Learn The Basics of Functional Programming

At this point, you should at least have heard about how cool functional programming is. There are a lot of concepts here, but at least the very basic ones you should keep in mind.

A lot of talks about functional programming come with weird words like "functors" and "monads". It doesn't hurt to know what they really mean (disclaimer: I still don't). But some other stuff coming from functional programming is actually easy to understand and grasp.

For example, immutability. This means that all your data can't change once it's created. Do you have a record with user information and the user changed their password? No, do not change the password field, create a new user record with the updated password and discard the old one. Sure, it does a lot of "create and destroy" sequences which makes absolutely no sense (why would you allocate memory for a new user, copy everything from the old one to the new one, update one field, and "deallocate" the memory from the old one? It makes no sense!) but, in the long run, it would prevent weird results, specially when you understand and start use threads.

(Basically, you're avoiding a shared state -- the memory -- between parts of your code.)

Another useful concept is pure functions. Pure functions are functions that, called with the same parameters, always return the same result, no matter how many times you call them. One example of a non pure function is random(): each time you call random(), you get a different number¹. An example of a pure function would be something like this in Python:

def mult(x):
   return x * 4

No matter how many times you call mult(2), it will always return 8. Another example could be our immutable password change above: You could easily write a function that receives a user record and returns a new user record with the password changed. You could call with the same record over and over again and it will always return the same resulting record.

Pure functions are useful 'cause they are, first most, easy to test.

Second, they are easy to chain, specially in a data flow design: Because they don't have an internal state (which is the real reason they are called pure functions), you can easily call one after the other and no matter how many times you pass things around, they still produce the same result. And because each function, given the same input, produce the same result, chaining them all also produces the same result given the same inputs.

Just those two concepts can make code longer (again, you're creating a new user record instead of simply changing one field), but the final result is a more robust code.