Functional programming, explained to my grandmother

Functional programming may remind you of your old school memories. You have no idea what a monad is, and you think that a “first-order function” is an insult. Nonetheless, you believe that lambdas have a certain elegance, and you have a feeling that immutability is a good thing. I will introduce the main concepts of functional programming to you, without cruelty or big words, and you will be able to apply them directly using the language of your choice.