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

[amindfv at gmail.com]

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 at 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 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 extension.
>>> 
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