Proposal for stand-alone deriving declarations?

[Bjorn Bringert]

What I implemented in GHC is an extension of the proposal below. The  
proposal just mentions:

deriving Class for Type

In GHC I also added a form for newtype deriving of multi-parameter  
type classes:

deriving (Class t1 ... tn) for Type

I think that it's close to what we ended up with when talking about  
it at the Hackathon. My intuition about this syntax is that except  
for the "for Type" part, it looks the same as a normal deriving  
clause. The "for" part is just there to connect it to a data/newtype  
declaration. This lets it pretty much use the normal code for  
deriving declarations.

Stand-alone deriving declarations are currently a little bit weaker  
than normal deriving clauses, since the current implementation does  
not let you reference the type arguments of a newtype in the  
arguments of an MPTC. See my response to Bulat on cvs-ghc at haskell.org  
for more details.

/Björn

On 5 okt 2006, at 10.36, Simon Peyton-Jones wrote:

> Thanks for doing this.
>
> Is this the syntax we settled on?  I remember we discussed it at  
> some length
>
> S
>
> | -----Original Message-----
> | From: haskell-prime-bounces at haskell.org [mailto:haskell-prime- 
> bounces at haskell.org] On Behalf Of
> | Bjorn Bringert
> | Sent: 05 October 2006 09:05
> | To: haskell-prime at haskell.org
> | Subject: Proposal for stand-alone deriving declarations?
> |
> | On http://hackage.haskell.org/trac/haskell-prime/wiki/
> | DerivedInstances it says:
> |
> | "- There is no way to derive an instance of a class for a data type
> | that is defined elsewhere (in another module)."
> |
> | Though there is no proposal to fix this. Would such a proposal be
> | appropriate for Haskell'?
> |
> |
> | If so, I propose to add a top-level declaration on the form:
> |
> | 'deriving' qtycls 'for' qtycon
> |
> | which produces the same instance as a deriving clause in the
> | declaration of the datatype or newtype would.
> |
> |
> | I have recently (thanks to the GHC Hackathon) implemented this in  
> GHC.
> |
> | /Björn
> |
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