[GHC] #15777: Ordering of code in file affects compilation

18 Oct 2018

      #15777: Ordering of code in file affects compilation
-------------------------------------+-------------------------------------
           Reporter:  chessai        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.2
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 consider the following module:

 {{{#!hs
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE UnboxedTuples #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}

 -- | Conversion between unlifted and lifted datatypes
 module Packed.Levity
   ( -- * Types
     Rep
   , Levity(..)
   ) where

 import Data.Kind (Type)
 import GHC.Types (TYPE, RuntimeRep(..), Int(..), Word(..))
 import GHC.Exts  (Int#, Word#, ByteArray#)

 type family Rep (a :: Type) :: RuntimeRep

 type instance Rep Int = IntRep
 type instance Rep Word = WordRep

 type Stuff# = (# Int#, Int# #)

 data Stuff = Stuff Int# Int#

 type instance Rep Stuff = TupleRep '[ 'IntRep, 'IntRep ]

 stuff# :: (# Int#, Int# #) -> Stuff
 stuff# (# x, y #) = Stuff x y

 unStuff# :: Stuff -> (# Int#, Int# #)
 unStuff# (Stuff x y) = (# x, y #)

 class Levity (a :: Type) where
   type Unlifted a :: TYPE (Rep a)
   box   :: Unlifted a -> a
   unbox :: a -> Unlifted a

 instance Levity Int where
   type Unlifted Int = Int#
   box = I#
   unbox (I# i) = i

 instance Levity Word where
   type Unlifted Word = Word#
   box = W#
   unbox (W# w) = w

 instance Levity Stuff where
   type Unlifted Stuff = Stuff#
   box = stuff#
   unbox = unStuff#
 }}}

 This succeeds to compile.

 Now, if we move everything from `type family Rep` to `unStuff# (` to the
 bottom of the module, it fails to compile.

 {{{#!hs
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE UnboxedTuples #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}

 -- | Conversion between unlifted and lifted datatypes
 module Packed.Levity
   ( -- * Types
     Rep
   , Levity(..)
   ) where

 import Data.Kind (Type)
 import GHC.Types (TYPE, RuntimeRep(..), Int(..), Word(..))
 import GHC.Exts  (Int#, Word#, ByteArray#)

 class Levity (a :: Type) where
   type Unlifted a :: TYPE (Rep a)
   box   :: Unlifted a -> a
   unbox :: a -> Unlifted a

 instance Levity Int where
   type Unlifted Int = Int#
   box = I#
   unbox (I# i) = i

 instance Levity Word where
   type Unlifted Word = Word#
   box = W#
   unbox (W# w) = w

 instance Levity Stuff where
   type Unlifted Stuff = Stuff#
   box = stuff#
   unbox = unStuff#

 type family Rep (a :: Type) :: RuntimeRep

 type instance Rep Int = IntRep
 type instance Rep Word = WordRep

 type Stuff# = (# Int#, Int# #)

 data Stuff = Stuff Int# Int#

 type instance Rep Stuff = TupleRep '[ 'IntRep, 'IntRep ]

 stuff# :: (# Int#, Int# #) -> Stuff
 stuff# (# x, y #) = Stuff x y

 unStuff# :: Stuff -> (# Int#, Int# #)
 unStuff# (Stuff x y) = (# x, y #)
 }}}

 {{{
 ts.hs:33:25-30: error:
     • Expected kind ‘TYPE (Rep Stuff)’,
         but ‘Stuff#’ has kind ‘TYPE ('TupleRep '['IntRep, 'IntRep])’
     • In the type ‘Stuff#’
       In the type instance declaration for ‘Unlifted’
       In the instance declaration for ‘Levity Stuff’
    |
 33 |   type Unlifted Stuff = Stuff#
    |                         ^^^^^^
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15777
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler