While imperative programming is a longstanding, familiar paradigm within the developer community, many tout declarative programming as a must-have in light of today's increasingly complex software development requirements. Both paradigms have pros and cons, which developers should carefully weigh before they choose between the two.
Let's examine some of the basic differences between declarative and imperative programming, as well as some ways developers can choose between these two approaches.
An intro to declarative vs. imperative programming
Declarative programming places much of its focus on the overall goal and intended outcome of a program's operations. Developers don't necessarily have to mind how that desired outcome is accomplished, nor hardcode the program's control flow.
Declarative code does, however, rely on descriptions of the logic behind various computations. Declarative code does not dictate the order of execution. Instead, it lists the number of available operations and the scenarios for program execution. HTML, CSS, Miranda and Prolog are some good examples of languages that take this approach.
Unlike the declarative approach, imperative programming emphasizes direct instruction on how the program executes functions. The code consists of a step-by-step sequence of command imperatives. For imperative programming, the order in which operations occur is crucial; it explicitly outlines the steps that dictate how the program implements desired functionality. Examples of imperative programming languages include C, C++, Java and Fortran.
Declarative vs. imperative programming: Technical details
The differences between declarative and imperative programming paradigms span several categories:
In declarative programming, developers do not create explicit computation instructions. Instead, the developer identifies the conditions that should trigger execution processes and allows the compiler to make decisions regarding the order of operations.
Alternatively, imperative programming revolves around control flow. To implement functionality, developers must explicitly describe each step in the process, and then command the compiler to carry out those steps.
Since the declarative programming doesn't place an emphasis on the order of execution and control flow, it can become difficult to understand the code.
On the other hand, the strict step-by-step structure of imperative programming can make the code fairly easy to understand, but can create an extensive amount of code to manage.
Flexibility and customization
It's fairly straightforward for developers to implement complex methods in declarative programming, as it mostly depends on an abstracted execution model. The complex syntax of declarative programming, however, can make it difficult to customize feature code, and demands a lot of specificity to execute complicated functions.
While imperative programming may not offer as much flexibility from a coding standpoint, it's great for closed-loop processes, and offers a straightforward way to customize code and functions. Unfortunately, heavy edits to code can lead to a high probability of errors, so frequent reviews of the code are a must.
To demonstrate the structure of declarative code, we'll use a program example that gives us all numbers less than six, but only within a range of 20 numbers. Even though the code doesn't specify the steps, the program should be able to figure out the order of operations:
small_nums = [x for x in range(20) if x < 6]
Alternatively, imperative code lays out a series of distinct steps. Using that same program example, the first step is to command the program to list all numbers within the range of 20. Then, developers need to tell the program to check each number to see if it is less than six. Finally, they'll instruct the program to display the desired numbers.
small_nums = 
for i in range(20):
if i < 6:
Which approach should you choose?
The imperative paradigm is a classic programming methodology, and still a dominant coding style. As such, some developers may find it hard to give up. But as programming evolves and software grows increasingly complex, declarative programming starts to lose its appeal. Practically speaking, developers are well served to learn declarative programming, and may find that they can write code with higher levels of readability and scalability.
Realistically, there is no concrete answer as to whether code should be declarative vs. imperative programming. However, it is often a question of functionality versus complexity. For instance, developers may find that they can still code the functionality they need through a declarative approach. If this is the case, they can avoid the complexity of variables, loops, conditionals and callbacks they'll have to sift through in order to review or debug the code. However, this doesn't rule out the chance that feature flexibility becomes more important over time, and the imperative approach may become a lot more valuable.