Interesting.
Can you summarise how ForeignPtr is defined right now? And point to any
documentation about the specification of ForeignPtr, or Notes about its
implementation?
So that we can compare with what your proposal.
What’s the connection with weak pointers?
Simon
From: ghc-devs <ghc-devs-boun...@haskell.org> On Behalf Of David Feuer
Sent: 05 September 2018 00:23
To: ghc-devs <ghc-devs@haskell.org>
Subject: ForeignPtr representation
I'm trying, and failing, to understand why ForeignPtr is defined the way it is.
It all seems rather complicated and redundant. I was thinking we might want to
switch to something simpler and more explicit, like this:
data ForeignPtr a = ForeignPtr {
fptr :: !(Ptr a) -- What we point to
froot :: MutableByteArray# RealWorld
fwk :: MutVar# RealWorld Any -- The finalizers
}
froot represents the object the ForeignPtr points into. When created by one of
the malloc variants, this will actually be the allocated byte array. Otherwise,
it will be a 0-length array.
fwk should be seen as having type
fwk :: MutVar# RealWorld (Weak# (MutableByteArray# RealWorld))
but we can't express that directly right now. fwk is keyed on froot, and in the
malloc case also points to it, to prevent it from being freed prematurely.
It seems that this representation requires a couple extra primitive features
from Weak#. I think that's okay: Weak# already has some primitive features
designed to support ForeignPtr.
1. An operation to add a Haskell finalizer to a Weak#, similar to the one that
adds a C finalizer.
2. A way to deal with mixed finalizers: either make the finalizer-adding
primops report distinguishable failure on mixing or arrange to run the C
finalizers after any Haskell finalizers (since C finalizers are much more
likely to actually invalidate the pointer).
3. A documented guarantee about the order in which the finalizers attached to a
particular Weak# run, compatible with the ForeignPtr documentation.
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