episode-008

Where we introduce functions

Setup

The following needs to be prepared

• A project with the name functions
• Repetition of printing hello and goodbye to two different subjects.

Script

Look at the following code

let mut subject = "world";

println!("Hello, {}!", subject);
println!("Goodbye, {}!", subject);

subject = "moon";

println!("Hello, {}!", subject);
println!("Goodbye, {}!", subject);

It looks very familiar. In fact in episode 004 we already saw something similar. The difference is that now we greet both world and moon, instead of one or the other.

Just as in episode 004 if we wanted to change the code, for example, change Hello into What's up we would have to change that in to places. This seems wastefull.

Lets see how a function could help in this situation.

A function is defined with the fn keyword, followed by the name of the function and its arguments.

fn greet(subject) {
  println!("Hello, {}", subject);
  println!("Goodbye, {}", subject);
}

When we try to compile this code, Rust will complain. When using variables Rust could infer the type of the variable for us. Rust will not do that when we are defining a function.

For now we will provide the type without going into details. They will be addressed in a different episode.

fn greet(subject: &str) {
  println!("Hello, {}", subject);
  println!("Goodbye, {}", subject);
}

This time Rust still complains, but now with a friendly reminder that we haven't used the greet function. So let's go ahead an use it.

let mut subject = "world";
greet(subject);

subject = "moon";
greet(subject);

And there you have it, we have introduced a function.