[GHC] #9429: Alternative to type family Any

[GHC]

#9429: Alternative to type family Any
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              Reporter:  mboes       |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.9
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:  9097, 9380
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by tibbe):

 There's another use of `Any` that I'm aware of that might not work
 anymore, to create unique tombstone values that can be compare using
 `reallyUnsafePtrEquality#`. See e.g.
 https://github.com/gregorycollins/hashtables/blob/8a1ed23450377360e4b53b83c531f7557fff7e79/src/Data/HashTable/Internal/UnsafeTricks.hs#L38.
 These tricks allow you to write a `Maybe a` represents null values (or any
 number of unary constructors) using a designated pointer instead of an
 indirection.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9429#comment:15>
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