ForeignPtr query

[Alastair Reid]

"Peter Gammie" <peteg at cse.unsw.EDU.AU> wants:
> a general "stick this arbitrary (small) C object in an
> optimiser-impervious box" generalisation of ForeignPtr.

Simon Marlow writes:
> There was a proposal for such a thing kicking around recently, and I
> think there's no real problems with it.

The discussion was in the 'ForeignPtr' thread back in November:

  http://haskell.org/pipermail/ffi/2002-November/thread.html

There's several designs that are variants on:

  data ForeignObj a            -- abstract
  instance Eq (ForeignObj a)
  
  type Finalizer a = FunPtr (a -> IO ())
  
  newForeignObj   :: a -> Finalizer a -> IO (ForeignObj a) 
  addFinalizer    :: ForeignObj a -> Finalizer a -> IO () 
  withForeignObj  :: ForeignObj a -> (a -> IO b) -> IO b 
  touchForeignObj :: ForeignObj a -> IO () 
  
[A variant on this is to have a 'Finalizable' constraint on the type
variable 'a' but I'm not sure why we need this.  Also, I'm not sure
what the status of the 'touch' operations is these days - IIRC, the
GHC folk say it is hard for them to implement correctly.]

I think it makes sense to do this or something very like it.  We
really ought to have done this when we switched from 'Addr' to 'Ptr a'
but we misgeneralized.

--
Alastair Reid

ps Note that the Eq instance I wrote is for equality of the ForeignObj
   container, not the contents.