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.
--S
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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