Simon Marlow wrote:
Brian Hulley wrote:
I've got the beginnings of an API for a GUI system as follows:
...
addTop :: ForeignPtr (Window a) -> IO ()
addTop w = withForeignPtr w duma_addTop
This works, but it seems a bit of a pain to have to manually convert
between ForeignPtr's and Ptr's all the time.
In particular, for the definition of addTop, I tried:
foreign import ccall "duma_addTop" addTop :: ForeignPtr (Window a)
-> IO ()
This is the way it used to be in GHC before the FFI. In the FFI we
moved to withForeignPtr instead. IIRC, the motivation was something
along these lines:
- allowing ForeignPtr to be passed directly to a foreign function
implies some magic that happens at the point of the foreign call
to convert the ForeignPtr to a Ptr.
- there also has to be some magic to ensure that the ForeignPtr
couldn't be finalized until the call returns. This amounted to
adding a touch# primitive to keep the ForeignPtr alive over the
call. So internally the compiler was doing something like
withForeignPtr anyway. This behaviour is quite hard to explain in
the spec, withForeignPtr is much simpler.
The above two things were what I was expecting the compiler to do for me
instead of me having to manually write a wrapper function in Haskell using
withForeignPtr for each wrapper function for each foreign function...
- We wanted withForeignPtr anyway, to avoid having to duplicate all
the marshalling operations that operate on Ptr.
So, given that we wanted withForeignPtr anyway, there was no need to
also have the compiler do its internal magic to allow ForeignPtr to be
used as an FFI argument. Also, this means GHC doesn't need a
primitive ForeignPtr type (the primitive Ptr type is enough).
Would it be possible to just treat ForeignPtr in foreign types as syntactic
sugar ie
foreign import ccall foo :: ForeignPtr a -> IO ()
would just be syntactic sugar for:
foreign import ccall "foo" foo' :: Ptr a -> IO ()
foo :: ForeignPtr a -> IO ()
foo x = withForeignPtr x foo'
The re-writing could be done at an early stage so that you'd still only need
to deal with marshalling Ptr, but would save the user from having to
manually create these extra wrappers, and would also give a simple way of
explaining the meaning of a ForeignPtr argument in a foreign function type.
Later on, we discovered that the withForeignPtr interface enables a
much more efficient representation of ForeignPtr. This is coming in
GHC 6.6.
Every cloud has a silver lining! :-)))
Thanks, Brian.
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