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

[Michael Burge]

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 at 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 at gmail.com> wrote:
>
>> For zero data constructors you need to use only EmptyDataDecls
>> <https://prime.haskell.org/wiki/EmptyDataDecls> extension.
>>
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