[Haskell] Classes with no data type

[Niklas Broberg]

|> Thus I have a typical classes problem, in that I have several
|> implementations of essentially the same function for different
|> circumstances.  The problem is, they must all operate on the same
|> data type, so I cannot define them as seperate instances.
|>
|> Anyone got any ideas how to type this?

Annotate the data type using a GADT:

> {-# OPTIONS_GHC -fglasgow-exts #-}
> module Foo where
>
> data One
> data Two
>
> data MyData a where
>  MyCon :: MyData a
>
> con1 :: MyData One
> con1 = MyCon
>
> con2 :: MyData Two
> con2 = MyCon
>
> class Display a where
 > display :: MyData a -> String
>
> instance Display One where
> display _ = "one"
>
> instance Display Two where
>  display _ = "two"

The annotations will tell you which constructor was used to create the
data, and you can implement destructor functions that can tell the
difference.

/Niklas