It’s a programming paradigm where every action is made up of functions, functions which have no side effects beyond their explicit outputs.

As others mentioned, it’s a programming paradigm. It and discussions around it have zero implications outside of programming.

People can write great applications using it or using any other paradigm. Same is true for terrible applications.

Some people love it, some people hate it, most are somewhere in between and think it has their merits and tradeoffs, and that it can be used where it makes sense, but shouldn’t where it doesn’t.

Heated discussions are very common in tech circles over things that have zero practical implications outside our own little world, and this is one of them. 😄

Nah nah nah, it’s just a different paradigm. It’s like… it’s a different meta, but for programming.

If I was to ELI5, I’d say it’s the difference between making a series of objects that interact with each other (OOP) and and creating a very large console with lots of buttons (functional)

Some problems are much easier object oriented. If you have a video game, building a projectile class and adding different types of projectiles makes things suuuper easy to build. Arrows move slow and deal x damage, beams move super fast and deal y.

Functional can be easier to troubleshoot, also easier to crank out novel things, easier to make secure, and even make simultaneous operations trivial. It has very different problems though. You need to put more effort into eliminating dependencies.

The two do not play well together at all they’re like fire and water. Sometimes you need water to soak something and make it easier to work, sometimes you need heat to melt something and make it easier to work.

Telecommunications like phones or military applications tend to the functional languages, data, video games, and a lot of the popular languages tend to OOP.

The transition between the two is jarring, and infuriating, but a knowledge of both can really improve your design skills.

Let me try getting this to ELI5 levels:

Most programming languages use a way of programming called “procedural programming” (or a variation of that, for example object-oriented programming). A series of instructions gets executed step by step to modify the program’s overall state. Imagine it like a cooking recipe. You have control structures like if, while and for that influence the order in which instructions are executed. There are functions (or methods in object-oriented programming) that let you reuse some instructions over and over but that’s not quite functional programming.

I’ll use a syntax similar to Java, C++ or C# as an example:

void squareAllElements(int[] array)
{
    for(int i = 0; i < array.size(); i++)
    {
        array[i] = array[i] * array[i];
    }
}

Functional programming is a bit more like what you know from maths class. There are still functions that take a list of inputs and produce an output but there is no state to be modified. Every operation must take everything it needs (apart from maybe constants) as parameters and produces new values/objects as an output. You say you know Java, so imagine that none of your methods returns void and every variable can only be assigned once. There are no loops but for example operations that let you separate the first element of a list from the rest or create a new list by applying an operation to all elements of an input. To make that easier, functions can take other functions as parameters.

So let me show the same example in a functional style. This is no specific language, just an example to help you imagine how it works.

int square(int x) => x * x;
int[] squareElements(int[] array) => applyToAllElements(array, square);

Functional programming has its own advantages and disadvantages. It can avoid a lot of bugs because functions can’t have unwanted side effects. They just take inputs, return outputs and leave the rest of the world (including their inputs) unmodified. On the other hand, that makes other use cases where you actually want to have some state that changes over time more difficult.

Hope that helps.

Edit: some cleanup and more precise explanations.

Edit: replaced “paradigm” with “way of programming” to make @palordrolap@fedia.io happy.