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

[James Brown]

In the Haskell report 2010, a data declaration has the format of:

<https://lh3.googleusercontent.com/-G-z92n-50B0/V63XKCCrJDI/AAAAAAAAAGA/w-Zdi9DaR18hX67n-OeQNkDB1iUR0IP7gCLcB/s1600/Screen%2BShot%2B2016-08-12%2Bat%2B10.03.00%2BAM.png>

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?
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