Elementary Examples of Functionals

Question

I'm working on a research project that's a little over my head, so forgive some simple questions.

Is a composite function $f(g(x))$ a functional? What are a handful of other simple examples of functionals?

Answer 1

https://en.wikipedia.org/wiki/Functional_%28mathematics%29

For example, the function which returns the length of a vector in Rn is a functional.