Phantom Types

[Kevin Meredith]

After reading the helpful, accepted answer on http://stackoverflow.com/questions/28247543/motivation-behind-phantom-types, I attempted to create a Phantom Type in Scala:

scala> case class Foo[A](x: Int)

defined class Foo

scala> def add(foo: Foo[Int], foo2: Foo[Int]): Int = foo.x + foo2.x

add: (foo: Foo[Int], foo2: Foo[Int])Int

scala> add(Foo[Int](1), Foo[Int](2))

res9: Int = 3

Note that I tried to add a `Foo[Int]` and `Foo[Double]`, but got a compile-time error, as expected:

scala> add(Foo[Int](1), Foo[Double](2))

<console>:15: error: type mismatch;

 found   : Foo[Double]

 required: Foo[Int]

       add(Foo[Int](1), Foo[Double](2))

                                   ^

Is my above `Foo` a correctly implemented Phantom Type? 

If so, must Foo's `A` be invariant to maintain it as a Phantom Type? 

[christi...@gmail.com]

No, this is no phantom type.

Please see the following links for examples:
https://github.com/dickwall/activator-scala-phantom/blob/master/src/main/scala/FakeDb.scala
https://ktoso.github.io/scala-types-of-types/#phantom-type
http://james-iry.blogspot.com/2010/10/phantom-types-in-haskell-and-scala.html

[Stephen Compall]

Yes, it's a phantom type.

No, it doesn't have to be invariant, but it usually helps preserve the API you want.