One of my high school students and I are going to try to do a port of Haskell's Parsec parser combinator library into Scala. (It has the advantage over Scala's built-in parsing library that you can pass state around fairly easily because all the parsers are monads.)

The first hitch I've come across is trying to figure out how Functor works in scalaz. Can someone explain how to convert this Haskell code:

data Reply s u a = Ok a !(State s u) ParseError
                 | Error ParseError


instance Functor (Reply s u) where
    fmap f (Ok x s e) = Ok (f x) s e
    fmap _ (Error e) = Error e -- XXX

into Scala (using Scalaz, I assume). I got as far as

sealed abstract class Reply[S, U, A]
case class Ok[S, U, A](a: A, state: State[S, U], error: ParseError)
    extends Reply[S, U, A]
case class Error[S, U, A](error: ParseError) extends Reply[S, U, A]

and know that I should make Reply extend the scalaz.Functor trait, but I can't figure out how to do that. (Mostly I'm having trouble figuring out what the F[_] parameter does.)

Any help appreciated!

Thanks, Todd