[GHC] #9115: The kind of (=>)

[GHC]

#9115: The kind of (=>)
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        Reporter:  kosmikus          |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  low               |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by goldfire):

 Here's a reason: normally, if we know `(a b c) ~ (d e f)`, it is
 reasonable to conclude `a ~ d`, `b ~ e` and `c ~ f`. But, what if `a` is
 really `(=>)`? Could `b` become `(Eq a, Eq [a])` and `e` become `Eq [a]`?
 Is it the case that `(Eq a, Eq [a]) ~ Eq [a]`? These are logically
 equivalent, but are they equal in the sense of `(~)`? I would hope not.
 Indeed, even asking whether `(a => b) ~ (c => d)` right now is forbidden.

 There may be a reasonable way forward here, but it's not abundantly
 obvious. If you want More Thought put into this, do you have a use case in
 mind?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9115#comment:1>
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