The parametric function classes of ROOT (TFormula and TF1) have been improved using the capabilities of Cling/LLVM. We will present how formula expressions can now be compiled on the fly using the just-in-time capabilities of LLVM/Cing. Furthermore using the new features of C++ 11, one can build complex function expressions by re-using the existing mathematical functions. We will show also the possibility of implementing auto-differentiation for having an automatic derivative computation of the functions.