[GHC] #13192: Ambiguity Caused By PolyKind and Not Helpful Error Messages

[GHC]

#13192: Ambiguity Caused By PolyKind and Not Helpful Error Messages
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        Reporter:  Shayan-Najd       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 In the original program, the application `F (I a)` is stuck because

 * `data I a` really means `data I k (a :: k) :: *`, i.e., `I` is
 polykinded

 * `type family F x` really means `type family F k (x :: *) :: *`, i.e.,
 `F` is ''not'' polykinded

 * `type instance F (I a) = a` really means `type instance F (I * a) = a`,
 because the instance is only well-kinded when `a` has kind `*`.

   But there's nothing stopping one from adding an additional instance
 `type instance F (I Maybe) = Char` which uses `I` at a different kind.

 * `identity :: F (I a) -> F (I a)` really means `identity :: forall k (a
 :: k). F (I k a) -> F (I k a)`, i.e., `identity` is polykinded. Now
 there's a problem because the instance we defined only applies when `k =
 *`, but here `k` could really be anything, like `* -> *`.

   And indeed if I added the `type instance F (I Maybe) = Char`, then I can
 write `identity @ Maybe` and it has type `Char -> Char` just like
 `identity @ Char` does. So the type really is ambiguous.

 What I'm confused about is why `F (I a)` is still stuck even when I make
 the result of `F` polykinded, so that the instance `type instance F (I a)
 = a` applies at all kinds.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13192#comment:4>
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