[GHC] #9761: A module can't usefully use both GeneralizedNewtypeDeriving and DeriveFoldable

3 Nov 2014


      #9761: A module can't usefully use both GeneralizedNewtypeDeriving and
DeriveFoldable
-------------------------------------+-------------------------------------
       Reporter:  dfeuer             |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.9
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:  GHC
     Blocked By:                     |  rejects valid program
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 If GND is enabled, and roles don't permit a `Foldable` instance to be
 derived, it won't fall back on `DeriveFoldable`; instead it just produces
 an error. I think ideally I'd want to use a pragma to state explicitly
 that I want to use `DeriveFoldable`-style deriving for this specific
 instance, but an implicit fallback would be acceptable.

 {{{#!hs
 {-# LANGUAGE
 --  GeneralizedNewtypeDeriving,
   DeriveFoldable #-}
 module Derive where
 import qualified Data.IntMap as M

 newtype UniqFM ele = UFM {unUFM :: M.IntMap ele}
   deriving (Foldable)
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9761
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