Declarative programming

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Declarative programming

What is declarative programming?

In Declarative Programming, you write the code describing what you want to do & allow the compiler to figure out how to do what you want to be done. Declarative programming is a high-level programming concept. It is generally used in databases and configuration management software.

Declarative programming is usually used along with a domain-specific language (DSL) to express what the user wants and shield them from low-level constructs that materialize the end state which is desired.

SQL and Prolog are examples of declarative programming languages. This declarative programming approach aids in simplifying the programming for quite a few parallel processing applications.

declarative programming
Source: Smashing Magazine

Declarative programming languages are also known as nonprocedural languages. The program essentially specifies what has to be done and not how it is to be done. In declarative (nonprocedural) programming languages, there is less of a difference between the specification of a program and its implementation than there is in procedural languages.

 Declarative programming describes a particular class of problems with language implementation figuring out the solution to these problems. 

What are the types of declarative languages?

The two most common types of declarative programming languages are: logic programming languages and functional languages. It is also possible to classify declarative programming languages are constraint programming, logic programming and constraint logic programming. But let’s just focus on logic programming languages and functional languages.

Logic programming languages

PROLOG (programming in logic) is the best known example of logic programming languages. These languages state a program as a set of logical relations (for example, a grandchild is the child of someone’s child). Logic programming languages are similar to the SQL database language. Programs gets executed by an inference engine that answers a query by searching these relations in a systematic manner for the purpose of making inferences that will answer a query. PROLOG has been used to a great extent in natural language processing (NLP) as well as other artificial intelligence programs.

Functional languages

These programming languages have a mathematical style. You can build a functional program by applying functions to arguments. Functional languages, like LISP, ML, and Haskell, are used as research tools in language development, in automated mathematical theorem provers, and even to some extent in commercial projects.


Declarative programming vs. imperative programming - what is the difference between them?

Imperative programming is instructional; it focuses on how to reach an end result. Declarative programming, on the other hand, concentrates on the end result and lets the compiler figure out how to achieve the stated outcome.

In declarative programming, you’re essentially saying something like this to the compiler: Here is what I want you to do, now it is up to you to decide how you will do it.”

While imperative programming concentrates on the intricacies of code setup, declarative programming allows the developer to concentrate on the actual resolution of the problem.

How does declarative programming work?

It depends on logic and constraints for defining the setup and the outcome. 

The constraints specify the properties that hold true in a particular programming scenario and are set up in software libraries or embedded in the language.

Logic programming expresses rules & facts regarding the domain in which the declarative programming is being used.

A domain-specific language (DSL) is used along with declarative programming so that the control flow is embedded in the language and exists separately from the logic.

In declarative programming, there are tools given to the programmers that make the abstraction of the implementation possible and assist in the concentration of the issue.

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What are the advantages of declarative programming?

Declarative programming has many advantages. Here are some of the main advantages of declarative programming:

1. Readability and usability

Since a DSL will tend to be more like a natural language than pseudocode, it is easier for non-programmers to read and learn it.

2. Reuse

Declarative programming makes it easy to create code that you can use for multiple purposes.

3. Commutativity

It is possible to express an end state, without needing to detail the order in which it will be implemented.

4. Error recovery

Declarative programming makes specifying constructs that stop at the first error much easier, which eliminates the need to add error listeners for all possible errors.

5. Idempotence

Declarative programming makes it possible for you to start with the end state and allow the program to figure it out for you.

6. Referential transparency

Expressions can be replaced with their corresponding value and the program’s behavior will not change.

Source: TopTal


Another major advantage that declarative programming languages bring to the table is that declarative programming helps in minimizing mutability. Immutable data structures aid in getting rid of hard-to-detect bugs and tend to be easier to handle. Because of this, programmers prefer working with declarative programming languages and approaches (and even consider it easier) than other programming languages and approaches that have unclear procedures, implicit dependencies or a lot of mutable states.

These programming languages even reduce state side effects because they favor the use of complex constructs such as higher order functions and pipelines and they even discourage variables.

Declarative programming also makes the code more understandable and scalable. With imperative programming, you cannot really understand what the code is doing without examining it line-by-line. That’s the main flaw of imperative programming — rather than declaring what you are trying to achieve, you would be telling the computer what it needs to do. Declarative programming makes the code far easier to understand.

Now let’s see how declarative programming makes the mode more scalable. Since declarative programs tend to be safer and simpler, it is easier to maintain these programs and it is more enjoyable for developers to work on them and scale them up.

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