/core/src/main/scala/scalaz/Monad.scala
http://github.com/scalaz/scalaz · Scala · 109 lines · 48 code · 17 blank · 44 comment · 4 complexity · c3508c9eef16e0feddb0c35c240e77cb MD5 · raw file
- package scalaz
- ////
- /**
- * Monad, an [[scalaz.Applicative]] that also supports [[scalaz.Bind]],
- * circumscribed by the monad laws.
- *
- * @see [[scalaz.Monad.MonadLaw]]
- */
- ////
- trait Monad[F[_]] extends Applicative[F] with Bind[F] { self =>
- ////
- override def map[A,B](fa: F[A])(f: A => B) = bind(fa)(a => point(f(a)))
- /**
- * Execute an action repeatedly as long as the given `Boolean` expression
- * returns `true`. The condition is evaluated before the loop body.
- * Collects the results into an arbitrary `MonadPlus` value, such as a `List`.
- */
- def whileM[G[_], A](p: F[Boolean], body: => F[A])(implicit G: MonadPlus[G]): F[G[A]] = {
- val f = Need(body)
- ifM(p, bind(f.value)(x => map(whileM(p, f.value))(xs => G.plus(G.point(x), xs))), point(G.empty))
- }
- /**
- * Execute an action repeatedly as long as the given `Boolean` expression
- * returns `true`. The condition is evaluated before the loop body.
- * Discards results.
- */
- def whileM_[A](p: F[Boolean], body: => F[A]): F[Unit] = {
- val f = Need(body)
- ifM(p, bind(f.value)(_ => whileM_(p, f.value)), point(()))
- }
- /**
- * Execute an action repeatedly until the `Boolean` condition returns `true`.
- * The condition is evaluated after the loop body. Collects results into an
- * arbitrary `MonadPlus` value, such as a `List`.
- */
- def untilM[G[_], A](f: F[A], cond: => F[Boolean])(implicit G: MonadPlus[G]): F[G[A]] = {
- val p = Need(cond)
- bind(f)(x => map(whileM(map(p.value)(!_), f))(xs => G.plus(G.point(x), xs)))
- }
- /**
- * Execute an action repeatedly until the `Boolean` condition returns `true`.
- * The condition is evaluated after the loop body. Discards results.
- */
- def untilM_[A](f: F[A], cond: => F[Boolean]): F[Unit] = {
- val p = Need(cond)
- bind(f)(_ => whileM_(map(p.value)(!_), f))
- }
- /**
- * Execute an action repeatedly until its result fails to satisfy the given predicate
- * and return that result, discarding all others.
- */
- def iterateWhile[A](f: F[A])(p: A => Boolean): F[A] =
- bind(f)(y => if (p(y)) iterateWhile(f)(p) else point(y))
- /**
- * Execute an action repeatedly until its result satisfies the given predicate
- * and return that result, discarding all others.
- */
- def iterateUntil[A](f: F[A])(p: A => Boolean): F[A] =
- bind(f)(y => if (p(y)) point(y) else iterateUntil(f)(p))
- /**The product of Monad `F` and `G`, `[x](F[x], G[x]])`, is a Monad */
- def product[G[_]](implicit G0: Monad[G]): Monad[λ[α => (F[α], G[α])]] =
- new ProductMonad[F, G] {
- def F = self
- def G = G0
- }
- trait MonadLaw extends ApplicativeLaw with BindLaw {
- /** Lifted `point` is a no-op. */
- def rightIdentity[A](a: F[A])(implicit FA: Equal[F[A]]): Boolean = FA.equal(bind(a)(point(_: A)), a)
- /** Lifted `f` applied to pure `a` is just `f(a)`. */
- def leftIdentity[A, B](a: A, f: A => F[B])(implicit FB: Equal[F[B]]): Boolean = FB.equal(bind(point(a))(f), f(a))
- }
- def monadLaw = new MonadLaw {}
- ////
- val monadSyntax: scalaz.syntax.MonadSyntax[F] =
- new scalaz.syntax.MonadSyntax[F] { def F = Monad.this }
- }
- object Monad {
- @inline def apply[F[_]](implicit F: Monad[F]): Monad[F] = F
- import Isomorphism._
- def fromIso[F[_], G[_]](D: F <~> G)(implicit E: Monad[G]): Monad[F] =
- new IsomorphismMonad[F, G] {
- override def G: Monad[G] = E
- override def iso: F <~> G = D
- }
- ////
- ////
- }
- trait IsomorphismMonad[F[_], G[_]] extends Monad[F] with IsomorphismApplicative[F, G] with IsomorphismBind[F, G]{
- implicit def G: Monad[G]
- ////
- ////
- }