One use is when you want an open sum type rather than a closed one. For example, you might want users of your library to add their own 'constructors'. Then, they serve a purpose similar to a constructor that takes no arguments: {-# LANGUAGE EmptyDataDecls #-} class Color a where rgb :: a -> (Int, Int, Int) data Red data Green data Blue instance Color Red where rgb _ = (255, 0, 0) instance Color Green where rgb _ = (0, 255, 0) instance Color Blue where rgb _ = (0, 0, 255) data ColorADT = Color_Red | Color_Green | Color_Blue rgb_adt Color_Red = (255, 0, 0) rgb_adt Color_Green = (0, 255, 0) rgb_adt Color_Blue = (0, 0, 255) On Fri, Aug 12, 2016 at 8:16 AM, James Brown <james.brown5374@gmail.com> wrote:
What's the purpose to allow data declarations with no constructors?
On Fri, Aug 12, 2016 at 11:08 AM, Sergey N. Yashin < yashin.sergey@gmail.com> wrote:
For zero data constructors you need to use only EmptyDataDecls <https://prime.haskell.org/wiki/EmptyDataDecls> extension.
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