[Haskell-cafe] example of FFI FunPtr

Hello, I want to model a Haskell function that is a callback from C. I have only found one example in the unix package's Semaphore.hsc, which apparently is not used. I want to be able to marshall a Haskell function that is a first class citizen residing in a Haskell data type and pass to a C function via FFI. Are there examples of this? Thanks, Vasili