Martin Hofmann wrote:

Thanks a lot, this helps a bit, but access to function bodies is exactly what I need.

Then perhaps you might like the method of reconstructing bodies (of possibly compiled) functions http://okmij.org/ftp/Computation/Generative.html#diff-th in the form of AST -- the template Haskell AST. The reconstructed bodies of functions can be arbitrarily manipulated (e.g., _symbolically_ differentiated or algebraically simplified) and then converted `back' to the compiled code.