Imperative vs. Declarative Programming - Pros and Cons

In software development, understanding different programming paradigms is crucial for writing efficient and maintainable code. Two of the most prevalent paradigms are imperative and declarative programming. This article delves into the core principles, advantages, and practical applications of each paradigm.

Read on and check out my presentation on programming paradigms.

A quick (historical) recap

The idea of imperative programming goes way back to 1950s, which is when the first high-level programming languages have been created. With 72 vacuum tube transistors and 18K of memory at disposal, the programs must have been as performant as possible. The only way to achieve it was to write programs in terms of step-by-step recipes for computer to execute.

FORTRAN and COBOL were designed to offer precise control over hardware with step-by-step instructions. In the 1960s, ALGOL introduced structured programming, which influenced many subsequent languages. The 1970s brought C, which became a cornerstone for system programming due to its efficiency and control over system resources.

The 1980s marked a pivotal shift with the rise of declarative programming. SQL, developed in the 1970s and standardized in the 1980s, allowed for efficient database querying by specifying what data to retrieve rather than how to retrieve it. Prolog, another influential language from the 1970s, focused on logic programming and became popular in artificial intelligence research. The 1990s and 2000s saw the advent of functional programming languages like Haskell and the growing popularity of JavaScript libraries and frameworks such as React, which embrace a declarative approach to UI development.

That's basically what imperative programming is all about — describing a program in terms of instructions which change its state. This 60-years-old style is still the most popular paradigm across many modern programming languages. JavaScript, Ruby, Objective-C and even the brand-new Swift — they are all imperative by default.

However, let's stop here and look at the following, typically imperative example:

This code seems overly complicated, considering it just downloads, parses and handles some remote data. As the project evolves and developers add more steps — the Pyramid of Doom quickly emerges.

Imperative Programming - Pros

Performance

Imperative programming offers optimal performance because it provides low-level control over system resources. This is crucial for performance-critical applications where every millisecond counts. By managing memory and processing power directly, developers can fine-tune their programs for maximum efficiency.

Control over System Resources

With imperative programming, developers have precise control over system resources. This allows for detailed optimization, which is essential in scenarios requiring specific hardware interactions or resource management, such as embedded systems or high-performance computing tasks.

Straightforward Debugging

Debugging in imperative programming can be more straightforward because the code execution follows a clear, step-by-step sequence. This linear flow makes it easier to trace the source of errors, as each instruction is executed in a predictable order, simplifying the identification of bugs.

Imperative Programming - Cons

Complexity

Code written in an imperative style can become complex and difficult to manage, especially as the project grows. The need to explicitly define each step can lead to lengthy and convoluted code, which becomes hard to read and maintain over time.

Harder to Scale

Scalability is a significant challenge in imperative programming due to the extensive use of mutable state and detailed control structures. As the codebase expands, maintaining and scaling the application becomes more cumbersome and error-prone, often requiring substantial refactoring.

Prone to Bugs

The reliance on mutable state in imperative programming increases the risk of bugs. Changes in one part of the code can inadvertently affect other parts, leading to unpredictable behavior and difficult-to-track errors. This interconnectedness can make debugging and maintenance particularly challenging.

What is the alternative, you might ask?

Declarative Programming - Pros

In contrast to the imperative one, declarative programming is about describing what you're trying to achieve, without instructing how to do it.

Let's convert the imperative example above into a declarative one (using promise-like constructs):

Woah, this is much more legible. What's even better — wanna add a step? No problem! Just add one line to the stream:

It minimizes mutability

Immutable objects are generally much easier to work with. Such objects can only be in one state, which cannot be modified across threads. You may share them, clone them, or even cache their data — all of that without worrying about them becoming stale.

You might be amazed how many easy-to-make and hard-to-detect bugs could be entirely eliminated by using immutable data structures.

It reduces state side-effects

Imagine you're debugging an unknown view controller, perhaps written by a team member. You find that a function F sometimes behaves differently for the same input parameters. After some examination, you find that it uses a public property x.

Then you look for places that modify x (inside and outside the view controller) and find that there are five of them. Good luck finding out which one is the true source of the bug. :)

This is why state is evil. Because literally anyone can change it, it cannot be relied upon.

Every time you had to restart an application or the computer to fix a problem, you were a victim of state gone out of control.

Declarative programming discourages usage of variables in favor of more sophisticated constructs, such as pipelines or higher-order functions.

It leads to more understandable code

Let's face it — it's not clear what the above code is doing without examining it line-by-line. This typical flaw is derived from the very definition of imperative programming — that instead of declaring what you are trying to achieve, you tell the computer what it is supposed to do.

In case you're wondering how do further simplify the above example, here's a more declarative version of it:

Much simpler, isn't it?

It is just more scalable

Because declarative programs are generally simpler and safer, such projects are more maintainable and enjoyable for developers to work on.

It's not easy for a new team member to jump into a project with lots of mutable state, unclear procedures and implicit dependencies, though.

Declarative Programming - Cons

Performance

The abstraction away from direct control of hardware resources in declarative programming can introduce performance overhead. This makes it less suitable for high-performance requirements where low-level optimizations are necessary to achieve desired performance metrics.

Less Intuitive for Complex State Management

Managing complex state transitions can be less intuitive in declarative programming. Developers accustomed to imperative paradigms may find it challenging to grasp functional or declarative concepts, posing a learning curve. This can make it harder to implement and debug complex state management logic without a solid understanding of the paradigm.

Transitioning Between Imperative vs Declarative Programming Paradigm

Transitioning from imperative to declarative programming can be a rewarding yet challenging journey. Transitioning between declarative and imperative programming requires understanding the core concepts of each paradigm. To start, familiarize yourself with the core concepts of declarative programming, such as immutability, higher-order functions, and pure functions.

Begin by refactoring small, isolated pieces of your imperative code into a declarative style, leveraging libraries and frameworks that support this paradigm, like React for UI development or functional programming libraries in JavaScript. One common challenge is shifting your mindset from focusing on step-by-step instructions to thinking about the desired outcome.

Overcome this by practicing with functional programming exercises and studying declarative codebases. It’s also beneficial to engage with the developer community through forums and coding groups to share experiences and solutions. Adopt best practices such as breaking down complex problems into smaller, reusable functions and avoiding mutable state. By gradually integrating these techniques into your projects, you’ll find declarative programming becomes more intuitive, leading to cleaner, more maintainable code.

Together

We’re all used to the imperative paradigm. It’s what all of us have been taught while learning to code, and that’s the main reason why this age-old style is still dominant in modern programming languages. However, as software projects become more and more complex, it is more important than ever to write safe, understandable and scalable code — something undoable using classic approaches.

I strongly encourage that we, the developer community, take a second look at our imperative-oriented habits, see its shortcomings and challenge ourselves to try something completely opposite. Markup languages like HTML and CSS are also examples of declarative languages that focus on specifying the desired result rather than the explicit sequence of commands.

So that next time we’re about to implement some functionality, we ask ourselves whether it can be written in a declarative manner, without any variables, loops, conditionals or callbacks.

You'd be surprised at how many times the answer is yes.

This is an excerpt from my talk at February's infoMEET conference: "Programming Paradigms — which one is the best?" Check it out below:

Netguru's developers often participate in various events both as listeners and speakers. Here's a few words about Mobile Central Europe conference by our iOS developer Patryk.