[GHC] #7346: Allow the use of `deriving` for GHC generics

[GHC]

#7346: Allow the use of `deriving` for GHC generics
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 Reporter:  kosmikus          |          Owner:                  
     Type:  feature request   |         Status:  new             
 Priority:  normal            |      Component:  Compiler        
  Version:  7.6.1             |       Keywords:                  
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  None/Unknown      |       Testcase:                  
Blockedby:                    |       Blocking:                  
  Related:                    |  
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 Currently, a class that makes use of generic default methods (via the
 `DefaultSignatures`) extension can be instantiated by providing an empty
 instance declaration.

 I propose to allow the use of `deriving` as well:

 (1) Standalone deriving should be usable for a class not only for the
 specific set of classes supported by GHC now, but in addition for any
 class, if (A) there's at least one generic default given for a method of
 the class, and (B) there are generic or normal default implementations for
 *all* methods of the class.

 There are a number of advantages of this solution over the empty instance
 declaration: we make it explicit that something generic is going on here;
 we ensure at compile time that we're not missing an implementation of a
 method; and we come syntactically closer to built-in derivable classes.

 In cases where a conflict arises between current GHC semantics and the
 proposed semantics (for example, when newtype-deriving is involved, I
 guess), I propose to stick with current GHC semantics, but I'm open for
 other suggestions.

 (2) I'd also like for normal `deriving` to be useful under the same
 conditions as above. For normal `deriving` GHC has to figure out the class
 context automatically. I propose that if normal `deriving` is used, GHC
 uses the same heuristic for figuring out the class context that it uses
 for `Eq` in the case of `*`-kinded classes, and for `Functor` in the case
 of `* -> *`-kinded classes. That may not be optimal or even wrong. But in
 such cases, standalone deriving can still be used.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7346>
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