module M where class C a where op :: a -> a instance C Int where op x = x+1 f x = Just (op x) Under your proposal, I'd infer f :: Int -> Maybe Int, on the grounds that C is closed and there is only one instance. Simon | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of MR | K P SCHUPKE | Sent: 12 August 2004 14:03 | To: haskell-cafe@haskell.org; qrczak@knm.org.pl | Subject: Re: [Haskell-cafe] closed classes [was: Re: exceptions vs. Either] | | Okay anybody whish to argue against these points: | | 1) closed or open is the same if no instances overlap | | 2) overlapping instances (open or closed) can break | code in modules which import the defining module | if a new instance is declared in any imported | module. | | 3) code changes in non-imported modules cannot have | any affect on _this_ module. (And here I count | any module in the import tree as imported - at | least in terms of recompilation dependancies) | | The conclusion appears to be it is overlapping instances | that cause code to be 'breakable' by simply defining a new | instance and not closing the class. Closing the class | would appear to have no adverse affects on existing | programs (as it allows better improvement rules) all existing | programs that compile without the better improvement rules | should still compile - just a few more programs will be | valid with the closed assumption? | | Keean. | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe