Obtain a function as tree in SML

Question

How to obtain syntactic representation of the function in PolyML? (Usually functions are hidden, cannot be pretty-printed.)

For my purposes I need to not only use functions, but also I need to print explicitly how they are written. (and maybe change them "on the fly", and execute)

I think it may be connected with CodeTree or Compiler-related structures, but I don't know how to organize it.