What is Functional Programming
Functional programming is quite different from Imperative programming in languages like C, C++ and Java.
Imperative programming languages
In Imperative languages code is written that specifies a sequential of instructions that complete a task. These instructions typically modifies program state until the desired result is achieved.
Variables typically represent memory addresses that are mutable (can be changed) by default.
Functional programming languages
In functional programming individual tasks are small and achieved by passing data to a function which returns a result. This function typically does not change the state of the system or other functions.
Functions are composed together to form more complex tasks and satify larger business logic. These composed functions pass the result of their evaluation to the next function, until all functions in the composition have been evaluated.
The entire functional program can be thought of as a single function defined in terms of smaller ones.
Program execution is an evaluation of expressions, with the nesting structure of function composition determining program flow.
Variables are typically immutable and represent values (in the mathematical sence).