#7346: Allow the use of `deriving` for GHC generics
------------------------------+---------------------------------------------
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: |
------------------------------+---------------------------------------------
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>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
Glasgow-haskell-bugs@haskell.org
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
