Extend course with functional programming

[pintergreg]

• https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-validate/ -- too advanced, requires to talk a lot about types
• https://kyleshevlin.com/just-enough-fp-pointfree/ -- maybe too advanced
• https://fsharpforfunandprofit.com/posts/recipe-part2/ -- railway oriented programming
• https://www.jamesparker.dev/the-principles-of-functional-programming/
• map-filter-reduce - https://elixirschool.com/en/lessons/basics/enum
• https://softwarepatternslexicon.com/functional-programming/essential-functional-programming-patterns/monad-pattern/