Create a function in Rust

The first thing we are going to write in Rust is a function.

A function is defined with the fn keyword, like this:

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fn main() {
    println!("Hello, world!");
}

Above our main() function, let’s create a new function called usage(). Inside it, we’ll use the same macro used to write "Hello, world!" to output the name and a short description of our tool:

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fn usage() {
    println!("tinymd, a markdown compiler written by <YOUR NAME>");
}

fn main() {
    println!("Hello, world!");
}

Go ahead and replace <YOUR NAME> with your name if you haven’t already done so.

Compile and run your project with cargo run -q, which will always automatically detect changes in your source files and rebuild your project if needed:

$ cargo run -q
Hello, world!

 $ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s
      Running `target\debug\tinymd.exe`
  Hello, world!

$ cargo run
Hello, world!

warning: function is never used: `usage`
 --> src/main.rs:1:4
  |
1 | fn usage() {
  |    ^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

If you don’t really care to look at the accompanying information (Finished dev and Running lines), you can pass the -q flag to Cargo, which tells it to be quiet:

$ cargo run -q
Hello, world!

To save space, that’s how we’ll be compiling, building, and running the code as we make changes to it—so don’t panic when you see the -q flag.

Now that we have a function to print the banner, let’s replace the println! command in the main function with a call to usage():

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fn usage() {
    println!("tinymd, a markdown compiler written by <YOUR NAME>");
}

fn main() {
    usage();
}

Finally, let’s compile, build, and run, all in one command:

$ cargo run -q
tinymd, a markdown compiler written by <YOUR NAME>

With our usage() function scaffolded, let’s iterate and have it return a simple value that we can write to the console window.