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

Sterling Clover s.clover at gmail.com
Sun Jan 18 16:02:31 EST 2009

The simple and H98 solution here is to give your typeclass a  
toSexpList method and a fromSexpList method of [a] -> Sexp and Sexp - 
 > Maybe [a] . The default implementation of this method is as in  
your instance for lists. For chars, you simply override this method  
to handle lists of chars differently. See the show and showList  
methods of the Show typeclass for an example of how this is done in  
the standard libraries.


On Jan 18, 2009, at 2:23 PM, Brian Hurt wrote:

> 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
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