[Haskell-cafe] type class question

[Derek Elkins]

Tim Docker wrote:
> I think this must almost be a FAQ, or at least a PAQ (Previously AQ)...
>  
> If I have a type class for conversion to a type X:
>  
>     class XType a where
>         toX   :: a -> X
>  
> I can define instances for 
>  
>     instance XType Int where toX  = ...
>     instance XType Double where toX  = ...
>     instance XType Tuple where toX  = ...
>  
> but not for Strings, given that they are a synonym for [Char]. Hence:
>  
>     instance XType String where toX  = ...
>  
> results in:
>  
>     Illegal instance declaration for `XType String'
>         (The instance type must be of form (T a b c)
>          where T is not a synonym, and a,b,c are distinct type
> variables)
>     In the instance declaration for `XType String'
>  
> Is there some type class cleverness that can make this work in haskell
> 98? I can create a new wrapper type for strings:
> 
>     newtype StringWrap = StringWrap String
> 
> and write an instance for that, but then I'll have to litter my code
> with calls to this constructor.
> 
> I'm aware of the approach taken by class Show in the prelude, which
> adds a extra method to the class:
>  
>     class XType a where
>         toX     :: a -> X
>         listToX :: [a] -> X
>  
> but I believe this says that whenever we can convert a to an X we can
> also
> convert [a] to an X, whereas I only want [Char] to be acceptable.

I believe there is a trick where essentially you end up with,
instance IsChar a => XType [a] where ...