Functional vs Object-Oriented vs Procedural Programming

During an interview this week I’ve been asked “what is the difference between Functional, Object-Oriented and Procedural Programming” and to be honest I did not really know how to answer to that question (and it was the first time I was hearing about Procedural Programming…). So today, I decided to do some research and (try to) understand what are the main differences.

Let’s start with some definitions:

Functional programming (FP) is a programming paradigm — a style of building the structure and elements of computer programs — that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data.” — Wikipedia

Object-oriented programming (OOP) is a programming paradigm based on the concept of “objects”, which may contain data, in the form of fields, often known as attributes; and code, in the form of procedures, often known as methods.” — Wikipedia

Procedural programming is a programming paradigm, derived from structured programming, based upon the concept of the procedure call. Procedures, also known as routines, subroutines, or functions, simply contain a series of computational steps to be carried out.” — Wikipedia

Ok, so that’s an introduction but let’s go in a little bit more details.

First, what is a programming paradigm? It is a style of programming, a way of thinking about software construction. A programming paradigm does not refer to a specific language but rather to a way to program, a methodology. Some languages make it easy to write in some paradigms but not others. For more details, see the Wikipedia article.

Now let’s dive into the three programming paradigms that interest us today:

Procedural programming (PP), also known as inline programming takes a top-down approach. It is about writing a list of instructions to tell the computer what to do step by step. It relies on procedures or routines.

Object-oriented programming (OOP) is about encapsulating data and behavior into objects. An OOP application will use a collection of objects which knows how to perform certain actions and how to interact with other elements of the application. For example an object could be a person. That person would have a name (that would be a property of the object), and would know how to walk (that would be a method). A method in OOP can be considered as a procedure in PP, but here it belongs to a specific object. Another important aspect of OOP are classes. A class can be considered as a blueprint for an object.

Functional programming (FP) is about passing data from function to function to function to get a result. In FP, functions are treated as data, meaning you can use them as parameters, return them, build functions from other functions, and build custom functions. Functions in FP have to be pure functions, they should avoid shared state, and side effects and data should be immutable. A pure function is a function that given the same type of input will always return the same output, it is not dependent on a local or global state. A shared state is a state that is shared between more than one function or more than one data-structure. So with shared state, in order to understand the effects of a function, you need to know all the details of every shared variable. It adds a lot of complexity and permits less modularity.


That’s it for today! Next time I (and hopefully you) am asked what is the difference between Functional, Object-Oriented and Procedural Programming, I will at least be able to explain the big lines of each.

If you have any additional explanations, or any comments please do not hesitate to add them below.