Cannot write the type signature of the main function because some type is not exposed

[lucamug]

I am getting these errors trying to write the type signature of main:

I cannot find a `Playground.Game` type
I cannot find a `Playground.Msg` type

I am going to send a PR about this issue. Feel free to discard it if you prefer not to expose these types or dealing with this in a different way

[cjduncana]

I just found this problem myself.