Why Haskell is the best choice for functional programming

Haskell is one of the most popular languages for functional programming. We tell you what features it has and how to get started with it.

The concept of functional programming (FP) is based on mathematical functions. This approach is fundamentally different from the imperative approach in which the key elements are code state changes and sequential execution of commands. In FP the main attention is paid to calculating some values through functions.

In functional languages it is easier to test, correct and maintain the code. Functions in FP are first class objects that are passed as arguments, can be returned from other functions and stored in variables.

Another characteristic feature of functional programming is more predictable, cleaner, and safer code. Since functions themselves do not change the state of the program, they are easier to work with. For this reason, FP is a more preferred tool for creating complex products in which code reliability and predictability are of paramount importance.

Haskell is among the most popular functional programming languages. It is characterized by full, strict and static typing and support for so-called lazy computing. The language was originally used as a tool for purely scientific mathematical research, but gradually became one of the most popular languages in practice.

This material is an introduction to functional programming in the Haskell language. We will tell you about its features and advantages and how it is applied in practice.

Basic concepts of FP

Haskell is considered a complex programming language – it uses concepts that do not exist in other languages. However, experts believe that for developers, mastering Haskell will be an extremely useful experience.

First, a brief historical background. The forerunner of Haskell was another functional language – Miranda with the concept of “lazy” calculations. The financial support for this language was quite solid, but it did not gain much popularity as it was distributed under a private license.

As an affordable alternative to Miranda, other languages were created, among which Haskell became the most popular. The language got its name in honor of Haskell Curry, a mathematician from the USA, the creator of combinatorial logic.

In 1990 the first version was published, 8 years later the creators presented the standard The Haskell 98 Report. This version is still the basic version, which does not cancel the constant development and improvement of the language.

The main advantage of functional programming in Haskell is the open nature of the tool. Developers accept suggestions from anyone who wishes to improve and extend the functionality.

A program written in Haskell consists of a set of mathematical functions – certain correspondences between elements. There is no point in delving into algebraic terms, it is only worth knowing that for calculating functions, only the initial data is important.

In Haskell programming, the actual function of some set is the main structural element of the code. The developer’s task is to describe the function in such a way that the automatic compiler, which translates the text into machine code, understands all relevant points:

• what parameters can become a function;
• what actions must be performed;
• what the final code will look like.

Despite its conceptual complexity, Haskell is among the top most widely used languages with support for deferred (aka lazy) computation. It has built-in support for parallel programming and an extensive toolkit, including automatic checking and debugging tools and thousands of libraries created by users all over the world.

Characteristics of Haskell

Before moving on to more narrow topics, it’s worth considering the most important characteristics of Haskell:

Laziness (deferral)

This means that functions are computed as needed. If the value of a particular function is not needed to execute the program, the calculation is postponed until later.

An example of practical application of the laziness principle is an ordinary calculator on a website, which contains a lot of functions, but only the functions needed by the user are used at a particular moment.

The advantage of this principle is that the developer has the ability to interact with almost infinite data structures. You can compare this to reading scientific articles that have formulas in them. When the reader gets to such places, he can skip them and come back later, once he is ready to concentrate.

When learning Haskell, it really doesn’t matter that much if the programmer doesn’t quite understand some of the syntax. The important thing is to grasp the general idea of the code.

Static Typing

A data type refers to a certain category of elements to which some operations can be applied. A type defines the characteristics of the data, its possible values, and the operations that can be performed on it.

The type system in Haskell is strict and static. This means that they have clear distinctions that are defined at compile time, not at program execution.

Having said that, there is a special data type in Haskell called monads. This is a kind of container that contains values of an arbitrary type. By means of monads in Haskell, you can perform a number of actions characteristic of imperative programming. For example, you can define sequential operations, use functions with side values.

Functionality

Developers who have worked only with imperative languages will have to learn many new concepts. However, certain difficulties will only benefit the programmer – he will acquire new practical skills. Experts claim that after working with Haskell it becomes easier to code in C++, Java and other languages.

Cleanliness

The Haskell functional language works exclusively with pure functions. These are characterized by properties such as:

strict determinism – one value of an argument corresponds to one value of a program function;
absence of side-effects – i.e. the function’s influence on the execution environment.
There are exceptions: some tasks require both nondeterminism and side effects. These possibilities are realized by means of monads.

In general, the basic law of Haskell applies here: using a function with the same parameters leads to the same result.

Modularity

All programs in the Haskell functional language are a set of autonomous blocks, each of which performs its own task. The code is divided into independent files that contain modules.

This structure greatly simplifies the creation and testing of program products. If you need to improve the code or fix an error, you can do it in a separate module without affecting the whole program. Modularity speeds up software development by dividing separate blocks between different development teams.

Parametric polymorphism

In Haskell, you can operate on values of different types in the same way. This property makes the language more expressive and allows you to repeatedly use a particular code fragment to work with different data, which greatly simplifies software development.

In practice, this means that the programmer does not need to write different code for each type of data – you can use the appropriate one from those that are already available.

Advantages and disadvantages

The key advantages of the Haskell functional language are:

• The layers of the language are clearly divided into “clean” and “unclean”, so it is easier for developers to work with each block separately. Thanks to this division, the code is better understood not only by those who have worked with it, but also by third-party programmers.
• Dividing the code into independent blocks greatly simplifies product testing. It is easier to find and fix errors in independent modules without having to rewrite the whole code. The program stability against errors increases because they are localized in separate modules.
• Thanks to free access, thousands of instructions, libraries and recommendations for beginners and experienced Haskell programmers have been created. An active community of pros and amateurs can help solve problems in the programming process – you can always ask for help or find a ready-made solution.
• Since types are assigned and checked at the compilation stage, you don’t have to do it while the program is running. Thanks to laziness, the software does not waste resources on calculating functions that are not involved at the moment. The speed of code implementation increases, it is easier to enable parallel computation and work in multitasking mode.
• Many different tools have been created for Haskell to test, debug, and integrate with products in other languages. For this reason, Haskell is successfully used for practical purposes, including working with math problems and developing utility products.

Experts note the disadvantages as well:

• Complex syntax. It may seem irrational at first, especially to experienced developers accustomed to programming languages with standard syntax. Oddly enough, novice programmers will have an easier time mastering Haskell because they have no specific preferences in language paradigms.
• Uneven development on different platforms. Developers initially focused on Linux and macOS, so those who work on Windows face a certain lack of tools and other problems.
• Difficulty in solving non-standard tasks. There are many solutions in repositories and libraries, but most of them are devoted to working with typical problems. You have to solve non-standard problems by yourself or ask for help from the community.

A serious problem is the lack of effective development environments. Those that are available have a limited set of options like autocomplete or syntax highlighting. Features such as navigation or refactoring (reformatting the structure without changing the behavior of the program) are practically absent.

Haskell language basics

So, a pure function always shows stable results with the same input data. It does not change the code state and does not depend on external influences. Such code is easier to test because of its predictability.

add :: Int -> Int -> Int
add x y = x + y

In the example, the add function demonstrates that for the same values of “x” and “y” the total will always be the same. This feature of pure functions is extremely important and useful in FP.

Data immutability (aka immutability) means that the created values cannot be changed, only new values can be entered. This property eliminates errors related to status changes and makes the code more predictable.

let x = 5
let y = x + 1 // x остается равным 5

In the example, “x” does not change even if a new value “y” is created. This property simplifies code handling – you don’t have to worry about values changing in unpredictable ways.

Thanks to higher-order functions that use other functions as arguments, developers publish more abstract but more efficient code. Such programs are easier to adapt and extend.

applyTwice :: (a -> a) -> a -> a
applyTwice f x = f (f x)

In the example, the applyTwice function takes the value “x”. In this way, the function is used to develop more powerful structures.

The process of mapping is the conversion of a function that takes a number of arguments into a function that takes a single argument. The procedure increases the flexibility of program elements.

add :: Int -> Int -> Int
add x y = x + y
addFive :: Int -> Int
addFive = add 5

Here, the “add” function is curried, which leads to the creation of another function “addFive” that always adds 5 to the argument. This way, the function can be used in different contexts.

Lambda expressions and higher functions

In writing Haskell code, functions are defined by the “let” command, followed by specifying the name, argument type, and body of the function.

let add x y = x + y

The example is a simple add function that sums two values. More complex programs are created on a similar basis.

Iterations are performed in Haskell using recursive functions rather than loops, as in more familiar imperative languages.

factorial :: Int -> Int
factorial 0 = 1
factorial n = n * factorial (n – 1)

In the example, the factorial is calculated using recursion.

Important language structures include lists. They are used to represent sequences using various functions.

let numbers = (1, 2, 3, 4, 5)
let doubled = map (*2) numbers

Here the list contains numbers, and the function is used to double the elements. Operations with lists are the first thing developers should master.

Anonymous lambda functions are used for short-term operations. They can be used to create functions without a name.

A simple example of using a lambda function to sum two numbers:

let add = \x z -> x + z

The tool is useful for creating local functions with limited usage.

Practical application of Haskell

In specialized literature, one can meet the statement that Haskell is the language of geniuses and academicians. This is true only partially. The first functional language is indeed not easy to master, but only as long as you try to understand its concept within the paradigm of imperative languages. If you approach it with an open mind, understanding Haskell is much easier.

Practical applications of Haskell include developing video servers, processing complex data sets, and other multi-component tasks.

Large banks and investment companies use the functional language to develop personal working tools. Haskell guarantees the accuracy and correctness of calculations regardless of the level of their complexity.

In addition, this language is used by parsing and text editing programs, some search services and programs for removing spam and advertising. The structure of Haskell is most convenient for displaying grammar and other rules of the most complex languages.

To start working with Haskell, you will need to install the Glasgow Haskell Compiler with the Stack package manager – it simplifies building projects.

Once installed, open a terminal and type the command “ghci” to launch the language’s interactive shell. A few simple expressions will make sure the system works:

Prelude> 2 + 2
4
Prelude> let square x = x * x
Prelude> square 3
9

The interactive shell allows you to work freely with the code and test it. Its use is mandatory for learning the language.

To create projects, you will need the Stack manager. To start a new project, type “stack new my-project” – this command will provide the structure and set the necessary dependencies.

The main file is Main.hs. It needs to be opened and added:

module Main where

main :: IO ()
main = do
    putStrLn "Enter your name:"
    name <- getLine
    putStrLn ("Hello, " ++ name ++ "!")

This simple program recognizes the user’s name and greets the user.

Commands are used to compile and run it:

stack build
stack exec my-project-exe

Stack manager is the most convenient tool for coding in Haskell.

Summary

The Haskell functional programming language contains powerful and efficient tools for writing clean code. Beginning developers, once they have mastered the basic concepts, can gradually move on to the more complex aspects of the functional paradigm. The next step is to create robust and predictable systems used in a variety of programming industries.