[GHC] #14111: strange error when using data families with levity polymorphism and unboxed sums and data families

13 Aug 2017

      #14111: strange error when using data families with levity polymorphism and unboxed
sums and data families
-------------------------------------+-------------------------------------
           Reporter:  carter         |             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:
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 I've the following small example

 {{{
 {-# LANGUAGE MagicHash, UnboxedSums, NoImplicitPrelude #-}
 {-# LANGUAGE TypeFamilies #-}
 -- {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE GADTs ,ExplicitNamespaces#-}
 {-# LANGUAGE UnboxedTuples #-}

 module Data.Unboxed.Maybe where

 import GHC.Exts
 import GHC.Types
 import Prelude (undefined)
 import Data.Void

 data family Maybe(x :: TYPE (r :: RuntimeRep))

 data instance Maybe (a :: * ) where
   MaybeSum :: (# a  | (# #) #) -> Maybe a
 data instance Maybe (x :: TYPE 'UnliftedRep) where
   MaybeSumU :: (# x | (# #) #) -> Maybe x
 }}}

 and then i get the error (made much saner to read by use of printing
 explicit kinds)

 {{{
 Prelude> :r
 [1 of 1] Compiling Data.Unboxed.Maybe ( src/Data/Unboxed/Maybe.hs,
 interpreted )

 src/Data/Unboxed/Maybe.hs:22:3: error:
     • Data constructor ‘MaybeSumU’ returns type ‘Maybe 'LiftedRep x’
         instead of an instance of its parent type ‘Maybe 'UnliftedRep x’
     • In the definition of data constructor ‘MaybeSumU’
       In the data instance declaration for ‘Maybe’
    |
 22 |   MaybeSumU :: (# x | (# #) #) -> Maybe x
 }}}

 this is

 a) a case where printing runtime reps makes things easier to debug :)

 b) a very confusing type error since the data instance clearly says "x ::
 TYPE 'UnliftedRep "

 is there something i'm overlooking, or is this a bug?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14111
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