calling a function on comsol 3.5a

Hi all,
I am using the comsol 3.5a and wrote a function of a linear intrpolation of a table called myFun. now myFun is not a function of x (the geometrical component) it is a function of the variable fi.
when I use this function on my model's globel expressions how do I call the function? myFun(fi)?
how do I define fi to be the function's variable?