[GHC] #14190: Typeable imposes seemingly redundant constraints on polykinded instances

[GHC]

#14190: Typeable imposes seemingly redundant constraints on polykinded instances
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
  TypeableReflection                 |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 To derive `Data` for `Const`, we need

 {{{#!hs
 deriving instance (Typeable k, Data a, Typeable (b :: k)) => Data (Const a
 b)
 }}}

 Where's that `Typeable k` constraint come from? It turns out that for
 reasons I haven't looked into, we can only obtain `Typeable (Const a (b ::
 k))` if we have `Typeable k`; `(Typeable a, Typeable b)` is insufficient.
 Is there a reason for that?

 Annoyingly, we can actually ''get'' that:

 {{{#!hs
 weGotThat :: forall k (a :: k). Typeable a :- Typeable k
 weGotThat = Sub $ withTypeable (typeRepKind (typeRep :: TypeRep a)) Dict
 }}}

 But of course that doesn't help us.

 Could we use `UndecidableSuperClasses` to work around this problem? I
 think we likely can, although I'm concerned about the performance
 implications:

 {{{#!hs
 class (Typeable a, Typeable' k) => Typeable' (a :: k)
 instance (Typeable a, Typeable' k) => Typeable' (a :: k)

 withTypeable' :: forall k (a :: k) r. TypeRep a -> (Typeable' a => r) -> r
 withTypeable' tr f = withTypeable tr $ withTypeable (typeRepKind (typeRep
 @a)) f
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14190>
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