDirectory.value
+    if (CrossVersion.partialVersion(scalaVersion.value) exists (_._2 >= 12))
+      CrossType.Pure.sharedSrcDir(bd, "main").toList map (f => file(f.getPath + "-2.12+"))
+    else
+      CrossType.Pure.sharedSrcDir(bd, "main").toList map (f => file(f.getPath + "-2.11-"))
+  },
   unmanagedSourceDirectories in Test ++= {
     val bd = baseDirectory.value
     if (CrossVersion.partialVersion(scalaVersion.value) exists (_._2 >= 11))

core/src/main/scala-2.11-/cats/internals/FutureShims.scala

@@ -0,0 +1,20 @@
+package cats.internals
+
+import scala.concurrent.{ExecutionContext, Future}
+
+private[cats] object FutureShims {
+  /**
+   * Exposes the new `Future#transformWith` from Scala 2.12 in a way
+   * that's compatible with Scala 2.11.
+   */
+  def redeemWith[A, B](fa: Future[A])(fe: Throwable => Future[B], fs: A => Future[B])
+    (implicit ec: ExecutionContext): Future[B] =
+    attempt(fa).flatMap(_.fold(fe, fs))
+
+  /**
+   * Exposes an optimized `attempt` for `Future` whose implementation
+   * depends on the Scala version.
+   */
+  def attempt[A](fa: Future[A])(implicit ec: ExecutionContext): Future[Either[Throwable, A]] =
+    fa.map(Right[Throwable, A]).recover { case e => Left(e) }
+}

core/src/main/scala-2.12+/cats/internals/FutureShims.scala

@@ -0,0 +1,30 @@
+package cats.internals
+
+import scala.concurrent.{ExecutionContext, Future}
+import scala.util.{Failure, Success}
+
+private[cats] object FutureShims {
+  /**
+   * Exposes the new `Future#transformWith` from Scala 2.12 in a way
+   * that's compatible with Scala 2.11.
+   */
+  def redeemWith[A, B](fa: Future[A])(fe: Throwable => Future[B], fs: A => Future[B])
+    (implicit ec: ExecutionContext): Future[B] = {
+
+    fa.transformWith {
+      case Success(a) => fs(a)
+      case Failure(e) => fe(e)
+    }
+  }
+
+  /**
+   * Exposes an optimized `attempt` for `Future` whose implementation
+   * depends on the Scala version.
+   */
+  def attempt[A](fa: Future[A])(implicit ec: ExecutionContext): Future[Either[Throwable, A]] =
+    fa.transformWith(r => Future.successful(
+      r match {
+        case Success(a) => Right(a)
+        case Failure(e) => Left(e)
+      }))
+}

core/src/main/scala/cats/ApplicativeError.scala

@@ -3,6 +3,7 @@ package cats
 import cats.data.EitherT
 import scala.util.{ Failure, Success, Try }
 import scala.util.control.NonFatal
+import cats.internals.EitherUtil.rightBox
 
 /**
  * An applicative that also allows you to raise and or handle an error value.
@@ -62,9 +63,8 @@ trait ApplicativeError[F[_], E] extends Applicative[F] {
    *
    * All non-fatal errors should be handled by this method.
    */
-  def attempt[A](fa: F[A]): F[Either[E, A]] = handleErrorWith(
-    map(fa)(Right(_): Either[E, A])
-  )(e => pure(Left(e)))
+  def attempt[A](fa: F[A]): F[Either[E, A]] =
+    handleErrorWith(map(fa)(rightBox[E, A]))(e => pure(Left[E, A](e)))
 
   /**
    * Similar to [[attempt]], but wraps the result in a [[cats.data.EitherT]] for

core/src/main/scala/cats/MonadError.scala

@@ -61,6 +61,76 @@ trait MonadError[F[_], E] extends ApplicativeError[F, E] with Monad[F] {
    */
   def rethrow[A](fa: F[Either[E, A]]): F[A] =
     flatMap(fa)(_.fold(raiseError, pure))
+
+  /**
+   * Returns a new value that transforms the result of the source,
+   * given the `recover` or `map` functions, which get executed depending
+   * on whether the result is successful or if it ends in error.
+   *
+   * This is an optimization on usage of [[attempt]] and [[map]],
+   * this equivalence being available:
+   *
+   * {{{
+   *   fa.redeem(fe, fs) <-> fa.attempt.map(_.fold(fe, fs))
+   * }}}
+   *
+   * Usage of `redeem` subsumes [[handleError]] because:
+   *
+   * {{{
+   *   fa.redeem(fe, id) <-> fa.handleError(fe)
+   * }}}
+   *
+   * Implementations are free to override it in order to optimize
+   * error recovery.
+   *
+   * @see [[redeemWith]], [[attempt]] and [[handleError]]
+   *
+   * @param fa is the source whose result is going to get transformed
+   * @param recover is the function that gets called to recover the source
+   *        in case of error
+   * @param map is the function that gets to transform the source
+   *        in case of success
+   */
+  def redeem[A, B](fa: F[A])(recover: E => B, map: A => B): F[B] =
+    redeemWith(fa)(recover.andThen(pure), map.andThen(pure))
+
+  /**
+   * Returns a new value that transforms the result of the source,
+   * given the `recover` or `bind` functions, which get executed depending
+   * on whether the result is successful or if it ends in error.
+   *
+   * This is an optimization on usage of [[attempt]] and [[flatMap]],
+   * this equivalence being available:
+   *
+   * {{{
+   *   fa.redeemWith(fe, fs) <-> fa.attempt.flatMap(_.fold(fe, fs))
+   * }}}
+   *
+   * Usage of `redeemWith` subsumes [[handleErrorWith]] because:
+   *
+   * {{{
+   *   fa.redeemWith(fe, F.pure) <-> fa.handleErrorWith(fe)
+   * }}}
+   *
+   * Usage of `redeemWith` also subsumes [[flatMap]] because:
+   *
+   * {{{
+   *   fa.redeemWith(F.raiseError, fs) <-> fa.flatMap(fs)
+   * }}}
+   *
+   * Implementations are free to override it in order to optimize
+   * error recovery.
+   *
+   * @see [[redeem]], [[attempt]] and [[handleErrorWith]]
+   *
+   * @param fa is the source whose result is going to get transformed
+   * @param recover is the function that gets called to recover the source
+   *        in case of error
+   * @param bind is the function that gets to transform the source
+   *        in case of success
+   */
+  def redeemWith[A, B](fa: F[A])(recover: E => F[B], bind: A => F[B]): F[B] =
+    flatMap(attempt(fa))(_.fold(recover, bind))
 }
 
 object MonadError {

core/src/main/scala/cats/Parallel.scala

@@ -78,30 +78,32 @@ trait Parallel[M[_], F[_]] extends NonEmptyParallel[M, F] {
     */
   def applicativeError[E](implicit E: MonadError[M, E]): ApplicativeError[F, E] = new ApplicativeError[F, E] {
 
-    def raiseError[A](e: E): F[A] =
+    override def raiseError[A](e: E): F[A] =
       parallel(MonadError[M, E].raiseError(e))
 
-    def handleErrorWith[A](fa: F[A])(f: (E) => F[A]): F[A] = {
+    override def handleErrorWith[A](fa: F[A])(f: (E) => F[A]): F[A] = {
       val ma = MonadError[M, E].handleErrorWith(sequential(fa))(f andThen sequential.apply)
       parallel(ma)
     }
 
-    def pure[A](x: A): F[A] = applicative.pure(x)
-
-    def ap[A, B](ff: F[(A) => B])(fa: F[A]): F[B] = applicative.ap(ff)(fa)
-
-    override def map[A, B](fa: F[A])(f: (A) => B): F[B] = applicative.map(fa)(f)
-
-    override def product[A, B](fa: F[A], fb: F[B]): F[(A, B)] = applicative.product(fa, fb)
-
-    override def map2[A, B, Z](fa: F[A], fb: F[B])(f: (A, B) => Z): F[Z] = applicative.map2(fa, fb)(f)
-
+    override def pure[A](x: A): F[A] =
+      applicative.pure(x)
+    override def ap[A, B](ff: F[(A) => B])(fa: F[A]): F[B] =
+      applicative.ap(ff)(fa)
+    override def attempt[A](fa: F[A]): F[Either[E, A]] =
+      parallel(MonadError[M, E].attempt(sequential(fa)))
+    override def map[A, B](fa: F[A])(f: (A) => B): F[B] =
+      applicative.map(fa)(f)
+    override def product[A, B](fa: F[A], fb: F[B]): F[(A, B)] =
+      applicative.product(fa, fb)
+    override def map2[A, B, Z](fa: F[A], fb: F[B])(f: (A, B) => Z): F[Z] =
+      applicative.map2(fa, fb)(f)
     override def map2Eval[A, B, Z](fa: F[A], fb: Eval[F[B]])(f: (A, B) => Z): Eval[F[Z]] =
       applicative.map2Eval(fa, fb)(f)
-
-    override def unlessA[A](cond: Boolean)(f: => F[A]): F[Unit] = applicative.unlessA(cond)(f)
-
-    override def whenA[A](cond: Boolean)(f: => F[A]): F[Unit] = applicative.whenA(cond)(f)
+    override def unlessA[A](cond: Boolean)(f: => F[A]): F[Unit] =
+      applicative.unlessA(cond)(f)
+    override def whenA[A](cond: Boolean)(f: => F[A]): F[Unit] =
+      applicative.whenA(cond)(f)
   }
 }