[GHC] #13546: Kind error with type equality

[GHC]

#13546: Kind error with type equality
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        Reporter:  int-index         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I agree with GHC here.

 The problem is that `UnT` takes `k` as a parameter. It does ''not'' look
 at the kind at which its argument is used and return something of that
 kind.

 So, in the type of `tunt`, the first `T` is applied at an unknown kind,
 which is also the return kind of `UnT`. Of course, you want this to be
 `Symbol`, but GHC doesn't know this. Adding a kind signature fixes the
 problem:

 {{{#!hs
 tunt :: Dict (T (UnT (T "a") :: Symbol) ~ T "a")
 tunt = Dict
 }}}

 Please close the ticket if you agree with my analysis. Thanks!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13546#comment:1>
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