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

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 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)

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On Fri, Aug 12, 2016 at 8:16 AM, James Brown wrote: What's the purpose to allow data declarations with no constructors?

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On Fri, Aug 12, 2016 at 11:08 AM, Sergey N. Yashin wrote: For zero data constructors you need to use only EmptyDataDecls extension.

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