[Haskell-cafe] Re: Type classes

[Oscar Picasso]

Forget it.  It's more clear reading further.

Sorry for the noise.

On Sat, Dec 27, 2008 at 1:02 AM, Oscar Picasso <oscarpicasso at gmail.com>wrote:

> 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
>
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