Proposal for stand-alone deriving declarations?

[John Meacham]

On Thu, Oct 05, 2006 at 08:58:28PM +0200, Bjorn Bringert wrote:
> - 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 have run into this restriction multiple times, and since it only
arises due to the syntax, it would be good to fix that too with any
proposed change to the deriving syntax.

A related issue is that you currently cannot 'co-derive' a
multiparameter typeclass among multiple newtype definitions. such as the
following:

> class SetLike m k  where
>         insert :: k -> m -> m
>         ...     
> 
> instance SetLike IntSet Int where
>         ...
> 
> newtype Id = Id Int
> 
> newtype IdSet = IdSet IntSet

now, ideally you should be able to derive MapLike (IdMap v) Id v, but
there is no way to do it with the current syntax.

if we make the instance we are using to derive the newtypes instance
from explicit however, it solves both problems nicely.

allow toplevel definitions like the following:
> deriving (SetLike IntSet Int => SetLike IdSet Id)


        John


-- 
John Meacham - ⑆repetae.net⑆john⑈