[GHC] #9429: Alternative to type family Any

[GHC]

#9429: Alternative to type family Any
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       Reporter:  mboes              |                   Owner:
           Type:  feature request    |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.9
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:  9097, 9380         |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 In GHC HEAD, `Any` is no longer a datatype. There are good reasons for
 this change, one of which was explained in #9097, the original ticket, and
 another in #9380. Unfortunately, a casualty of this change is that it is
 no longer easy to generalize the
 [https://hackage.haskell.org/package/rank1dynamic|rank1dynamic package] to
 rank-1 types with type variables of arbitrary kind (not just `*`). We
 submitted a way to do so [here], that exploits the fact that `Any` has the
 very magical property of inhabiting *all* kinds, including closed ones.

 This works for GHC 7.8, but won't work in HEAD, because we require that
 there exists a `Typeable` instance for `Any`, just as there are `Typeable`
 instances for any other type one wishes to have instances for. The reason
 is that now that `Any` is a type family, `Any` is no longer a legal
 instance head.

 There are several possible solutions that I see:
 * while it's clearly dangerous for the compiler to be inserting the old
 `Any` during typechecking from the moment that we have computation in
 types over closed kinds, we may still want the old `Any`, say under a
 different name, as a backdoor. It wouldn't be used by the compiler during
 type checking - only by packages such as rank1dynamic. I believe that
 furthermore, if we restrict the old `Any` to not inhabit closed kinds,
 then none of the problems cited in the above tickets arise.
 * Instead of making the new `Any` a type family, keep it a datatype, but
 ban it from inhabiting closed kinds. I don't know if such an `Any` would
 be sufficient for the purposes of GHC, however.
 * Hardwire a `Typeable` instance for the `Any` type family (not sure if
 this makes sense).

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