Hello. I've got the following problem: I have class Structure, that says that its members have some internal parts that can be read from/written to Strings:
data StrItem a=forall b c.(Show b,Parse c)=>StrItem String (a->b) (a->c->a)
class Structure a where strName::a->String strEmpty::a strItems::[StrItem a]
Now I would like to use it to create instances of Show and Parse (and perhaps others (for input/output from various formats, ...)). The approaches I have tried: 1)
instance Structure a => Show a where showsPrec _ w = showsStruct (strName w) (map showsPair strItems) where showsPair (StrItem fName fGet _)=(fName,shows $ fGet w)
This is nice, but it only works in ghc with allowed overlapping and undecidable instances. It may not seem too important, when I use existential types anyway, but existential types are supported by at least one more implementation (hugs) and they seem to me to be a well defined extension that has good chances to become part of standard Haskell later; handling of overlapping and undecidable instances in ghc seems quite too much ad-hoc and experimental for me. 2)
newtype Str a=Str a instance Structure a => Show (Str a) where showsPrec _ (Str w) = showsStruct (strName w) (map showsPair strItems) where showsPair (StrItem fName fGet _)=(fName,shows $ fGet w)
This seems to be a standard way how to do it. The problem is, that I don't get Show instance for a, but for isomorphic type Str a; this is a bit annoying when I want to use it; also I may no longer use deriving Show on types containing such structure. Is there some better way how to do this? Zdenek Dvorak _________________________________________________________________ Send and receive Hotmail on your mobile device: http://mobile.msn.com