You can see the rules here http://www.haskell.org/ghc/dist/current/docs/users_guide/type-extensions .html#instance-overlap GHC 6.6's story is that an instance declaration can only be overlapped if you compile that module with -fallow-overlapping-instances. Since the list instance for Show was not compiled in this way, you can't overlap it. You may think this is annoying -- but it does mean that you can look at an instance declaration and see whether it might be overlapped, just by look at the flags for that module. Currently there is no workaround; you have to compile the instance decl that you want to overlap with the -fallow-overlapping-instances flag. It would be possible to relax this restriction, so that instances can overlapped if *either* you use -fallow-overlapping-instances when you compile the instance declaration, *or* you use that flag at the usage site. (But the exact usage site can be hard to pin down.) I'd be interested in feedback on this. Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org] | On Behalf Of Serge D. Mechveliani | Sent: 02 September 2006 12:30 | To: glasgow-haskell-users@haskell.org | Subject: overlapping instances in 6.6 candidate | | Dear GHC developers, | | Can you, please, explain what has happened in 6.5.20060831 | with treating of overlapping instances? | | It seems to understand them differently than ghc-6.4.1. | For example, I compile the module | | ------------------------ | class C a where c :: a -> Bool | | instance C Int where c n = n > 0 | | instance C a => C [a] where c xs = and $ map c xs | | instance C [Int] where c xs = c $ head xs | | main = putStr (shows (c [1, (-2 :: Int)]) "\n") | | instance Show [Bool] where showsPrec _ xs = shows (head xs) | ------------------------ | | with the options | $dmCpOpt = | -fglasgow-exts -fallow-undecidable-instances | -fallow-overlapping-instances -fno-warn-overlapping-patterns | -fwarn-unused-binds -fwarn-unused-matches -fwarn-unused-imports | : | > ghc $dmCpOpt --make Main | | It reports | | -------------------------------------------------------- | [1 of 1] Compiling Main ( Main.hs, Main.o ) | | Main.hs:11:0: | Overlapping instances for Show [Bool] | arising from use of `GHC.Show.$dmshowList' at Main.hs:11:0 | Matching instances: | instance (Show a) => Show [a] -- Defined in GHC.Show | instance [overlap ok] Show [Bool] -- Defined at Main.hs:11:0 | In the expression: GHC.Show.$dmshowList | In the definition of `showList': showList = GHC.Show.$dmshowList | In the definition for method `showList' | -------------------------------------------- | | And it takes as all right the overlaps for the class C. | | [Bool] is a substitutional instance of [a], and according to the GHC | specification, the Show instance for [Bool] must override the more generic | one for Show a => [a]. | | Thank you in advance for explanation. | | ----------------- | Serge Mechveliani | mechvel@botik.ru | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users