[Haskell-cafe] Existentially-quantified constructors, Eq and Show

[Greg Buchholz]

Joel Reymont wrote:
> Folks,
> 
> Is there a less verbose way of doing this:
> 
> data State a
>     = Start
>     | Stop
>     | (Show a, Eq a) => State a
>

   I'm curious, what is the difference between the above and...

        data State a = Start 
                     | Stop  
                     | State a  deriving (Show, Eq)
                     
...Does it give better error messages at compile time or something?


Thanks,

Greg Buchholz