>From Real World Haskell:

data JValue = JString String
            | JNumber Double
            | JBool Bool
            | JNull
            | JObject [(String, JValue)]
            | JArray [JValue]
              deriving (Eq, Ord, Show)

....
type JSONError = String

class JSON a where
    toJValue :: a -> JValue
    fromJValue :: JValue -> Either JSONError a

instance JSON JValue where
    toJValue = id -- Really ?
    fromJValue = Right

Now, instead of applying a constructor like JNumber to a value to wrap it, we apply the toJValue function. If we change a value's type, the compiler will choose a suitable implementation of toJValue to use with it.

Actually it does not work. And I don't see how it could with this toJValue implementation. Is it possible to make it work by providing another implementation?

Oscar