Unlifted data types

[Richard Eisenberg]

I have put up an alternate set of proposals on 

  https://ghc.haskell.org/trac/ghc/wiki/UnliftedDataTypes

These sidestep around `Force` and `suspend` but probably have other problems. They make heavy use of levity polymorphism.

Back story: this all was developed in a late-evening Haskell Symposium session that took place in the hotel bar. It seems Edward and I walked away with quite different understandings of what had taken place. I've written up my understanding. Most likely, the Right Idea is a combination of this all!

See what you think.

Thanks!
Richard

On Sep 8, 2015, at 3:52 AM, Simon Peyton Jones <simonpj at microsoft.com> wrote:

> |  And to
> |  be honest, I'm not sure we need arbitrary data types in Unlifted;
> |  Force (which would be primitive) might be enough.
> 
> That's an interesting thought.  But presumably you'd have to use 'suspend' (a terrible name) a lot:
> 
> type StrictList a = Force (StrictList' a)
> data StrictList' a = Nil | Cons !a (StrictList a)
> 
> mapStrict :: (a -> b) -> StrictList a -> StrictList b
> mapStrict f xs = mapStrict' f (suspend xs)
> 
> mapStrict' :: (a -> b) -> StrictList' a -> StrictList' b
> mapStrict' f Nil = Nil
> mapStrict' f (Cons x xs) = Cons (f x) (mapStrict f xs)
> 
> 
> That doesn't look terribly convenient.
> 
> | ensure that threads don't simply
> |  pass thunks between each other. But, if you have unlifted types, then
> |  you can have:
> |  
> |      data UMVar (a :: Unlifted)
> |  
> |  and then the type rules out the possibility of passing thunks through
> |  a reference (at least at the top level).
> 
> Really? Presumably UMVar is a new primitive? With a family of operations like MVar?  If so can't we just define
> 	newtype UMVar a = UMV (MVar a)
> 	putUMVar :: UMVar a -> a -> IO ()
> 	putUMVar (UMVar v) x = x `seq` putMVar v x
> 
> I don't see Force helping here.
> 
> Simon
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