[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 simonpj):

 I hate the fact that a type-system question forces us to lose a potential
 optimisation.  But I do not know any well-typed way around this particular
 one.

 I'm absolutely not willing for the compiler to add `unsafeCoerce`s to the
 code!  One of GHC's most unique and distinctive features is its commitment
 to a solidly-typed intermediate language, and I'm not willing to
 compromise that property.  There is a long tradition in Haskell of doing
 things the Right Way and using the pain to drive innovation.  (Monads,
 System FC, kind polymorphism, etc etc.)

 All that said, STG code is un-typed and I would be quite happy with STG-
 level optimisations that transformed
 {{{
   case e of y {
     ...
     C a b -> C a b
     ...
   }

 =====>

   case e of y {
     ...
     C a b -> y
     ...
   }
 }}}
 and then commoned identical branches.  (The above transformation is done
 in Core too, but not when the types don't match.  In STG that doesn't
 matter.)

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9291#comment:13>
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