[GHC] #14674: Deferring more levity polymorphism checks in indefinite backpack modules

[GHC]

#14674: Deferring more levity polymorphism checks in indefinite backpack modules
-------------------------------------+-------------------------------------
           Reporter:  andrewthad     |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.4.1-alpha1
           Keywords:  backpack       |  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 is kind of a follow up issue to issue #13955. I'm able to get along a
 little further with the thing I'm trying to do. Here's a minimal example.
 The `number-example-a` and `number-example-b` sections are not really
 needed to trigger the problem. They are included as examples of how the
 signature might be instantiated.

 {{{
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE TypeFamilies #-}

 unit the-fam where
   module TheFamily where
     import Data.Kind
     import GHC.Types
     import GHC.Prim
     data Liftedness = Lifted | Unlifted
     data SingLiftedness (x :: Liftedness) where
       SingLifted :: SingLiftedness 'Lifted
       SingUnlifted :: SingLiftedness 'Unlifted
     type family LiftednessIntRep (x :: Liftedness) :: RuntimeRep where
       LiftednessIntRep 'Lifted = LiftedRep
       LiftednessIntRep 'Unlifted = IntRep
     type family LiftednessInt (x :: Liftedness) :: TYPE (LiftednessIntRep
 x) where
       LiftednessInt 'Lifted = Int
       LiftednessInt 'Unlifted = Int#

 unit number-indefinite where
   dependency the-fam
   signature NumberUnknown where
     import GHC.Types
     import TheFamily
     data MyLiftedness :: Liftedness
   module NumberStuff where
     import NumberUnknown
     import TheFamily
     identityInt :: LiftednessInt MyLiftedness -> LiftednessInt
 MyLiftedness
     identityInt i = i

 unit number-example-a where
   dependency the-fam
   module NumberUnknown where
     import GHC.Types
     import TheFamily
     type MyLiftedness = Lifted
     identityInt :: Int -> Int
     identityInt x = x

 unit number-example-b where
   dependency the-fam
   module NumberUnknown where
     import GHC.Types
     import GHC.Prim
     import TheFamily
     type MyLiftedness = Unlifted
     identityInt :: Int# -> Int#
     identityInt i = i

 unit main where
   dependency number-indefinite[NumberUnknown=number-
 example-a:NumberUnknown]
   module Main where
     import NumberStuff
     import GHC.Types
     main = print (identityInt 5)
 }}}

 Attempting to compile with the ghc 8.4 release candidate gives:

 {{{
 > /usr/local/bin/ghc-8.4.0.20171214 --backpack small_levity_backpack.bkp
 small_levity_backpack.bkp:35:17: error:
     A levity-polymorphic type is not allowed here:
       Type: LiftednessInt MyLiftedness
       Kind: TYPE (LiftednessIntRep MyLiftedness)
     In the type of binder ‘i’
    |
 35 |     identityInt i = i
    |                 ^
 }}}

 I disagree with this error. The type of `i` is not actually levity
 polymorphic in either of the example instantiations of the signature. If
 this check were omitted from the type-checking of the indefinite module, I
 suspect that this code should be able to compile and run fine.

 I'm not sure if there's a good general rule for when to suppress these
 checks in indefinite modules and when to not suppress them. Clearly, there
 are some cases were a function will with a levity-polymorphic binder no
 matter how the signature is fulfilled, and those ideally should continue
 to be rejected. And there are other cases like the one I raise here where
 it's guaranteed to be safe. There are other cases where the whether or not
 there's a levity-polymorphic binder depends on the instantiation:

 {{{
 data X = X1 | X2
 type family Rep (a :: X) (r :: RuntimeRep) :: RuntimeRep
   Rep X1 r = r
   Rep X2 r = UnliftedRep
 }}}

 And then finally, there are situations where it's always going to lead to
 a levity-polymorphic binder, but it isn't feasible for the compiler to
 figure that out. A sufficiently tricky type family would cause this.

 Sorry this was a bit of a ramble. I think there actually is a good rule
 for this. Defer the levity-polymorphic binder check in indefinite modules
 any time the type-checker encounters a type variable whose RuntimeRep is a
 type family that cannot be reduced. The check will happen later any way,
 and more correct code will be accepted.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14674>
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