Before we jump into writing code, let's discuss the ideas and motivations behind functional programming.
Functional programming makes it significantly easier to think of each component in isolation. This includes things such as types, functions, and methods. If we can wrap our minds around everything that is input into these code components and everything that should be returned from them, we could analyze the code easily to ensure that there are no bugs and it performs well. Every type is created with a certain number of parameters and each method and function in a program has a certain number of parameters and return values. Normally, we think about these as the only inputs and outputs, but the reality is that often there are more. We refer to these extra inputs and outputs as state.
In a more general sense, state is any stored information, however temporary, that can be changed. Let's consider a simple double function:
func double(input...