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?