[GHC] #13955: Backpack does not handle unlifted types

10 Jul 2017

      #13955: Backpack does not handle unlifted types
-------------------------------------+-------------------------------------
           Reporter:  andrewthad     |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc2
           Keywords:  backpack       |  Operating System:  Unknown/Multiple
  LevityPolymorphism                 |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 In the code snippet below, I attempt to use backpack with levity
 polymorphism:

 {{{
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE DataKinds #-}

 unit number-unknown where
   signature NumberUnknown where
     import GHC.Types
     data Number
     plus :: Number -> Number -> Number
     multiply :: Number -> Number -> Number
   module NumberStuff where
     import NumberUnknown
     funcA :: Number -> Number -> Number
     funcA x y = plus x (multiply x y)

 unit number-int where
   module NumberUnknown where
     type Number = Int
     plus :: Int -> Int -> Int
     plus = (+)
     multiply :: Int -> Int -> Int
     multiply = (*)

 unit number-unboxed-int where
   module NumberUnknown where
     import GHC.Prim
     type Number = Int#
     plus :: Int# -> Int# -> Int#
     plus = (+#)
     multiply :: Int# -> Int# -> Int#
     multiply = (*#)

 unit main where
   dependency number-unknown[NumberUnknown=number-unboxed-
 int:NumberUnknown]
   module Main where
     import NumberStuff
     main = putStrLn "Hello world!"
 }}}

 Compiling this with `ghc --backpack packer.bkp` fails with the following
 error:

 {{{
     - Type constructor ‘Number’ has conflicting definitions in the module
       and its hsig file
       Main module: type Number = GHC.Prim.Int# :: TYPE 'GHC.Types.IntRep
       Hsig file:  data Number
       The types have different kinds
     - while checking that number-unboxed-int:NumberUnknown implements
 signature NumberUnknown in number-unknown[NumberUnknown=number-unboxed-
 int:NumberUnknown]

         type Number = Int#
 }}}

 The error is pretty clear: `Number` can only be instantiated by types of
 kind `Type` (aka `TYPE LiftedRep`). Even while remaining levity
 monomorphic, there doesn't seem to be a way to pick a different kind. For
 example, redefining `Number` in the signature as

 {{{
 data Number :: TYPE IntRep
 }}}

 leads to the following immediate failure:

 {{{
 Kind signature on data type declaration has non-* return kind TYPE 'IntRep
 }}}

 I do not understand any of the internals of backpack, so I do not
 understand if there's anything fundamental that makes this impossible.
 Going one step further, I would like to be able to do something like this
 (the syntax here is not even currently valid for a backpack signature):

 {{{
 type MyRep :: RuntimeRep
 data Number :: TYPE MyRep
 }}}

 This may be instantiated with something like this:

 {{{
 type MyRep = IntRep
 type Number = Int#
 }}}

 And then end users would be able to monomorphize levity-polymorphic
 functions. This would be really neat because there is currently no way to
 do this in GHC.

 So, I guess there are really two feature requests in here. One is the
 ability to use unlifted data types with backpack. The other is the ability
 to use backpack to monomorphize levity-polymorphic functions.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13955
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