Another use:

class Foo a e | a -> e where
   foo :: a -> Either e a

instance Foo Int Void where
   foo :: Int -> Either Void Int
   foo x = Right (x + 1)

Even though it's an "Either", we're saying we will never (ignoring bottom) return a "Left" value.

Tom


El 12 ago 2016, a las 11:28, Michael Burge <michaelburge@pobox.com> escribió:

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 extension.

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