Proposal for stand-alone deriving declarations?

[Bjorn Bringert]

We considered that syntax, but decided against it. Stand-alone  
deriving declarations are made to be as similar as possible to the  
current deriving mechanism, rather than be similar to instance  
declarations. The basic reason for maintaining a syntactic  
distinction between instance declarations and deriving declarations  
is to make the programmer aware of the restrictions of the deriving  
mechanism.

These are some things that make deriving declarations different from  
instance declarations:

- You can only derive instances for data types and newtypes.
- For deriving declarations, the compiler figures out the  
constraints, whereas the programmer writes them for instance  
declarations.
- In GHC, you can declare non-Haskell98 instances such as Eq (C X)  
where X is a concrete type, but you can't do deriving for them.
- When deriving instances of multi-parameter type classes (again non- 
standard), the newtype for which the deriving is made must be the  
last argument to the class. If the syntax were "deriving (Class  
T1 ... Tn)", it might not be clear to the reader what type the  
deriving is for.

I can't see any technical reason not to do as you propose. One  
advantage would be that it makes it possible to fully subsume GHC's  
current deriving extensions (though there are other ways to do this,  
see my recent e-mail to ghc-cvs). One slight disadvantage is that it  
does require a bit more footwork in the compiler to figure out which  
type to do the deriving for.

/Björn

On 5 okt 2006, at 19.58, Iavor Diatchki wrote:

> Hello,
> A question about the syntax:  would there be a problem if we made the
> 'deriving' declaration look like an instance?  Then we would not need
> the special identifier 'for', and also we have a more standard looking
> notation.  I am thinking something like:
> deriving Show SomeType
> deriving Eq (AnotherType a)
> -Iavor
>
>
>
> On 10/5/06, Björn Bringert <bringert at cs.chalmers.se> wrote:
>> Simon Peyton-Jones wrote:
>> > | What is not so nice is that you take a new keyword ('for'),  
>> which is
>> > | quite likely to have been used as a variable name in existing  
>> code.
>> > (Or
>> > | does it work out to use one of the 'special' names here?)
>> >
>> > The latter is what Bjorn has done.  That is, 'for' is only  
>> special in
>> > this one context.  You can use it freely otherwise.  As I  
>> understand it
>> > anyway.
>>
>> Yes. There is even a "for" function somewhere in the libraries (or  
>> was
>> it the testsuite, can't remeber), which tripped up one of my early
>> versions, before I had remembered to make "for" as a special ID in  
>> the
>> parser.
>>
>> > | I think it would be useful to write the proposal in complete  
>> detail up
>> > | on the Haskell' wiki.
>> >
>> > Yes please.  Bjorn?  (It may just be a qn of transcribing the user
>> > manual stuff you have written.)
>>
>> Sure. It seems that I have to be on the committee to write to the  
>> Wiki.
>> Can I join it?
>>
>> /Björn
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