Functional programming & distributed systems blog

Yuriy Netesov on programming and stuff

Scala trait with implicit bound

intro

Sometimes when we define traits with type parameters, we would like to have some implicit evidence for that type parameters. However traits do not allow us to have any parameters.

concrete example

Lets say we want to implement a Monad instance for something. We know that for something to be a monad, it should be a Functor first. So our natural desire would be to write something like

trait Monad[F[_]:Functor]

That will be desugared into

trait Monad[F[_]](implicit functor:Functor[F])

And that will produce a compile error because traits can't have parameters.

workaround

trait Monad[F[_]] {
    implicit def functor:Functor[F]
}

implicit def monadInstance[F[_]](implicit f:Functor[F]) = new Monad { functor = f}