Wow, that's a useful bit of information. Thank you!
On Sun, Dec 2, 2018, 3:22 PM GHC #15777: Ordering of code in file affects compilation
-------------------------------------+-------------------------------------
Reporter: chessai | Owner: (none)
Type: bug | Status: closed
Priority: normal | Milestone: 8.6.2
Component: Compiler | Version: 8.6.1
Resolution: duplicate | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: GHC rejects | Unknown/Multiple
valid program | Test Case:
Blocked By: | Blocking:
Related Tickets: #12088 | Differential Rev(s):
Wiki Page: |
-------------------------------------+------------------------------------- Comment (by RyanGlScott): One other trick worth noting (that I learned recently from #15561) is that
open and closed type families behave differently in SCC analysis, so
turning `Rep` into a closed type family actually makes this typecheck.
That is to say, the following compiles: {{{#!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 where
Rep Int = IntRep
Rep Word = WordRep
Rep Stuff = TupleRep '[ 'IntRep, 'IntRep ] type Stuff# = (# Int#, Int# #) data Stuff = Stuff Int# Int# stuff# :: (# Int#, Int# #) -> Stuff
stuff# (# x, y #) = Stuff x y unStuff# :: Stuff -> (# Int#, Int# #)
unStuff# (Stuff x y) = (# x, y #)
}}} You may not be able to get away with making `Rep` a closed type family in
the actual program that you're writing, but I thought I'd point it out
nonetheless, since I was myself unaware of this fact until today. --
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15777#comment:5
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