I want to write a Haskell interface to a C library that provides two data
structures A and B, where B is contained in A. That is, if A is freed,
then B is automatically freed, too.
There are two ways to obtain a Ptr B. Either construct it separately or as
part of A, i.e.
b0 :: Ptr B
b0 <- createB
b1 :: Ptr B
b1 <- getBofA =<< createA
Now I want to let the garbage collector manage the resources for B in a
ForeignPtr. The first case is easy:
fb0 :: ForeignPtr B
fb0 <- newForeignPtr deleteB =<< createB
But how to create an fb1?
fa1 :: ForeignPtr A
fa1 <- newForeignPtr deleteA =<< createA
fb1 :: ForeignPtr B
fb1 <- newForeignPtr ??? =<< withForeignPtr fa1 getBofA
Obviously, there is nothing I can insert for ???. fb1 must not have its
own finalizer, because fa1 already has one. But I have to make sure that
fa1 lives at least as long as fb1. How to do that?
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