[GHC] #14555: GHC Panic with TypeInType / levity polymorphism

4 Dec 2017

      #14555: GHC Panic with TypeInType / levity polymorphism
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.3
           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:
-------------------------------------+-------------------------------------
 {{{#!hs
 {-# Language TypeInType #-}
 {-# Language TypeOperators, DataKinds, PolyKinds #-}

 import Data.Kind
 import GHC.Types (TYPE)

 data Exp :: [TYPE rep] -> TYPE rep -> Type where
   Lam :: Exp (a:xs) b -> Exp xs (a -> b)
 }}}

 gives

 {{{
 $ ghci -ignore-dot-ghci hs/126-bug.hs
 GHCi, version 8.3.20171122: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( hs/126-bug.hs, interpreted )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.3.20171122 for x86_64-unknown-linux):
         zonkTcTyVarToVar
   'LiftedRep
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1147:37 in
 ghc:Outputable
         pprPanic, called at compiler/typecheck/TcType.hs:1640:17 in
 ghc:TcType

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 caused by `TypeInType`. Removing the first line, we get an error

 {{{
 GHCi, version 8.3.20171122: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( hs/126-bug.hs, interpreted )

 hs/126-bug.hs:7:19: error:
     • Expected kind ‘GHC.Types.RuntimeRep’, but ‘rep’ has kind ‘*’
     • In the first argument of ‘TYPE’, namely ‘rep’
       In the kind ‘[TYPE rep] -> TYPE rep -> Type’
   |
 7 | data Exp :: [TYPE rep] -> TYPE rep -> Type where
   |                   ^^^

 hs/126-bug.hs:7:32: error:
     • Expected kind ‘GHC.Types.RuntimeRep’, but ‘rep’ has kind ‘*’
     • In the first argument of ‘TYPE’, namely ‘rep’
       In the kind ‘[TYPE rep] -> TYPE rep -> Type’
   |
 7 | data Exp :: [TYPE rep] -> TYPE rep -> Type where
   |                                ^^^
 Failed, no modules loaded.
 Prelude>
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14555
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