[GHC] #15622: Generalize `E{0,1,2,3,6,9,12}` from `Data.Fixed`

[GHC]

#15622: Generalize `E{0,1,2,3,6,9,12}` from `Data.Fixed`
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        Reporter:  rockbmb           |                Owner:  rockbmb
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Core Libraries    |              Version:  8.4.3
      Resolution:                    |             Keywords:  base,
                                     |  Data.Fixed
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 RyanGlScott):

 That is still a breaking change, since it changes `E0`, `E1`, etc. from
 data types to type synonyms. Among other things, this will cause programs
 that declare instances against these types to stop compiling if they do
 not enable the `FlexibleInstances` extension.

 That change notwithstanding, I would also be interested to hear the
 community's feedback on the use of the `DataKinds` GHC extension in a
 prominent place in `base` like `Data.Fixed`. (`DataKinds` is already used
 in other places in `base`, but they're mostly sectioned off within the
 `GHC.*` namespace, where language extension experimentation is more
 readily tolerated.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15622#comment:9>
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