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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