Your best source will be “A Taste of Functional Programming” on p.74 of the newest, 4th edition of Programming in Lua. There, the creator of Lua adopts an example from the research report Haskell vs. Ada vs. C++ vs. Awk vs. ... An Experiment in Software Prototyping Productivity, by Paul Hudak and Mark P. Jones.
The point is, Lua provides the mechanisms by which to write programs using functional programming patterns and constructs. As said here, functional programming is a programming paradigm. A style. So, some programming languages are built for that paradigm exclusively. Others, like Lua, offer a variety of programming mechanisms.