Question

In short, I want to declare a trait like this:

trait Test {
    def test(amount: Int): A[Int] // where A must be a Monad
}

so that I can use it without knowing what monad that A is, like:

class Usecase {
    def someFun(t: Test) = for { i <- t.test(3) } yield i+1
}

---

more details…

essentially, I want to do something like this:

class MonadResultA extends SomeUnknownType {
    // the base function
    def test(s: String): Option[Int] = Some(3)
}

class MonadResultB(a: MonadResultA) extends SomeUnknownType {
    // added a layer of Writer on top of base function
    def test(s: String): WriterT[Option, String, Int] = WriterT.put(a.test(s))("the log")
}

class Process {
    def work(x: SomeUnknownType) {
        for {
            i <- x.test("key")
        } yield i+1
    }
}

I wanted to be able to pass any instances of MonadResultA or MonadResultB without making any changes to the function work.
The missing piece is that SomeUnknowType, which I guess should have a test like below to make the work function compiles.

trait SomeUnknowType {
    def test(s: String): T[Int] // where T must be some Monad
}

---

As I’ve said, I’m still learning this monad thing… if you find my code is not the right way to do it, you’re more than welcomed to point it out~

thanks a lot~~

Answer 1

Assuming you have a type class called Monad you can just write

def test[A:Monad](amount: Int): A[Int]

The compiler will require that there is an implicit of type Monad[A] in scope when test is called.

EDIT:
I’m still not sure what you’re looking for, but you could package up a monad value with its corresponding type class in a trait like this:

//trait that holds value and monad
trait ValueWithMonad[E] {
  type A[+E]
  type M <: Monad[A]
  val v:A[E]
  val m:M
}

object M {

  //example implementation of test method
  def test(amount:Int):ValueWithMonad[Int] = new ValueWithMonad[Int] {
    type A[+E] = Option[E]
    type M = Monad[Option]
    override val v = Option(amount)
    override val m = OptionMonad
  }

  //test can now be used like this
  def t {
    val vwm = test(1)
    vwm.m.bind(vwm.v, (x:Int) => {
      println(x)
      vwm.m.ret(x)
    })
  }
}

trait Monad[A[_]] {
  def bind[E,E2](m:A[E], f:E=>A[E2]):A[E2]
  def ret[E](e:E):A[E]
}

object OptionMonad extends Monad[Option] {
  override def bind[E,E2](m:Option[E], f:E=>Option[E2]) = m.flatMap(f)
  override def ret[E](e:E) = Some(e)
}