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As issue #33 (closed) points out, if a callback is saved into a C++ variable that persists past the lifetime of the GHC RTS, then it's too late to be calling back into Haskell to do freeHaskellFunPtr, and we get the following warning as the program shuts down:

hoppy-example: newBoundTask: RTS is not initialised; call hs_init() first

If we can check whether the RTS is alive from a callback's destructor, then we could do nothing if it's dead. Or perhaps we can use hs_free_fun_ptr(). One problem with both of these is that if the RTS is shut down and started up again as is explicitly allowed by hs_exit()/hs_init(), then our function pointers will be garbage, so it might be better to see whether we can use finalizers or some RTS shutdown hook to invalidate callbacks.