[GHC] #13603: Can't resolve levity polymorphic superclass

23 Apr 2017


      #13603: Can't resolve levity polymorphic superclass
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           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  LevityPolymorphism, TypeInType     |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This works

 {{{#!hs
 {-# Language PolyKinds, TypeInType #-}

 import GHC.Exts (TYPE, RuntimeRep)

 class        A (a :: TYPE rep)
 class A a => B (a :: TYPE rep)

 instance A b => A (a -> (b :: TYPE rep))
 instance B b => B (a -> b)
 }}}

 but the moment you add (`b :: TYPE rep`) to the last line it stops working


 {{{#!hs
 -- t3I7.hs:9:10-40: error: …
 --     • Could not deduce (A b)
 --         arising from the superclasses of an instance declaration
 --       from the context: B b
 --         bound by the instance declaration at /tmp/t3I7.hs:9:10-40
 --     • In the instance declaration for ‘B (a -> b)’
 -- Compilation failed.

 {-# Language PolyKinds, TypeInType #-}

 import GHC.Exts (TYPE, RuntimeRep)

 class        A (a :: TYPE rep)
 class A a => B (a :: TYPE rep)

 instance A b => A (a -> (b :: TYPE rep))
 instance B b => B (a -> (b :: TYPE rep))
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13603>
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