Scalaz's Equal type class (and associated handy === syntax) is a type-safe alternative to Scala's ==

gistfile1.scala

scala> case class UserId(id: Long)
defined class UserId

scala> def isSusan(id: UserId): Boolean = id == 3L // see the bug?
isSusan: (id: UserId)Boolean

scala> isSusan(UserId(3L))
res31: Boolean = false// shouldn't this be true?

// let's show the compiler how to check whether two UserIds are equal
scala> implicit val userIdEqual: Equal[UserId] = Equal.equalBy(_.id)
userIdEqual: scalaz.Equal[UserId] = scalaz.Order$$anon$7@33f02699

scala> def isSusan2(id: UserId): Boolean = id === 3L // now the bug is a compile error
<console>:23: error: could not find implicit value for parameter F0: scalaz.Equal[Any]
       def isSusan2(id: UserId): Boolean = id === 3L // now the bug is a compile error
                                           ^

scala> def isSusan3(id: UserId): Boolean = id === UserId(3L)
isSusan3: (id: UserId)Boolean

scala> isSusan3(UserId(3L))
res32: Boolean = true // ah there we go