What is the difference between declarative and imperative programming?

Antonio Cucciniello

October 11th, 2017

Declarative and imperative programming are methods or approaches to programming. Using either will depend on how you want to work on a given project or application, and what you want to achieve.

What is declarative programming?

Let us first start with declarative programming. This is the form or style of programming where we are most concerned with what we want as the answer, or what would be returned. Here, we as developers are not concerned with how we get there, simply concerned with the answer that is received.

What is imperative programming?

Next, let's take a look at imperative programming. This is the form and style of programming in which we care about how we get to an answer, step by step. We want the same result ultimately, but we are telling the complier to do things a certain way in order to achieve that correct answer we are looking for.

An analogy

If you are still confused, hopefully this analogy will clear things up for you. The analogy is comparing wanting to learn a topic or outsourcing work to have someone else do it.

If you are outsourcing work, you might not care about how the work is completed; rather you care just what the final product or result of the work looks like. This is likened to declarative programming. You know exactly what you want and you program a function to give you just that.

Let us say you did not want to outsource work for your business or project. Instead, you tell yourself that you want to learn this skill for the long term. Now, when attempting to complete the task, you care about how it is actually done. You need to know the individual steps along the way in order to get it working properly. This is similar to imperative programming.

Why you should use declarative programming

Reusability

Since the way the result is achieved does not necessarily matter here, it allows for the functions you build to be more general and could potentially be used for multiple purposes and not just one. Not rewriting code can speed up the program you are currently writing and any others that use the same functionality in the future.

Reducing Errors

Given that in declarative programming you tend to write functions that do not change state as you would in functional programming, the chances of errors arising are smaller and it allows for your application to become more stable. The removal of side effects from your functions allows you to know exactly what comes in and what comes out, allows for a more predictable program.

Potential drawbacks of declarative programming

Lack of Control

In declarative programming, you may use functions that someone else created, in order to achieve the desired results. But you may need specific things to be completed behind the scenes to make your result come out properly. You do not have this control in declarative programming as you would in imperative programming.

Inefficiency

When the implementation is controlled by something else, you may have problems making your code efficient. In applications where there may be a time constraint, you will need to program the individual steps in order to make sure your program is running as efficient as possible.

There are benefits and disadvantages to both forms. Overall, it is entirely up to you, the programmer, to decide which implementation you would like to follow in your code. If you are solely focused on the data, perhaps consider using the declarative programming style. If you care more about the implementation and how something works, maybe stick to an imperative programming approach. More importantly, you can have a mix of both styles. It is extremely flexible for you. You are in charge here.


Antonio Cucciniello is a Software Engineer with a background in C, C++ and JavaScript (Node.Js) from New Jersey.  His most recent project called Edit Docs is an Amazon Echo skill that allows users to edit Google Drive files using your voice.  He loves building cool things with software, reading books on self-help and improvement, finance, and entrepreneurship. Follow him on twitter @antocucciniello, and follow him on GitHub.