# Functional Programming: ideal for algorithms

Common programming languages usually allow for several programming paradigms, whereby a rough distinction is made between declarative and imperative programming. To put it simply, such paradigms form the basic approach when programming software. A sub-form of the declarative approach is so-called functional programming, which is predominantly used in the development of the following programs or codes:

• Technical and mathematical applications
• Artificial intelligence (AI)
• Compilers and parsers
• Algorithms

What makes the programming approach or functional programming languages so interesting for this type of computer application? And how does it differ from other concepts such as object-oriented programming?

## What is functional programming?

As the name suggests, the focus of the functional approach to programming is on functions. With a functional program, all elements can be considered a function, and the code can be executed with concatenated function calls. Conversely, there are no independent assignments of values. A function is best envisaged as a special variant of a subprogram. This is reusable and, unlike a procedure, directly returns a result.

There are, of course, functions in many higher-level programming languages which are defined and then applied, so this is not a special feature of functional programming. The thing that makes the functional approach so important for computer science, and at the same time appropriate for a wide range of applications, is the fact that functions within this programming paradigm can take on different "forms". This way they can be linked together just like data. Furthermore, use as a parameter as well as use as a function result are possible. This special handling of functions allows programmers to implement and process complex computational tasks (in particular those of a symbolic nature).

## Why functional programming is more relevant today than ever

Although the roots of functional programming date back to the 1930s (as part of basic mathematical research), the functional approach continues to be very popular, especially within technical and mathematical fields. There are several reasons for this:

• Extensive possibilities for algebraic program transformation
• Extensive possibilities for algebraic program synthesis
• Simple semantic analysis possibilities thanks to the renunciation of "inner states in the calculation process" and "by-effects".
• Elimination of internal conditions: unlike imperative programming, no internal states of a calculation process are required.
• Renunciation of side effects: the state changes which go hand-in-hand with the internal states, the so-called by-effects, can also be dispensed within functional tasks.

Functional programming offers a high degree of abstraction as it is based on the mathematical concept and principle of function. Clean application of this type of programming leads to very precise code. Many small, reusable and highly specialized units, as well as the functions, create a program to solve a much larger task.

Therefore, there are many practical reasons why functional programming and the functional programming languages working with this principle are very popular in computer science, in particular, when it comes to complex mathematical problems and algorithms. At the same time, the very specific application areas ensure functional programming languages play something of a niche role.

## At a glance: The most important functional programming languages

The most important programming languages based on the functional approach include the following:

• LISP
• ML
• OCaml
• F#
• Erlang
• Clojure
• Scala

Furthermore, there are well-known programming languages that allow functional programming as one of several possible paradigms:

• Perl
• Ruby
• Visual Basic .NET
• Dylan
• ECMAScript

Programs are stateless Data (e.g. variables) cannot be changed
Well-suited for parallelization Retrieval of large amounts of data not possible efficiently
Easily testable code Not recommended for connections to databases and servers
Easily verifiable code, even stateless functions can be verified Not suitable for many recursions of the same stack
Can be combined with imperative, object-oriented programming Recursive programming can lead to serious errors
More accurate, shorter code Not suitable for all tasks

The table provides an overview to determine whether the functional paradigm is the right approach for your software programming project or not. The decision in favor of a programming style is often strongly dependent on the personal preferences of the developer. For this reason, for example, object-oriented programming is a popular alternative to the functional approach. The two approaches should, therefore, be briefly compared in the following - including a final practical example.

### Trend or no trend? Object-oriented and functional programming in cross-comparison

Just as in fashion, there are various trends in programming. For quite some time now, object-oriented programming has been on-trend. It’s particularly popular in the development of web applications and computer games. In comparison to functional programming, one does not describe the individual elements as a function but as objects and classes when using this approach. In combination with an inheritance system, this offers the advantage that all components can be easily reused and expanded at any time. Functional code, on the other hand, is shorter, clearer, and preferred where testable and verifiable code is needed.

Incidentally, the choice between object-oriented and functional programming does not necessarily have to be made. Many modern programming languages support working with both programming styles. This way, they can be easily combined and developers benefit from the merits of both paradigms.

### Functional programming using the example of a parser

Parsers are software components that are often indispensable as supervisory mechanisms for compilers to translate programming language into machine language.

A parser can generally be implemented on the basis of various programming paradigms, for example, with an object-oriented language. However, the functional approach offers a set of useful advantages with regard to a parser’s code design:

• There are no global and changeable variables. Accordingly, no programming error can result from the so-called "mutable global state”, as can be the case with object-oriented projects. This benefits a parser as a central element of the program.
• Thanks to higher-order functions and manageable program code, larger data collections can be easily handled. This is very useful to a parser, which processes large amounts of data.
• A parser is a program element that is run frequently. It helps the overall program, provided that this central element is precisely programmed and runs efficiently, as is the case with functional programming.
• An error in the parsing process is usually fatal and must be avoided at all costs. While a program is running, however, there will certainly be a lot of semantic dependencies that can often lead to serious errors. When implemented properly, functional programming can help to minimize or even prevent such serious errors in execution.
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