I'm learning scalaz these days. When I compare scalaz with Haskell, I find that it's hard to customize my own monad transformer.
The code I can do in Haskell like this:
newtype Box a = Box a deriving Show
-- Monad instance of Box, it's simple, I pass it..
newtype BoxT m a = BoxT {runBoxT :: m (Box a)}
instance (Monad m) => Monad (BoxT m) where
return a = BoxT $ return $ Box a
x >>= f = BoxT $ do
(Box v) <- runBoxT x
runBoxT $ f v
instance (Monad m) => Applicative (BoxT m) where
pure = return
(<*>) = ap
instance (Monad m) => Functor (BoxT m) where
fmap = liftM
How can I do the same thing in scalaz ? I have tried many ways, but it looks like I can not figure out the type of scala's generic, like this:
case class Box[A](value:A)
case class BoxT[M[_],A](run: M[Box[A]])
implicit val monad = Monad[BoxT] { // goes wrong, type `BoxT[M[_],A]` doesn't match F[_]
... //don't know how to override functions..
}
Thanks for @Iva Kam's comment about kind projector.
First of all, enabled kind projector in build.sbt : https://github.com/typelevel/kind-projector
addCompilerPlugin("org.typelevel" %% "kind-projector" % "0.11.0" cross CrossVersion.full)
Then the code can work like this:
case class Box[A](value:A)
case class BoxT[M[_],A](run: M[Box[A]])
implicit def monad[M[_]](implicit m : Monad[M]): Monad[BoxT[M, ?]] = new Monad[BoxT[M,?]] {
override def point[A](a: => A): BoxT[M,A] = BoxT(m.point(Box(a)))
override def bind[A, B](fa: BoxT[M,A])(f: A => BoxT[M,B]): BoxT[M,B] = BoxT(m.bind(fa.run){ b: Box[A] =>
val v = b.value
f(v).run
})
}
def main(args: Array[String]): Unit = {
val a = Monad[BoxT[Option,?]].point(2)
val b = for {
v <- a //v should be a Int, but idea can't analyze it, maybe I should update, still in 2018.2
} yield {v * 4}
val c = a >> b //this line works, but idea can't analyze it :(
println(b)
println(c)
}
But it looks like when I use BoxT's Monad, I should take ?
instead of A
of Box's inner type.