Re: [GHC] #14661: Cannot derive (newtype I a b = I (F a -> F b) deriving newtype Category) for type family F

23 Jan 2018

      #14661: Cannot derive (newtype I a b = I (F a -> F b) deriving newtype Category)
for type family F
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.4.1-alpha1
      Resolution:  wontfix           |             Keywords:
                                     |  DerivingStrategies, deriving,
                                     |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 I don't know if this should be a separate ticket but it is another case
 that can be derived only by expanding constructors

 {{{#!hs
 import Data.Monoid (Endo)

 newtype ENDO = ENDO (forall xx. Endo xx)

 instance Semigroup ENDO where
   (<>) :: ENDO -> ENDO -> ENDO
   ENDO f <> ENDO g = ENDO (f <> g)

 instance Monoid ENDO where
   mempty :: ENDO
   mempty = ENDO mempty
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14661#comment:12
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