Overloading generic implicit conversions
- by raichoo
Hi
I'm having a little scala (version 2.8.0RC1) problem with implicit conversions. Whenever importing more than one implicit conversion the first one gets shadowed. Here is the code where the problem shows up:
// containers
class Maybe[T]
case class Nothing[T]() extends Maybe[T]
case class Just[T](value: T) extends Maybe[T]
case class Value[T](value: T)
trait Monad[C[_]] {
def >>=[A, B](a: C[A], f: A => C[B]): C[B]
def pure[A](a: A): C[A]
}
// implicit converter
trait Extender[C[_]] {
class Wrapper[A](c: C[A]) {
def >>=[B](f: A => C[B])(implicit m: Monad[C]): C[B] = {
m >>= (c, f)
}
def >>[B](b: C[B])(implicit m: Monad[C]): C[B] = {
m >>= (c, { (x: A) => b } )
}
}
implicit def extendToMonad[A](c: C[A]) = new Wrapper[A](c)
}
// instance maybe
object maybemonad extends Extender[Maybe] {
implicit object MaybeMonad extends Monad[Maybe] {
override def >>=[A, B](a: Maybe[A], f: A => Maybe[B]): Maybe[B] = {
a match {
case Just(x) => f(x)
case Nothing() => Nothing()
}
}
override def pure[A](a: A): Maybe[A] = Just(a)
}
}
// instance value
object identitymonad extends Extender[Value] {
implicit object IdentityMonad extends Monad[Value] {
override def >>=[A, B](a: Value[A], f: A => Value[B]): Value[B] = {
a match {
case Value(x) => f(x)
}
}
override def pure[A](a: A): Value[A] = Value(a)
}
}
import maybemonad._
//import identitymonad._
object Main {
def main(args: Array[String]): Unit = {
println(Just(1) >>= { (x: Int) => MaybeMonad.pure(x) })
}
}
When uncommenting the second import statement everything goes wrong since the first "extendToMonad" is shadowed.
However, this one works:
object Main {
implicit def foo(a: Int) = new {
def foobar(): Unit = {
println("Foobar")
}
}
implicit def foo(a: String) = new {
def foobar(): Unit = {
println(a)
}
}
def main(args: Array[String]): Unit = {
1 foobar()
"bla" foobar()
}
}
So, where is the catch? What am I missing?
Regards,
raichoo
