Ordering of code in file affects compilation

consider the following module:

{-# 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.

{-# 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#
   |                         ^^^^^^

Trac metadata

Trac field	Value
Version	8.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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