A Pitfall in Scala Implicits

Consider the following code:

import cats._

case class MyClass(a: Int)

object Main {
  implicit val myMonoid = Monoid[MyClass]

  def main(args: Array[String]): Unit = {
    println(myMonoid)
  }
}

For those unfamiliar with Cats/Scalaz, Monoid is a simulacrum-generated typeclass, so its apply method simply resolves the implicit in scope. It looks something like this:

object Monoid {
  def apply[T](implicit m: Monoid[T]): Monoid[T] = m
  // ...
}

This code will not compile because the Monoid cannot be implicitly resolved. However:

import cats._

case class MyClass(a: Int)

object Main {
  implicit val myMonoid: Monoid[MyClass] = Monoid[MyClass]

  def main(args: Array[String]): Unit = {
    println(myMonoid)
  }
}

This will compile and print null. It resolves the implicit using the default empty null value!

Be careful when exporting implicits in your objects.