[GHC] #9291: Don't reconstruct sum types if the type subtly changes

[GHC]

#9291: Don't reconstruct sum types if the type subtly changes
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        Reporter:  schyler  |            Owner:
            Type:  feature  |           Status:  new
  request                   |        Milestone:
        Priority:  normal   |          Version:  7.8.2
       Component:           |         Keywords:
  Compiler                  |     Architecture:  Unknown/Multiple
      Resolution:           |       Difficulty:  Moderate (less than a day)
Operating System:           |       Blocked By:
  Unknown/Multiple          |  Related Tickets:
 Type of failure:           |
  None/Unknown              |
       Test Case:           |
        Blocking:           |
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Comment (by nomeata):

 (Even more crazy idea: One can even take this further. What about `case x
 of Right str -> Just str`. `Right` and `Just` can be used interchangeable
 at runtime (same constructor number, same number of arguments of the same
 shape). The compiler could create only one constructor for each such
 shape, and optimize the code above to  `case x of Right _ -> x`.
 Furthermore we could have representational coercions between datatypes
 that are α-equal... Although I doubt that the run-time benefit of that
 will be high, and we’d lose all hopes to implement something like `vacuum`
 or `ghc-vis`.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9291#comment:9>
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