Scala implicit parameter

Question

Okay, I finally managed to understand the use case of implicit in function parameters and got my type classes working. Let's have a look at my monad type class:

trait Monad[M[_]] extends Functor[M] { self =>
  def pure[a](value: a): M[a]

  def flatMap[a, b](ma: M[a])(f: a => M[b]): M[b]
}

I created a type class instance for my parser monad (which is just a dummy for now):

case class Parser[a](value: (Unit => a)) {
  def >>=[b](f: a => Parser[b])(implicit p: Monad[Parser]): Parser[b] =
    p.flatMap(this)(f)

  def fmap[b](f: a => Parser[b])(implicit p: Monad[Parser]): Parser[b] =
    p.map(this)(f)
}

object Parser {
  implicit object MonadInstance extends Monad[Parser] {
    override def pure[a](value: a): Parser[a] =
      new Parser[a]({ _ => value })

    override def flatMap[a, b](ma: Parser[a])(f: (a) => Parser[b]): Parser[b] =
      f(ma.value())

    override def map[a, b](fa: Parser[a])(f: (a) => b): Parser[b] =
      new Parser[b]({ _ => f(fa.value()) })
  }
}

I'm just wondering, if I can use the monad instance without having to declare an implicit parameter wherever I use my monads. Is there some way to "import" the functionality globally?

Answer 1

There's a method in Predef called implicitly that is designed for this use case:

def implicitly[T](implicit t: T):T = t

which can be used like this:

val mParser = implicitly[Monad[Parser]]

Original:

In scala, implicits are automatically looked for in the companion objects of the relevant types. So implicitly[Monad[Parser]] ought to just work, because it ought to look in object Parser.

Note that because of the way the interpreter works, it's a bit of a pain to define companion objects in the repl. The easiest thing is to wrap them in an object, or to not start object Parser on a new line (just for the repl).