[Haskell-cafe] Stupid question, re: overloaded type classes

[Brian Hurt]

So, I'm working with this simplistic S-expression library of my own design 
(yes, I know, reinventing the wheel).  Basically, I have the type:

data Sexp =
 	List of [ Sexp ]
 	| Atom of String

with the associated parsers and printers which really aren't relevent to 
the question at hand.  Then, I want to define the type class of types I 
can convert to and from s-expressions, like:

class Sexpable a where
 	toSexp :: a -> Sexp
 	fromSexp :: Sexp -> Maybe a

here, fromSexp can return Nothing is the s-expression isn't the right form 
to be parsed into a whatever.

Now, here's the problem.  I want to define a bunch of default instances, 
and two in particular I want to define are:

instance Sexpable String where
 	toSexp s = Atom s
 	fromSexp (Atom s) = Just s
 	fromSexp _ = Nothing

instance Sexpable a => Sexpable [ a ] where
 	toSexp lst = List $ map toSexp lst
 	fromSexp (List lst) = mapM fromSexp lst
 	fromSexp _ = Nothing

Note that I am not implementing Sexpable Char anywhere, so the only valid 
transform for [Char] should be the String one.  But this still causes a 
compiler error due to the overloaded instances on [Char].

There are two solutions to this that I already know of.  One is to play 
games with newtype, which I don't like because it simply adds complexity 
in my case and doesn't help anything else.  The second possibility is to 
compile with -fallow-incoherent-instances, which I'm slightly afraid of 
because I'm not sure what (if any) possible errors adding this option 
might allow.

So my question is twofold: 1) what errors might be allowed if I add 
-fallow-incoherent-instances, and 2) is there some third choice that 
avoids both solutions I already know about?

Thanks.

Brian