Tagged type : type mismatch

Question

I try to use Tagged Type from scalaz to strengthen type safety.

I encountered a warning and an error that I don't understand.

Can you explain me both ?

Here is the output of the console :

scala> sealed trait PostId
defined trait PostId

scala> def PostId(l: Long) : Long @@ PostId = Tag[Long, PostId](l)
PostId: (l: Long)scalaz.@@[Long,PostId]
warning: previously defined trait PostId is not a companion to method PostId.
Companions must be defined together; you may wish to use :paste mode for this.

scala> case class Post(id: PostId)
defined class Post

scala> Post(PostId(2l))
<console>:26: error: type mismatch;
 found   : scalaz.@@[Long,PostId]
    (which expands to)  Long with AnyRef{type Tag = PostId}
 required: PostId
              Post(PostId(2l))

Answer 1

In your example, PostId is just a tag type. The actual tagg-ed type (the type that you should manipulate) is Long @@ PostId.

The error is that you have defined Post to take an instance of PostId when you really meant to have it take an instance of Long @@ PostId, hence the type mismatch.

I suggest renaming PostId to PostIdTag and defining PostId as an alias to Long @@ PostId:

sealed trait PostIdTag
type PostId = Long @@ PostIdTag

def PostId(l: Long) : PostId = Tag[Long, PostIdTag](l)

Then you can keep your Post definition as is.

UPDATE: As it turns out, scalaz tagged types seem to only work with types <: AnyRef, ie one cannot create a tagged type from an AnyVal sub-type.

The solution then is to replace Long with java.lang.Long (which works transparently because scala will automatically convert java.lang.Long values to Long):

sealed trait PostIdTag
type PostId = java.lang.Long @@ PostIdTag
def PostId(l: java.lang.Long) : PostId = Tag[java.lang.Long, PostIdTag](l)
case class Post(id: PostId)
Post(PostId(2l))