Proposal for standalone deriving declarations?
Simon PeytonJones
simonpj at microsoft.com
Wed Nov 1 04:10:44 EST 2006
The thread about "standalone" deriving is longish 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: haskellprimebounces at haskell.org [mailto:haskellprimebounces at haskell.org] On Behalf Of
 John Meacham
 Sent: 30 October 2006 05:17
 To: haskellprime at haskell.org
 Subject: Re: Proposal for standalone deriving declarations?

 On Fri, Oct 06, 2006 at 10:39:39AM 0500, Michael Shulman wrote:
 > On 10/6/06, John Hughes <rjmh at cs.chalmers.se> 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⑈
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