[Haskell-cafe] Typeclasses question in "Real World Haskell" book

[S. Doaitse Swierstra]

How about:

*Main> fromJValue (JBool True) :: Either JSONError Bool
Right True
*Main> 

Doaitse


On 26 jul 2010, at 15:16, Angel de Vicente wrote:

> 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
>    fromJValue = Right
> 
> instance JSON Bool where
>    toJValue = JBool
>    fromJValue (JBool b) = Right b
>    fromJValue _ = Left "not a JSON boolean"