Functional Programming

https://en.wikipedia.org/wiki/Pure_function

Pure function: https://www.youtube.com/watch?v=o9cWPrOMuyU

https://en.wikipedia.org/wiki/Referential_transparency

https://en.wikipedia.org/wiki/Lambda_calculus

FP vs OOP: https://stackoverflow.com/questions/2078978/functional-programming-vs-object-oriented-programming/2079678#2079678

OO vs FP, Uncle Bob: https://blog.cleancoder.com/uncle-bob/2014/11/24/FPvsOO.html