[GHC] #14155: GHC mentions unlifted types out of the blue (to me anyway)

24 Aug 2017

      #14155: GHC mentions unlifted types out of the blue (to me anyway)
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This one does me 'ead in, I accidentally type `Ran` instead of `Swap`

 {{{#!hs
 {-# Language RankNTypes, DerivingStrategies, TypeApplications,
 ScopedTypeVariables, GADTs, GeneralizedNewtypeDeriving, InstanceSigs,
 PolyKinds #-}

 import Data.Coerce

 newtype Ran g h a = Ran (forall b. (a -> g b) -> h b)

 newtype Swap p f g a where
   Swap :: p g f a -> Swap p f g a
   deriving newtype
     Show

 class IxPointed m where
   ireturn :: a -> m i i a

 instance IxPointed f => IxPointed (Swap f) where
   ireturn :: forall a i. a -> Swap f i i a
   ireturn a = Ran (ireturn a)
 }}}

 and get this error

 {{{
 $ ghci -ignore-dot-ghci /tmp/bug.hs
 GHCi, version 8.3.20170605: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( /tmp/bug.hs, interpreted )

 /tmp/bug.hs:17:15: error:
     • Couldn't match expected type ‘Swap f i i a’
                   with actual type ‘Ran g0 h0 a0’
     • In the expression: Ran (ireturn a)
       In an equation for ‘ireturn’: ireturn a = Ran (ireturn a)
       In the instance declaration for ‘IxPointed (Swap f)’
     • Relevant bindings include
         a :: a (bound at /tmp/bug.hs:17:11)
         ireturn :: a -> Swap f i i a (bound at /tmp/bug.hs:17:3)
    |
 17 |   ireturn a = Ran (ireturn a)
    |               ^^^^^^^^^^^^^^^

 /tmp/bug.hs:17:20: error:
     • Couldn't match a lifted type with an unlifted type
       Expected type: (a0 -> g0 b) -> h0 b
         Actual type: (->) (a0 -> g0 b) (a0 -> g0 b) a
     • In the first argument of ‘Ran’, namely ‘(ireturn a)’
       In the expression: Ran (ireturn a)
       In an equation for ‘ireturn’: ireturn a = Ran (ireturn a)
    |
 17 |   ireturn a = Ran (ireturn a)
    |                    ^^^^^^^^^
 Failed, modules loaded: none.
 }}}

 Is GHC right to bring up unlifted types? I would guess this is due to the
 newly added levity polymorphism of `(->) :: TYPE rep -> TYPE rep' -> Type`

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14155
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler