• Programs are constructed by applying and composing functions

• Functions are treated as first-class citizens

• Functional programming avoids side effects

• Functional programming provides referential transparency

• Instead of loop use map and recurrence

• Functions can be bound to names (including local identifiers), passed as arguments, and returned from other functions, just as any other data type can 1

• Imperative program will use a loop to traverse and modify a list, while a functional program, would prefer using a higher-order map function that takes a function and a list, generating and returning a new list by applying the function to each list item 4

• Restricting side effects, can decrease number of bugs, be easier to debug and test, and be more suited to formal verification 3 2

• Functional Design Patterns - Scott Wlaschin https://www.youtube.com/watch?v=srQt1NAHYC0

• The Functional Programmer's Toolkit - Scott Wlaschin - https://www.youtube.com/watch?v=Nrp_LZ-XGsY

• Source: 6

• Comprehensibility: Pure functions don't change states and are entirely dependent on input, and are consequently simple to understand.

• Concurrency: As pure functions avoid changing variables or any data outside it, concurrency implementation is easier.

• Lazy evaluation: Functional programming encourages lazy evaluation, which means that the value is evaluated and stored only when required.

• Easier debugging and testing: Pure functions take arguments once and produce unchangeable output. With immutability and no hidden output, debugging and testing becomes easier.

• Source: 6

• Potentially poorer performance: Immutable values combined with recursion might lead to a reduction in performance.

• Coding difficulties:Though writing pure functions is easy, combining it with the rest of the application and I/O operations can be tough.

• No loops can be challenging:Writing programs in a recursive style instead of loops can be a daunting task.

## 4.1.3. Applications¶

• Source: 6

Generally, functional programming is widely employed in applications focusing on concurrency or parallelism, and carrying out mathematical computations.

Functional programming languages are often preferred for academic purposes, rather than commercial software development. Nevertheless, several prominent functional languages like Clojure, Erlang, F#, Haskell, and Racket, are used for developing a variety of commercial and industrial applications.

For example, WhatsApp makes use of Erlang, a programming language following the functional programming paradigm, to manage data belonging to over 1.5 billion people.

Another important torchbearer of the functional programming style is Haskell, which is used by Facebook in its anti-spam system. Even JavaScript, one of the most widely used programming languages, flaunts the properties of a dynamically typed functional language.

Moreover, the functional style of programming is essential for various programming languages to lead in distinct domains - like R in statistics and J, K, and Q in financial analysis. Even used by domain-specific declarative languages such as Lex/Yacc and SQL for eschewing mutable values.

## 4.1.5. References¶

1

Functional programming. Retrieved: 2020-10-09. URL: https://en.wikipedia.org/wiki/Functional_programming

2

Hudak, Paul. "Conception, evolution, and application of functional programming languages". ACM Computing Surveys. 21 (3): 359–411. doi:10.1145/72551.72554. S2CID 207637854. 1989.

3

Hughes, John. "Why Functional Programming Matters". Chalmers University of Technology. 1984.

4

Spiewak, Daniel. "Implementing Persistent Vectors in Scala". Code Commit. 2008.

5

Paulson, Larry C. "ML for the Working Programmer". Cambridge University Press. ISBN: 978-0-521-56543-1. Retrieved: 2013-02-10. 1996.

6(1,2,3)

Inouye, Jenna. "Functional Programming Languages: Concepts & Advantages". Year: 2022. Retrieved: 2022-07-28, URL: https://hackr.io/blog/functional-programming