The thread about "stand-alone" deriving is long-ish now, so I have summarised the issues here: http://haskell.org/haskellwiki/GHC/StandAloneDeriving Perhaps those who are interested can add their thoughts? Bjorn is busy at the moment, but I think he'll get back to the implementation in a while, so now is a good time to refine the design. I've tried to note all the issues that came up by email, but I might have missed. I put it on the GHC wiki, because I'm not proposing it for Haskell'. (Someone else is free to do so, of course.) Simon | -----Original Message----- | From: haskell-prime-bounces@haskell.org [mailto:haskell-prime-bounces@haskell.org] On Behalf Of | John Meacham | Sent: 30 October 2006 05:17 | To: haskell-prime@haskell.org | Subject: Re: Proposal for stand-alone deriving declarations? | | On Fri, Oct 06, 2006 at 10:39:39AM -0500, Michael Shulman wrote: | > On 10/6/06, John Hughes wrote: | > >deriving (Eq Foo, Ord Foo) | > > | > >instead of | > > | > >deriving (Eq, Ord) for Foo | > | > So what does | > | > newtype Foo a = Foo a | > newtype Bar b = Bar b | > class C a b | > deriving (C (Foo a) (Bar b)) | > | > mean? I could see it meaning any or all of the following: | > | > instance (C (Foo a) b) => (C (Foo a) (Bar b)) | > instance (C a (Bar b)) => (C (Foo a) (Bar b)) | > instance (C a b) => (C (Foo a) (Bar b)) | | | this is why we should make this explicit when deriving "complex" newtype | instances, so we would write exactly the instance we want to derive: | | > deriving (C (Foo a) b) => (C (Foo a) (Bar b)) | > deriving (C a (Bar b)) => (C (Foo a) (Bar b)) | > deriving (C a b) => (C (Foo a) (Bar b)) | | respectively. | | John | | -- | John Meacham - ⑆repetae.net⑆john⑈ | _______________________________________________ | Haskell-prime mailing list | Haskell-prime@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-prime