Re: [GHC] #10598: DeriveAnyClass and GND don't work well together

2 Jul 2015

      #10598: DeriveAnyClass and GND don't work well together
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        Reporter:  osa1              |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by goldfire):

 It sounds to me, from the original summary, that GHC is behaving as
 documented, with the exception of a poor choice of error message. Fixing
 the error message is one way forward. But -- if you're keen on this sort
 of thing -- what would be even better is to come up with a way for users
 to direct GHC in this matter.

 GHC now has 3 distinct ways of `deriving` classes: the built-in way
 (extended beyond Haskell 2010 by various extensions), the GND way, and the
 !DeriveAnyClass way. In your example, they are '''all''' applicable (at
 least with `-XDeriveFunctor` on). And, I believe they will each produce
 different instances! What I've wanted here is to have some way to control
 GHC's choice of `deriving` mechanism, per instance. Something like

 {{{
 newtype MyMaybe a = Mk (Maybe a)
   deriving( {-# GND #-} Functor
           , {-# BuiltIn #-} Show
           , {-# GND #-} Read
           , {-# Any #-} FromJSON )
 }}}

 Now, the user chooses what facility provides the instances. Note that I've
 done something currently impossible: I've used GND for the `Read` class.
 Normally, we don't want this behavior, and (to my knowledge) there's no
 way to convince GHC to use GND to derive a `Read` or `Show` instance. But
 maybe some user out there does want it.

 This, of course, needs to be extended to standalone-deriving, and all the
 details (particularly, what are the default choices for the `deriving`
 mechanism) need to be worked out. If you (or anyone else) wants to have a
 go at this, I'd happily lend a hand.

 Or, if you're not ready to tackle feature design, we'd gladly welcome a
 patch just to fix the (clearly broken) error message originally reported.
 Thanks!

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10598#comment:3
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