How to get started with functional programming — The Endeavour: "Someone asked me this weekend how to get started with functional programming. My answer: Start by writing pure functions in the programming language you’re currently using.
The only input to a pure function is its argument list and the only output is its return value. If you haven’t seen this before, you might think all functions are pure. After all, any function takes in values and returns a value. But in conventional programming there are typically out-of-band ways for information to flow in or out of a function. For example, an impure function may depend on a global variable or class member data. In that case, it’s behavior is not entirely determined by its arguments. Similarly, an impure function might set a global variable or write to a database. In that case the function has a side effect in addition to its return value."
