[GHC] #13689: Data.Either doesn't export INLINABLE short functions like "rights"

[GHC]

#13689: Data.Either doesn't export INLINABLE short functions like "rights"
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        Reporter:  varosi            |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect API     |  Unknown/Multiple
  annotation                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3576
       Wiki Page:                    |
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Comment (by nomeata):

 JFTR: The definition
 {{{
 lefts = concatMap go
   where go (Left x) = [x]
         go (Right _) = []
 }}}
 results in the same code (GHC HEAD even CSE’s it with the other definition
 if defined in the same module), and also does not get an unfolding
 automatically, and with `INLINEABLE` the unfolding is the large one with
 `foldr` and `build`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13689#comment:19>
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