Actually it depends who you ask. There's kind of two school of thought. One says FP is about using functions with closures, higher order functions and anonymou

Actually it depends who you ask. There's kind of two school of thought. One says FP is about using functions with closures, higher order functions and anonymous functions as the main building blocks for modeling your program logic. This style is championed by Scheme and other Lisps. It is also slowly creeping up in all major languages. The other is about immutable data and pure functions, and modeling your program using those as your main building blocks. Either as much as possible, and sometimes even going as far as restricting yourself only to this.