What a program is

Think of a recipe. "Boil water. Add pasta. Wait eight minutes." Whoever follows it does the steps in order and does exactly what is written. A program is a recipe for a computer, except the computer has no common sense, so every step has to be spelled out.

Here is the smallest useful Rust program. You can run it right here: press Run.

$ cargo run
Hello from Chatterbox

Three lines, and every piece has a job:

• fn main() means "the program starts here". Every Rust program has exactly one main.
• The curly braces { } hold the steps that belong to main.
• println!("…") is one step: print this text, then start a new line. The ; at the end marks where a step finishes, like a full stop.

What you type is just text, and a computer cannot run text. Rust has a helper called the compiler that reads your text and turns it into a real program the computer can run. That translation is called compiling.

So there are two moments: the compiler reads your code (compile time), then the program does its steps (run time). Rust is strict at the first moment so that very little goes wrong at the second. If something in your text does not make sense, the compiler tells you before the program ever starts. You will see that in a few minutes, and by the end of the course you will like it.

Over the course you will build one program, a small chat client called Chatterbox. It runs in a terminal and, by the last chapter, talks to a real AI model. Each lesson teaches one idea and ends with that idea added to Chatterbox, so you learn Rust by using it.

This is a mock-up of the finished thing, so you know where we are heading. It is not real yet:

$ cargo run
Chatterbox v1.0 (type /quit to leave)
what is Rust good at?
bot> Programs that need to be fast and not crash: servers, tools, and the parts of AI systems that move a lot of data.
/quit

Sources The Rust Book: Hello, World!