[GHC] #12731: Generic type class has type family; leads to big dep_finsts

[GHC]

#12731: Generic type class has type family; leads to big dep_finsts
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        Reporter:  ezyang            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
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 ezyang):

 A quick grep of Hackage suggests it happens very rarely. I found one case
 in module-management:

 {{{
 #if MIN_VERSION_Cabal(1,21,1)
 instance (Ord k, Generic k, Generic v) => Generic (Map k v) where
     type Rep (Map k v) = Rep [(k, v)]
     from = from . toList
     to = fromList . to
 #else
 }}}

 This is actually a pretty reasonable instance as far as things go, but I
 am sure I would not lose sleep if it got axed.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12731#comment:2>
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