Accesing R c-code

Hi r-devel,

I am working on a R extension. My package is writen on C++ and in my code
I require a R function object. I received the R function object, then a
point x in which the "function" is going to be evaluated is generated in
some way, then I evalue the "function" at x and I repete this process
several thousand of times. Since I am using the function eval(SEXP fn,
SEXP env) my code is really slow. Due to my major concern is speed, I
wonder to know when it is posible to access to the parse tree of the R
function object and build my own c++ parser tree in order to have c++
doing the evaluations of the function instead of R, as it is now working
in my c++ code. What I want to avoid is to have to develope a complete
parser mainly because it would take me too much time since I am a newe in
compilers and parsers.

It would also help me to know how R mcmc packages work, because it is more
or less the same situation. In mcmc packages a target density function is
required, and I suppose this density function is evaluated many many
times, but this packages are not too slow.