Proposal for stand-alone deriving declarations?

[Simon Peyton-Jones]

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 at haskell.org [mailto:haskell-prime-bounces at haskell.org] On Behalf Of
| John Meacham
| Sent: 30 October 2006 05:17
| To: haskell-prime at 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 <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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