Functional programming is generally used in situations where a large number of operations have to be performed on only one set of data. While complicated, these languages are essential in software development, and most developers aspire to master most if not all relevant functional programming languages.

But the most important question for any developer would be about which functional programming languages are really worth pursuing? Which are the best and are most relevant for businesses for the time to come.

After working on hundreds of projects, I and my Indian developers at ValueCoders have observed that there are more than 50 languages that support functional programming. Honestly, each of them has impressed us in their own way. However, if I consider the challenges we faced, then the story turns different.

As you know the programming task is indeed quite challenging. However, we developers manage to express that idea in our code. But let’s also face the reality. There are situations when legacy codes, scaling of codes, and tricky codes slow down our pace. Then, while managing a large amount of data, we spend hours of effort on that.

Hence, to cut short your development time, I have made a detailed list of top functional programming languages that you can use for software development in 2020.

Top Functional Programming Languages for Web App Development in 2020

HasKell

Haskell is often considered as the purest functional programming language. Programs made with Haskell include a series of highly generalizable functions that define the purpose of the purpose, i.e., what it’s supposed to do.

While Haskell as a functional programming language is suited for almost all use cases, its primary uses come in the field of data analysis, business logic, and rapid prototyping. The Haskell programming has been dipping down in terms of popularity over the past few years. However, that will soon change with a major Haskell update set to be released in 2020.

What makes Haskell one of the most sought-after functional programming languages of 2020?

Haskell is expected to regain its significance in 2020 due to the hype around its upcoming major standard update.
While it certainly isn’t among the most popular programming languages today, it remains undisputed as the purest functional language out there.

Key features of Haskell:

Statically typed
Known as the most purely functional programming language
Type inference
Concurrent
Use of Lazy functions
Availability of a wide range of packages

Functional Programming in Haskell:

Being a purely functional programming language, the best thing about Haskell is its conciseness and the clean code. Let’s see an example.



In the above screenshot, we see a list created of all even numbers up to 100, and another list deleting the first five of them.

Even if you’re not an experienced developer, you could definitely see how cleaner and more direct the Haskell code is, and what benefits it could deliver. The relative improvement as seen in this small example only increases with more complicated problems.

Clojure

The Clojure functional programming language combines the approachability and interactive development of a scripting language with an efficient and robust infrastructure suitable for multithreaded programming. Despite being a compiled language, Clojure remains completely dynamic and has all of its features supported at runtime. Clojure provides easy access to the Java frameworks, with optional type hints and type inference, to ensure that calls to Java can avoid reflection.

Clojure is a dialect of Lisp and a lot of stuff with the Lisp family of programming languages. Clojure is a functional programming language and features a rich set of immutable, persistent data structures. When a mutable state is needed, Clojure offers a software-based transactional memory system and reactive Agent system that ensure clean, correct, multithreaded designs.

Features and things about Clojure that make it a sought-after functional programming language in 2020:

— The simplicity of the language, which is kept largely intact since its introduction in the year 1958.
Java Interoperability.

— REPL (Read Eval Print Loop): A tool that enables developers to interact with the program and develop it rapidly.
Riddance from duplicate codes through the use of Macros.

— Ease of writing concurrent programs

— An active, highly experienced developer community.

Functional Programming in Clojure:

The Clojure functional programming language identifies two different types of structures:

Literal representation of data structures in the form of maps, numbers, vectors, strings, etc.
Operations in the form of opening and closing parenthesis, operators, operands, closing parenthesis.

Let’s consider an example of a simple math calculation using Clojure:

user=> (+ 1 2 3 4 5)
15
user=> (str "append " "the " "string")
"append the string"


Other languages have different structures of different operations. Clojure’s structural uniformity ensures that the structure remains the same despite changes in operations.

Elm

Elm is a functional programming language that compiles to JavaScript. Elm is best for building website and web app development. The language places a strong emphasis on simplicity and a high-quality tooling system.

Benefits by using Elm functional programming language:

There are several benefits that a functional programming language such as Elm offers over other JS libraries, which are as follows:

— No runtime errors in practice
— Developer-friendly error notifications
— Super-reliable refactoring
— Semantic versioning is automatically enforced for all Elm packages

Functional Programming in Elm:

Let’s try an example to show how functional programming works in Elm:

The below code is for a simple program that allows the user to increment and decrement a number.

import Browser
import Html exposing (Html, button, div, text)
import Html.Events exposing (onClick)

main =
  Browser.sandbox { init = 0, update = update, view = view }

type Msg = Increment | Decrement

update msg model =
  case msg of
    Increment ->
      model + 1

    Decrement ->
      model - 1

view model =
  div []
    [ button [ onClick Decrement ] [ text "-" ]
    , div [] [ text (String.fromInt model) ]
    , button [ onClick Increment ] [ text "+" ]
    ]

Racket

You must be thinking, why Racket is on the upper part of the list not being a purely functional programming language. Yes, that’s right! It's not a PURE functional language, but it's certainly functional. Of Course, I am not comparing it with Haskell, but it’s really helpful in many cases.

In fact, some Racket developers prefer the Scheme-style tail recursion method of iteration (via the named let or letrec) to the looping constructs provided in the library, even when for loops would be just as effective.

In the same way, not all Common LISP programmers like the loop macro, and some (e.g. pg) actually use Common LISP in a style that resembles functional programming.

However, don't think of Racket as a pure functional programming language as I said before. That's as misleading as calling C++ a procedural one, even though you could write all your code C-style without ever using objects.

Racket is a LISP, which means it can be adapted to fit virtually any paradigm. Racket is far closer to Common LISP and Clojure than it is to literally any non-LISP.

Here’s an example of a functional code done right with Racket.

;; Given two sorted lists l1 and l2, merge them into a sorted list l3
(define (merge l1 l2)
(cond
[(= 0 (length l1)) l2]
[(= 0 (length l2)) l1]
[(< (first l1) (first l2)) (cons (first l1) (merge (rest l1) l2))]
[else (cons (first l2) (merge l1 (rest l2)))]))

(test-case "merge"
(check-equal? (merge `(1 2 3) `(4 5 6)) `(1 2 3 4 5 6))
(check-equal? (merge `(1 3 5) `(2 4 6)) `(1 2 3 4 5 6))
(check-equal? (merge `(2 4 6) `(1 3 5 7)) `(1 2 3 4 5 6 7))
(check-equal? (merge `(2 4 6) `(2 4 6)) `(2 2 4 4 6 6))
)

(define (merge-sort l)
(cond
[(< (length l) 2) l]
[else (let-values ([(l1 l2) (split-at l (quotient (length l) 2))])
(merge (merge-sort l1) (merge-sort l2)))]))

(test-case "merge-sort"
(check-equal? (merge-sort `(6 5 4 3 2 1)) `(1 2 3 4 5 6))
(check-equal? (merge-sort `(5 4 3 2 1)) `(1 2 3 4 5))
(check-equal? (merge-sort `()) `())
(check-equal? (merge-sort `(2)) `(2))
(check-equal? (merge-sort `(6 1 4 3 2 5)) `(1 2 3 4 5 6))
)


Scheme

Scheme is a dialect of LISP, the second-oldest programming language that is still widely used today (after Fortran). The community of Scheme programmers has continued to thrive for decades and when we are talking about functional programming languages, the significant contribution of Scheme is undeniable.

Here are a few significant strengths of Scheme

Useful expressions
Strong definitions
Compound values
Use of Symbolic Data
Turtle graphics & Block structure

While coding in Scheme, you might find some functions which are quite primitive yet still useful in modern times. Here is an example of a block structure

(define (hofstadter-male-female n)
(letrec ((female (lambda (n)
(if (= n 0)
1
(- n (male (female (- n 1)))))))
(male (lambda (n)
(if (= n 0)
0
(- n (female (male (- n 1))))))))
(let loop ((i 0))
(if (> i n)
'()
(cons (cons (female i)
(male i))
(loop (+ i 1)))))))

(hofstadter-male-female 8)

===> ((1 . 0) (1 . 0) (2 . 1) (2 . 2) (3 . 2) (3 . 3) (4 . 4) (5 . 4) (5 . 5))


PureScript

This again is a pure functional programming language as PureScript describes itself as “A strongly-typed functional programming language that compiles to JavaScript”. As PureScript refers to JavaScript and JavaScript and Haskel are quite similar, so codes in PureScript matches Haskell very much.

Let's make a module in Purescript that allows us to solve this same problem. We will start by writing a Pythagoras.purs module. Here is the code we would write to see how PureScript works.

module Pythagoras where
import Data.List (List, range, filter)
import Data.Int (pow)
import Prelude
sourceList :: List Int
sourceList = range 1 100
data Triple = Triple
  { a :: Int
  , b :: Int
  , c :: Int
  }
allTriples :: List Triple
allTriples = do
  a <- sourceList
  b <- sourceList
  c <- sourceList
  pure $ Triple {a: a, b: b, c: c}
isPythagorean :: Triple -> Boolean
isPythagorean (Triple triple) =
  (pow triple.a 2) + (pow triple.b 2) == (pow triple.c 2)
isSmallEnough :: Triple -> Boolean
isSmallEnough (Triple triple) =
  (triple.a) + (triple.b) + (triple.c) < 100
finalAnswer :: List Triple
finalAnswer = filter
  (\triple -> isPythagorean triple && isSmallEnough triple) 
  allTriples

As you can see, it’s pretty similar to Haskell. Hence, if you are into Haskel or JavaScript, you can easily get into this.

Kotlin

The Kotlin programming language began to gain popularity as soon as google began to introduce it as an alternative to Java for Android App development. However, Kotlin’s popularity began to skyrocket after this year’s Google I/O 2019, when Google announced Kotlin as the official programming language for android app development.

Kotlin is based on Java but offers several advantages over the latter in the form of more concise coding, riddance from the annoying nullpointerexceptions, and most importantly, easy ways for developers to convert existing Java code into Kotlin language, thereby making their adaptation to the new language even easier.

Here’s why Kotlin is expected to be one of the top functional programming languages in 2020:

A look at PYPL index would tell you how Kotlin’s popularity has skyrocketed since 2017. More and more developers and web app development companies are switching to Kotlin for building mobile apps.
Google is going all-out to remove Java from its ecosystem thanks to the legal battles between Oracle and Google. So with the internet giant itself rallying behind the language, there’s no way we’ll see Kotlin’s popularity and its applications stall anytime soon.

Key features of Kotlin:

More concise code as compared to Java.
Easy conversion from Java to Kotlin
Riddance from annoying nullpointerexceptions

Functional Programming in Kotlin:

Quicksort example:

We will implement a Quicksort algorithm. It’s quite easy to understand: we choose an element (pivot) and distribute all other elements to the list with bigger and smaller elements than the pivot. Then we repetitively sort these sub-arrays. Then we add this sorted list of smaller elements, the pivot, and the sorted list of bigger elements. Here are the results:

fun <T : Comparable<T>> List<T>.quickSort(): List<T> = 
    if(size < 2) this
    else {
        val pivot = first()
        val (smaller, greater) = drop(1).partition { it <= pivot}
        smaller.quickSort() + pivot + greater.quickSort()
    }
// Usage
listOf(2,5,1).quickSort() // [1,2,5]

Golang

Golang is a multi-paradigm language. When using this language for functional purposes please mind that using functional programming does not mean that it is all or nothing. You can always use functional programming concepts to complement the object-oriented or imperative concepts in Go.

The benefits of functional programming can be used whenever possible, regardless of the paradigm or language used. And that is exactly what we are going to see.

First-class and higher-order functions

A function can be considered as a higher order function only if it takes one or more functions as parameters or returns another function as a result.

func main() {
    var list = []string{"Orange", "Apple", "Banana", "Grape"}
    // we are passing the array and a function as arguments to mapForEach method.
    var out = mapForEach(list, func(it string) int {
        return len(it)
    })
    fmt.Println(out) // [6, 5, 6, 5]

}

// The higher-order-function takes an array and a function as arguments
func mapForEach(arr []string, fn func(it string) int) []int {
    var newArray = []int{}
    for _, it := range arr {
        // We are executing the method passed
        newArray = append(newArray, fn(it))
    }
    return newArray
}

Pure functions

This is quite simple, take the following, this is a pure function. It will always return the same output for the given input and its behavior is highly predictable. We can cache the method securely if necessary.

<cut/> 
func sum(a, b int) int {
    return a + b
}

If we add an extra line in this function, the behavior becomes unpredictable as it now has a side effect that affects an external state.

var holder = map[string]int{}

func sum(a, b int) int {
    c := a + b
    holder[fmt.Sprintf("%d+%d", a, b)] = c
    return c
}

Hence, try to keep your functions pure and simple.

JavaScript:

JavaScript is an omnipresent language spread across all of the Internet. It is used on almost every website we visit and has applications on both the front-end and back-end.

Javascript is a high-level interpreted programming language. It is a multi-paradigm language with support for object-oriented and functional programming. It is possible to create mobile/web applications with JavaScript when used along with CSS, HTML, and AJAX.

There is no death of libraries and frameworks to build JavaScript-based web apps. Some of these are PhoneGap, jQueryMobile and Ionic.

Creating mobile/web apps in Javascript is easier as developers only have to code once, which then can be released on multiple platforms such as Android, iOS, and Windows.

What makes Python one of the top functional programming languages for web app development in 2020?

Similar to Python, JavaScript reached its peak popularity since 2001 on the TIOBE Index in December 2019.

JavaScript is only second to Python in the PYPL index, though it should be noted that its popularity is seen on a decreasing trend this year.

As per data from GitHut 2.0, JavaScript retains its place as the top programming language at the end of this year. But as seen in the PYPL Index, its popularity is gradually decreasing.

Key Features of JavaScript:

Can be learned in a few days
Fast and efficient
Used for creating animations

Functional Programming In JavaScript:

Pure Functions are the pillars of functional programming. A pure function does not modify nor depend on the state of variables out of its scope.

Here’s a code snippet below to present the implementation of Pure Functions in Javascript:

Python:

Let’s begin by talking about one of the most obvious inclusions in this list of the most popular functional programming languages, i.e., Python.

Python is a high-level programming language, used not just for web app development but also for other purposes such as machine learning, desktop applications, web servers, media tools, etc. It is used in analyzing and computing data for various scientific and numeric fields, creating desktop GUIs, and of course software development.

Here are some facts that make Python one of the top functional programming languages for web development in 2020:

In the well-known TIOBE Index, Python has just reached its peak position since 2001 in the month of December 2019. It was already declared as the language of the year by TIOBE back in 2018. What better indicator to show Python’s continued popularity?

As per the PYPL Index, Python has retained its spot as the most popular programming language as of December 2019. In fact, Python’s popularity has grown over 19% in the last five years!

Data from Github statistics using GitHut 2.0 reveals that Python has gained a significant amount of popularity since 2018, though its position as the second-most popular functional programming language (after JavaScript) remains constant.

Here are some popular apps developed in Python:

BitTorrent
Quora
Reddit
Spotify
Instagram
YouTube

Key features of Python programming language:

Processed at runtime by the interpreter
Object-oriented language
Easy-to-learn, easy to read and master
Interactive language
Highly Scalable
Supports GUI applications
Runs on Windows, Mac, Unix, and Linux

Functional Programming In Python:
Functional programming in Python works right from the get-go without adding any special features or libraries.

Let’s see how Functional Programming works in Python with the below code snippet.


We will create a Pure Function to multiply numbers by 2.

The above snippet is considered as a pure function because the numbers are not changed and there’s no need to refer to any other variables outside of the function.

Fun fact: the creator of Python didn’t intend Python to have functional features, but nevertheless appreciated the advantages that came with having them. So it must be considered that the Python language implementation is not directly optimized for functional programming.

Hence, these are the top 10 functional programming languages you can certainly think about if you are starting your new project. Adding some expertise in these languages will enrich your respiratory highly.