State Machine

A state machine is just a fancy word for a system that can be in one of many different states. For example, this function is not a state machine:

func add(a, b int) int {
    return a + b
}

Because it doesn't have any internal state. It just takes two numbers and returns their sum. But this function is a state machine:

type Counter struct {
    count int
}

func (c *Counter) Add(a int) int {
    c.count += a
    return c.count
}

Because it has internal state (the count field), and each time we call Add, it changes the state of the Counter struct. It's not a pure function, it's a stateful function.