No, all functions do that. Consider the functions f(x)=x^2, and g(x). f(2)=4, but f(g(x))=g(x)^2. A more intuitive example would be abs(x)=|x|. For any numeric input, you get a numeric output, but for a functional input, g(x), you get g`(x) as an output, where g`(x) is a new function simmilar to, but possibly different from, g(x).