What is Haskell? A portrait of the Haskell functional programming language

Different approaches have always existed in software development: Programming paradigms, for example, determine how programs are written and the way in which program codes are structured. The most popular approaches include functional programming. This, in turn, is a sub-form of the declarative approach, in which only results, and not the steps leading to them, are described. An established representative of this category is the programming language Haskell named after the American mathematician Haskell Brooks Curry.

Below, you will find out what Haskell is, how the language differs from other programming languages, and for which projects it is best suited.

What is Haskell?

Haskell is a purely functional programming language, the first version of which was published in 1990. The namesake was the mathematician Haskell Brooks Curry, who laid the foundation for functional programming languages with his work on combinatory logic (between 1920 and 1960). Haskell is based on lambda calculus (a formal language for examining functions), which is why the Greek letter Lambda is also part of the language’s official logo.

Programs written in Haskell are always represented as mathematical functions. These functions never have secondary or side effects. With the same input, each function used always delivers the same result and never changes the status of a program. The value of an expression or the result of a function therefore only depends on the current input parameters. There are no imperative language constructions for programming a sequence of instructions in Haskell.

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After its publication, Haskell became a kind of standard for functional programming languages. As a result, numerous derivatives such as Parallel Haskell, Eager Haskell, Haskell ++, and Eden were developed that are closely related to Haskell. Some entirely new programming languages are also based on Haskell. The universal language Python, one of the most important modern Internet programming languages, has adopted the Lambda notation and the Haskell list processing syntax, for example.

What can be done with Haskell as a programming language?

It takes a lot of work, time, and money to develop and maintain large software projects. Functional programming languages such as Haskell can provide relief. Haskell is a particularly attractive option thanks to the following advantages:

  • Developer productivity can be significantly increased.
  • The code for Haskell software is short, clear, and easy to maintain.
  • Haskell applications are less prone to errors and offer high reliability.
  • The 'semantic' gap between programmer and language is minimal.

As such, Haskell is suitable as a programming language for a wide range of applications. The functional language is particularly predestined for programs that should offer a high degree of modifiability and ease of maintenance. Haskell’s strengths also come into play when it comes to developing specifications and prototypes that can actually be executed, and thus tested and debugged.

In industries in which the exact mapping of mathematical algorithms is required, Haskell is commonly chosen as a programming language. Typical examples are applications for network security, specification frameworks for embedded systems, and programs for modelling complex mathematical calculations.


Haskell is not a suitable choice for programming simple programs.

How does Haskell differ from other programming languages?

As a purely functional programming language, Haskell stands out from many other languages. In particular, its differences from languages that are based on the imperative programming paradigm are significant: Programs that are written in imperative languages execute sequences of instructions. The status of these instructions can change during execution by changing variables, for example. Control flow structures ensure that instructions can be executed multiple times.

In the functional approach on which Haskell is based, the software does not give the computer any direct commands as to how it should do something. Instead, the problem or its solution is described. Variables are undynamic: A variable with the value ‘1’ has a permanent value of ‘1’ in Haskell and other functional languages, and cannot just be easily changed. Functions only serve the purpose of calculating something and returning the corresponding result.

Learning Haskell: requirements, tips, and a first example

Learning Haskell can pay off for several reasons: First, you can subsequently program your own software solutions in Haskell, provided that the language is suitable as a basis. In addition, you are well prepared if you have to deal with third-party applications that are written in Haskell. Since Haskell is also a kind of standard for functional programming languages, learning it is also worthwhile if you want to build up a general know-how of functional programming.

As with many other languages, you have two options for processing Haskell code: in batches with a compiler or interactively with an interpreter. An interactive system has the advantage of providing a suitable command line in which you can experiment directly and evaluate expressions. For a simple introduction to the Haskell programming language, this is preferable in any case. One of the best known examples is the interpreter Hugs, which is no longer actively developed. Alternatively, GHC (Glasgow Haskell Compiler) provides the practical complete package of a Haskell interpreter and compiler.


Exactly how the operations and functionalities of interpreters and compilers differ is revealed in our article 'Compiler vs. Interpreter – Definition and Differences'.

The following code snippet provides a simple example of how Haskell works:

add :: Integer -> Integer -> Integer   --function declaration
add x y =  x + y                       --function definition
main = do
   putStrLn "The sum of the two numbers is:"
   print(add 2 5)    --calling a function 

In line 1 of the code, the Haskell function is declared, which should have integers (whole numbers) as input and output values. The function is specifically defined in line 2: Two arguments are added, and the addition result should be presented. The two values '2' and '5' are supplied as input. The execution of this snippet leads to the following output:


The sum of the two numbers is: 7

Further tips and tricks for getting started with the syntax and functionality of Haskell are available in our extensive Haskell tutorial.

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