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Monad implementation in Scala
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object MonadInterface { | |
import scala.language.higherKinds | |
trait Functor[T[_]] { | |
def map[A, B](f: A => B)(x: T[A]): T[B] | |
} | |
trait Applicative[T[_]] extends Functor[T] { | |
def pure[A](v: A): T[A] | |
def apply[A, B](f: T[A => B])(a: T[A]): T[B] | |
} | |
trait Monad[T[_]] extends Applicative[T] { | |
def bind[A, B](m: T[A])(f: A => T[B]): T[B] | |
def join[A](m: T[T[A]]): T[A] | |
} | |
abstract class MonadOps[T[_], A](m: T[A]) { | |
def $[B](f: A => B)(implicit monad: Monad[T]): T[B] = | |
monad.map(f)(m) | |
def *[B](f: T[A => B])(implicit monad: Monad[T]): T[B] = | |
monad.apply(f)(m) | |
def >>=[B](f: A => T[B])(implicit monad: Monad[T]): T[B] = | |
monad.bind(m)(f) | |
} | |
} | |
object MaybeMonad { | |
import MonadInterface._ | |
sealed trait Maybe[+A] | |
case class Just[A](v: A) extends Maybe[A] | |
case class Empty() extends Maybe[Nothing] | |
implicit val MaybeMonad = new Monad[Maybe] { | |
def map[A, B](f: A => B)(x: Maybe[A]): Maybe[B] = { | |
x match { | |
case Just(v) => Just(f(v)) | |
case Empty() => Empty() | |
} | |
} | |
def pure[A](v: A): Maybe[A] = Just(v) | |
def apply[A, B](f: Maybe[A => B])(a: Maybe[A]): Maybe[B] = { | |
f match { | |
case Just(fn) => a match { | |
case Just(v) => Just(fn(v)) | |
case Empty() => Empty() | |
} | |
case Empty() => Empty() | |
} | |
} | |
def bind[A, B](m: Maybe[A])(f: A => Maybe[B]): Maybe[B] = { | |
m match { | |
case Just(v) => f(v) | |
case Empty() => Empty() | |
} | |
} | |
def join[A](m: Maybe[Maybe[A]]): Maybe[A] = { | |
m match { | |
case Just(v) => v | |
case Empty() => Empty() | |
} | |
} | |
} | |
implicit class MaybeMonadOps[A](m: Maybe[A]) extends MonadOps(m) {} | |
} | |
object Main extends App { | |
import MaybeMonad._ | |
assert((Just(5) $ (x => x * x)) == Just(25)) | |
assert((Empty() $ ((x: Int) => x + x)) == Empty()) | |
def f(x: Int)(y: Int)(z: Int): Int = x * y * z | |
assert(Just(5) * (Just(5) * (Just(5) $ f)) == Just(125)) | |
assert(Just(5) * (Empty() * (Just(5) $ f)) == Empty()) | |
assert((Just(3) >>= ( | |
x => Just(4) >>= ( | |
y => Just(x * y)))) == Just(12)) | |
assert((Just(3) >>= ( | |
x => Empty() >>= ( | |
(y: Int) => Just(x + y)))) == Empty()) | |
import scala.language.postfixOps | |
println(Just(5) $ (_ toString) $ (_ length)) | |
} |
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