We saw a couple of functions, in passing, in the last chapter when we looked at the automatically generated boilerplate code created by cargo new. What were we actually seeing, though?

A function is a sequence of instructions for the computer to follow. It's sort of like a recipe. We don't have to tell a person how much flour, sugar, and milk to use to bake cookies, if we know that they already have a cookie recipe. We can just say: Bake some cookies, please. It's similar with a function. We don't have to tell the computer exactly how to save some information to a database; if there's a save_to_database function, we can use it to do the job.

In Rust, instructions that can tell the computer to take action can only be written inside of functions. It all starts with a function called main, which can cause other functions to run, which can in turn cause yet more functions to run, and so on. Using our recipe analogy again, it's like a pie recipe saying: Use the recipe on page 57 to make dough for the crust.