[Haskell-cafe] Need an example of new type constructor with zero data constructors

In the Haskell report 2010, a data declaration has the format of: https://lh3.googleusercontent.com/-G-z92n-50B0/V63XKCCrJDI/AAAAAAAAAGA/w-Zdi... For a new type, there could be zero or more data constructors. What's the use case for a new type that has zero data constructor?