[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:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Moderate (less
  Unknown/Multiple                   |  than a day)
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by rwbarton):

 Replying to [comment:9 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`.)

 This could make `toInteger :: Int -> Integer` a no-op, for example.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9291#comment:16>
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