scalaz.MaybeT.scala Maven / Gradle / Ivy
package scalaz
/**
* monad transformer for Maybe
*/
final case class MaybeT[F[_], A](run: F[Maybe[A]]) {
self =>
import Maybe._
def map[B](f: A => B)(implicit F: Functor[F]): MaybeT[F, B] = new MaybeT[F, B](mapO(_ map f))
def mapT[G[_], B](f: F[Maybe[A]] => G[Maybe[B]]): MaybeT[G, B] =
MaybeT(f(run))
def flatMap[B](f: A => MaybeT[F, B])(implicit F: Monad[F]): MaybeT[F, B] = new MaybeT[F, B](
F.bind(self.run)(_.cata(f(_).run, F.point(empty)))
)
def flatMapF[B](f: A => F[B])(implicit F: Monad[F]): MaybeT[F, B] = new MaybeT[F, B](
F.bind(self.run)(_.cata((a => F.map(f(a))(just)), F.point(empty)))
)
def foldRight[Z](z: => Z)(f: (A, => Z) => Z)(implicit F: Foldable[F]): Z = {
F.foldRight[Maybe[A], Z](run, z)((a, b) => Foldable[Maybe].foldRight[A, Z](a, b)(f))
}
def traverse[G[_], B](f: A => G[B])(implicit F: Traverse[F], G: Applicative[G]): G[MaybeT[F, B]] = {
G.map(F.traverse(run)(o => Traverse[Maybe].traverse(o)(f)))(MaybeT.apply)
}
def ap[B](f: => MaybeT[F, A => B])(implicit F: Monad[F]): MaybeT[F, B] =
MaybeT(F.bind(f.run)(_.cata(ff => F.map(run)(_ map ff), F.point(empty))))
/** Apply a function in the environment of both maybes, containing
* both `F`s. It is not compatible with `Monad#bind`.
*/
def app[B](f: => MaybeT[F, A => B])(implicit F: Apply[F]): MaybeT[F, B] =
MaybeT(F.apply2(f.run, run) {
case (ff, aa) => maybeInstance.ap(aa)(ff)
})
def isJust(implicit F: Functor[F]): F[Boolean] = mapO(_.isJust)
def isDefined(implicit F: Functor[F]): F[Boolean] = mapO(_.isJust)
def isEmpty(implicit F: Functor[F]): F[Boolean] = mapO(_.isEmpty)
def filter(f: A => Boolean)(implicit F: Functor[F]): MaybeT[F, A] = MaybeT(F.map(self.run) { _ filter f })
def cata[X](just: A => X, empty: => X)(implicit F: Functor[F]): F[X] = mapO(_.cata(just, empty))
def getOrElse(default: => A)(implicit F: Functor[F]): F[A] = mapO(_.getOrElse(default))
/** Alias for `getOrElse`. */
def |(default: => A)(implicit F: Functor[F]): F[A] =
getOrElse(default)
def getOrElseF(default: => F[A])(implicit F: Monad[F]): F[A] =
F.bind(self.run)(_.cata(F.point(_), default))
def orZero(implicit F0: Functor[F], M0: Monoid[A]): F[A] = getOrElse(M0.zero)
def unary_~(implicit F0: Functor[F], M0: Monoid[A]): F[A] = orZero
def exists(f: A => Boolean)(implicit F: Functor[F]): F[Boolean] = mapO(_.exists(f))
def forall(f: A => Boolean)(implicit F: Functor[F]): F[Boolean] = mapO(_.forall(f))
def orElse(a: => MaybeT[F, A])(implicit F: Monad[F]): MaybeT[F, A] =
MaybeT(F.bind(run)(_.cata(a => F.point(just(a)), a.run)))
def |||(a: => MaybeT[F, A])(implicit F: Monad[F]): MaybeT[F, A] =
orElse(a)
def toRight[E](e: => E)(implicit F: Functor[F]): EitherT[F,E,A] = EitherT(F.map(run)(_.toRight(e)))
def toLeft[B](b: => B)(implicit F: Functor[F]): EitherT[F,A,B] = EitherT(F.map(run)(_.toLeft(b)))
private def mapO[B](f: Maybe[A] => B)(implicit F: Functor[F]) = F.map(run)(f)
}
//
// Prioritized Implicits for type class instances
//
sealed abstract class MaybeTInstances3 {
implicit def maybeTFunctor[F[_]](implicit F0: Functor[F]): Functor[MaybeT[F, ?]] =
new MaybeTFunctor[F] {
implicit def F: Functor[F] = F0
}
}
sealed abstract class MaybeTInstances2 extends MaybeTInstances3 {
implicit def maybeTMonadError[F[_], E](implicit F0: MonadError[F, E]): MonadError[MaybeT[F, ?], E] =
new MaybeTMonadError[F, E] {
def F = F0
}
}
sealed abstract class MaybeTInstances1 extends MaybeTInstances2 {
implicit def maybeTFoldable[F[_]](implicit F0: Foldable[F]): Foldable[MaybeT[F, ?]] =
new MaybeTFoldable[F] {
implicit def F: Foldable[F] = F0
}
implicit def maybeTBindRec[F[_]](implicit F0: BindRec[F], F1: Monad[F]): BindRec[MaybeT[F, ?]] =
new MaybeTBindRec[F] {
implicit def B: BindRec[F] = F0
implicit def F: Monad[F] = F1
}
}
sealed abstract class MaybeTInstances0 extends MaybeTInstances1 {
implicit def maybeTMonadPlus[F[_]](implicit F0: Monad[F]): MonadPlus[MaybeT[F, ?]] =
new MaybeTMonadPlus[F] {
implicit def F: Monad[F] = F0
}
}
sealed abstract class MaybeTInstances extends MaybeTInstances0 {
implicit val maybeTMonadTrans: Hoist[MaybeT] = new MaybeTHoist {}
implicit def maybeTTraverse[F[_]](implicit F0: Traverse[F]): Traverse[MaybeT[F, ?]] =
new MaybeTTraverse[F] {
implicit def F: Traverse[F] = F0
}
implicit def maybeTEqual[F[_], A](implicit F0: Equal[F[Maybe[A]]]): Equal[MaybeT[F, A]] =
F0.contramap((_: MaybeT[F, A]).run)
}
object MaybeT extends MaybeTInstances {
def maybeT[M[_]] =
λ[λ[α => M[Maybe[α]]] ~> MaybeT[M, ?]](
new MaybeT(_)
)
def just[M[_], A](v: => A)(implicit M: Applicative[M]): MaybeT[M, A] =
MaybeT.maybeT[M].apply[A](M.point(Maybe.just(v)))
def empty[M[_], A](implicit M: Applicative[M]): MaybeT[M, A] =
MaybeT.maybeT[M].apply[A](M.point(Maybe.empty))
def monadTell[F[_], W, A](implicit MT0: MonadTell[F, W]): MonadTell[MaybeT[F, ?], W] =
new MaybeTMonadTell[F, W] {
def MT = MT0
}
def monadListen[F[_], W, A](implicit ML0: MonadListen[F, W]): MonadListen[MaybeT[F, ?], W] =
new MaybeTMonadListen[F, W] {
def MT = ML0
}
}
//
// Implementation traits for type class instances
//
private trait MaybeTFunctor[F[_]] extends Functor[MaybeT[F, ?]] {
implicit def F: Functor[F]
override final def map[A, B](fa: MaybeT[F, A])(f: A => B): MaybeT[F, B] = fa map f
}
private trait MaybeTMonad[F[_]] extends Monad[MaybeT[F, ?]] {
implicit def F: Monad[F]
override final def ap[A, B](fa: => MaybeT[F, A])(f: => MaybeT[F, A => B]): MaybeT[F, B] = fa ap f
final def point[A](a: => A): MaybeT[F, A] = MaybeT[F, A](F.point(Maybe.just(a)))
final def bind[A, B](fa: MaybeT[F, A])(f: A => MaybeT[F, B]): MaybeT[F, B] = fa flatMap f
}
private trait MaybeTBindRec[F[_]] extends BindRec[MaybeT[F, ?]] with MaybeTMonad[F] {
implicit def B: BindRec[F]
final def tailrecM[A, B](a: A)(f: A => MaybeT[F, A \/ B]): MaybeT[F, B] =
MaybeT(
B.tailrecM[A, Maybe[B]](a)(a => F.map(f(a).run) {
_.cata(_.map(Maybe.just), \/-(Maybe.empty))
})
)
}
private trait MaybeTFoldable[F[_]] extends Foldable.FromFoldr[MaybeT[F, ?]] {
implicit def F: Foldable[F]
override def foldRight[A, B](fa: MaybeT[F, A], z: => B)(f: (A, => B) => B): B = fa.foldRight(z)(f)
}
private trait MaybeTTraverse[F[_]] extends Traverse[MaybeT[F, ?]] with MaybeTFoldable[F] with MaybeTFunctor[F]{
implicit def F: Traverse[F]
def traverseImpl[G[_] : Applicative, A, B](fa: MaybeT[F, A])(f: A => G[B]): G[MaybeT[F, B]] = fa traverse f
}
private trait MaybeTHoist extends Hoist[MaybeT] {
def liftM[G[_], A](a: G[A])(implicit G: Monad[G]): MaybeT[G, A] =
MaybeT[G, A](G.map[A, Maybe[A]](a)((a: A) => Maybe.just(a)))
def hoist[M[_]: Monad, N[_]](f: M ~> N) =
λ[MaybeT[M, ?] ~> MaybeT[N, ?]](_ mapT f)
implicit def apply[G[_] : Monad]: Monad[MaybeT[G, ?]] =
MaybeT.maybeTMonadPlus[G]
}
private trait MaybeTMonadPlus[F[_]] extends MonadPlus[MaybeT[F, ?]] with MaybeTMonad[F] {
implicit def F: Monad[F]
def empty[A]: MaybeT[F, A] = MaybeT(F point Maybe.empty)
def plus[A](a: MaybeT[F, A], b: => MaybeT[F, A]): MaybeT[F, A] = a orElse b
}
private trait MaybeTMonadError[F[_], E] extends MonadError[MaybeT[F, ?], E] with MaybeTMonad[F] {
override def F: MonadError[F, E]
override def raiseError[A](e: E) =
MaybeT[F, A](F.map(F.raiseError[A](e))(Maybe.just))
override def handleError[A](fa: MaybeT[F, A])(f: E => MaybeT[F, A]) =
MaybeT[F, A](F.handleError(fa.run)(f(_).run))
}
private trait MaybeTMonadTell[F[_], W] extends MonadTell[MaybeT[F, ?], W] with MaybeTMonad[F] with MaybeTHoist {
def MT: MonadTell[F, W]
implicit def F = MT
def writer[A](w: W, v: A): MaybeT[F, A] =
liftM[F, A](MT.writer(w, v))
}
private trait MaybeTMonadListen[F[_], W] extends MonadListen[MaybeT[F, ?], W] with MaybeTMonadTell[F, W] {
def MT: MonadListen[F, W]
def listen[A](ma: MaybeT[F, A]): MaybeT[F, (A, W)] = {
val tmp = MT.bind[(Maybe[A], W), Maybe[(A, W)]](MT.listen(ma.run)) {
case (m, w) => m.cata(j => MT.point(Maybe.just((j, w))), MT.point(Maybe.empty))
}
MaybeT.maybeT[F].apply[(A, W)](tmp)
}
}
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