[GHC] #12928: Too easy to trigger CUSK condition using TH

[GHC]

#12928: Too easy to trigger CUSK condition using TH
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        Reporter:  int-index         |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
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):

 Another way to see the difference is that GHC uses the definition of a
 type to inform its kinds. That is,

 {{{
 data T a = MkT (a Int)
 }}}

 will infer `a :: Type -> Type`.

 On the other hand, term-level type signatures stand entirely on their own,
 as Simon says.
 So,

 {{{
 x :: forall a. Proxy a
 x = Proxy :: Proxy (a :: Type)
 }}}

 is rejected, even though choosing `a :: Type` in the type signature would
 work here.

 I've been favoring having standalone type-level kind signatures for some
 time, and am waiting for the right time to introduce a ghc-proposal to
 this end. CUSKs are terribly fiddly and very unexpected to users!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12928#comment:9>
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