Given the following:
>
> import Data.Maybe ( fromMaybe )
>
> data None
> none = error "none evaluated" :: None
these class and instance declarations:
> class ToMaybe2 b a where toMaybe2 :: a -> Maybe b
> instance ToMaybe2 a None where toMaybe2 = const Nothing
> instance ToMaybe2 a a where toMaybe2 = Just
compile fine (given -fallow-undecidable-instances and
-fallow-overlapping-instances). And the following code:
> mbPutStrLn :: ToMaybe2 String a => a -> IO ()
> mbPutStrLn = putStrLn . fromMaybe "None." . toMaybe2
> main = mbPutStrLn none >> mbPutStrLn "Hello, world!"
works as I'd expect. However, if the parameters to the class
are reversed:
> {- uncomment to see error message
> class ToMaybe1 a b where toMaybe1 :: a -> Maybe b
> instance ToMaybe1 None a where toMaybe1 = const Nothing
> instance ToMaybe1 a a where toMaybe1 = Just
> -}
I get the following error message (from GHC{,i} 6.2):
Overlapping instance declarations:
OverlapOrder.lhs:27: ToMaybe1 None a
OverlapOrder.lhs:28: ToMaybe1 a a
Why is this?
thanks,
mike
_______________________________________________
Glasgow-haskell-users mailing list
[EMAIL PROTECTED]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
