Using a view bounds with scalaz

Question

I'm taking my first foray into scalaz by converting an existing class to use the Monoid trait. What I am trying to achieve is to set a view bound on my class type parameter to ensure that it can only be used with types that can be implicitly converted to a Monoid. My (simplified) class definition is thus:

import scalaz._
import Scalaz._

case class Foo[T <% Monoid[T]](v: T)

new Foo(42)

Compiling this simple example gives the compiler error:

error: No implicit view available from Int => scalaz.Monoid[Int].

Previously this view bound was defined against my own custom trait with an implicit conversion from T to the trait and this worked fine.

What am I missing now that I have converted this to scalaz?

Thanks, Chris

Answer 1

You are supposed to be using a context bound, and not a view bound there.

import scalaz._
import Scalaz._

case class Foo[T : Monoid](v: T)

new Foo(42)

The T : Monoid notation means that there is an implicit of type Monoid[T] in scope. In fact, it desugars to the following:

case class Foo[T](v: T)(implicit ev: Monoid[T])

This is known as type class pattern and you can read more about it here.