Handling of unlifted types

[Simon Peyton Jones]

Suppose

  f1 :: forall (a :: TYPE LiftedPtrRep). a -> Closure
  f2 :: forall (a :: TYPE UnliftedPtrRep). a -> Closure
  f3 :: forall (a :: TYPE FloatRep). a -> Closure

f3 takes its argument in a floating point register, whereas f1 and f2 take their arguments in a pointer register; so you can't have a function polymorphic in all three.  See the paper (have you read it?)

You might think that f1 and f2 are more compatible, since they both take a pointer, but consider the call
	f1 (g x)
	f2 (h x)

For f1 we'll build a thunk for (g x) and pass it to f1.  For f2 we'll evaluate (h x) and pass the result to f2.  So the calls are different.

In short, there is a good reason that we can't be polymorphic here. You'll have to use three different functions.

Simon


|  -----Original Message-----
|  From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Erik de
|  Castro Lopo
|  Sent: 05 February 2017 08:49
|  To: ghc-devs at haskell.org
|  Subject: Handling of unlifted types
|  
|  Hi all,
|  
|  I'm working on pulling the core parts of Joachim Breitner's ghc-heap-view
|  library into GHC. The WIP Phab review is here:
|  
|      https://phabricator.haskell.org/D3055
|  
|  Currently that library has a function:
|  
|      getClosureData :: a -> IO Closure
|  
|  which works fine for lifted types. However, some of the supported closure
|  types in
|  
|  
|  https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fhackage.hask
|  ell.org%2Ftrac%2Fghc%2Fbrowser%2Fincludes%2Frts%2Fstorage%2FInfoTables.h&dat
|  a=02%7C01%7Csimonpj%40microsoft.com%7Ca5585e761d5149d4f3c608d44da3f3dc%7C72f
|  988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636218813965987006&sdata=5ze3nbAjXYm
|  Ey7H6M4Imtyz7dVH1bjEtloUJWQ2ItWo%3D&reserved=0
|  
|  actually refer to unlifted types, like the `MUT_ARR_PTRS_*` and
|  `SMALL_MUT_ARR_PTRS_*`.
|  
|  I've had a look at the levity polymorphism stuff implemented in ghc 8.0 and
|  came up with a new type for `getClosureData`:
|  
|      getClosureData :: forall (r :: RuntimeRep) (a :: TYPE r). a -> IO
|  Closure
|  
|  but that results in:
|  
|      A representation-polymorphic type is not allowed here:
|        Type: a
|        Kind: TYPE r
|      In the type of binder ‘x’
|  
|  Am I barking up the wrong tree here? Any one have a clue on how I might make
|  progress on this?
|  
|  
|  Erik
|  --
|  ----------------------------------------------------------------------
|  Erik de Castro Lopo
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