instance overlapping

[Nguyen Phan Dung]

Hi,

If I have the folowwing declaration:

class Soup where
...

instance Soup String where
...

instance Soup t => Soup [t] where
...

This will lead to an error: "instance overlapping".

Is there anyway to solve this? 
(I could not make an instance for Char)

Thank you very much !
Phan Dung.