Functional Programming: A PragPub Anthology

It’s About Functions

At its core, functional programming is about programming with pure, side-effect-free functions.

This is a pure function:

f(x) = <raise power="2">x</raise>

So is this:

public int incrementCounter(int counter) {
  return counter++;
}

This is not:

private int counter = 0;
public void incrementMutableCounter() {
  counter++;
}

The first two examples increment a counter by returning a new integer that’s one higher than the passed-in integer. The third example does the same thing but does it by mutating a bit of state that may be shared across many pieces of a program.

A definition: a function like incrementCounter that doesn’t rely on mutating state is called a pure function. And this purity ...