episode-005

Where we discover immutabillity

Setup

The following needs to be prepared

• A project with name immutabillity
• A variable subject with value moon
• A println! statement

Script

We used a variable to reuse a value multiple times.

let subject = "world";

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

If we want to greet the world and the moon, we could try to copy the println! statements and reassign the subject variable.

let subject = "world";


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

subject = "moon";

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

But Rust won't let us. Instead it reports a compile error

main.rs:7:5: 7:21 error: re-assignment of immutable variable `subject` [E0384]
main.rs:7     subject = "moon";
              ^~~~~~~~~~~~~~~~
main.rs:2:6: 2:13 note: prior assignment occurs here
main.rs:2 	let subject = "world";
          	    ^~~~~~~
error: aborting due to previous error
Could not compile `immutabillity`.

To learn more, run the command again with --verbose.

When we define a variable in Rust the default is to make the variable immutable. That means that we are not allowed to change the binding. This is what Rust complains about.

It could seems strange to make variable immutable by default, but it is a blessing in disguise. Immutability makes it easier to reason about programs.

If we really want to change a variable binding, Rust allows us if we tell when we declare our variable.

let mut subject = "world";

We can use the mut keyword when we define the variable to signal Rust that we might want to rebind the variable later in the program.

And there you have it, we discovered immutability.