Small Example

Here is a small example program in Rust:

1. fn main() {              // Program entry point
2. let mut x: i32 = 6;  // Mutable variable binding
3. print!("{x}");       // Macro for printing, like printf
4. while x != 1 {       // No parenthesis around expression
5. if x % 2 == 0 {  // Math like in other languages
6. x = x / 2;
7. } else {
8. x = 3 * x + 1;
9. }
10. print!(" -> {x}");
11. }
12. println!();
13. }

The code implements the Collatz conjecture: it is believed that the loop will always end, but this is not yet proved. Edit the code and play with different inputs.

Key points:

• Explain that all variables are statically typed. Try removing i32 to trigger type inference. Try with i8 instead and trigger a runtime integer overflow.
• Change let mut x to let x, discuss the compiler error.
• Show how print! gives a compilation error if the arguments don’t match the format string.
• Show how you need to use {} as a placeholder if you want to print an expression which is more complex than just a single variable.
• Show the students the standard library, show them how to search for std::fmt which has the rules of the formatting mini-language. It’s important that the students become familiar with searching in the standard library.