By the end of this lesson
Give a function typed parameters and return an expression.
Before you start
- Rust 1.90.0 or later, a terminal and a plain-text editor. Examples were tested with Rust 1.90.0, Edition 2024 on Linux.
- Complete lesson 5, or be comfortable explaining and running its examples.
Three words to know
- Parameter
- A named input in a function definition.
- Return type
- The type declared after ->.
- Expression
- Code that evaluates to a value.
1. Understand the idea
A function groups one operation behind a name. Parameters and return types make its contract visible. Rust blocks can evaluate to a value: a final expression without a semicolon becomes the block result. Adding a semicolon turns it into a statement and usually makes the block evaluate to (), the unit value. Keep small calculations separate from printing so they are easy to test.
2. Save, run and observe
Save this complete program as lesson.rs in a fresh practice folder. It replaces that lesson file only; keep earlier examples separately. Compile it with rustc, then run the executable. Rust source goes in the editor; rustc and cargo commands go in the terminal.
fn total(open: u32, closed: u32) -> u32 {
open + closed
}
fn main() {
println!("total={}", total(2, 1));
}
What you should see · example output
total=3
rustc --edition=2024 lesson.rs -o lesson
./lesson3. Explain the result
The call supplies two u32 values. The final expression returns their sum. This beginner calculation assumes small inputs; it does not replace the checked arithmetic policy from lesson 4 for arbitrary values. Printing belongs in main rather than inside total.
YOUR TURN
A small check before you continue
What changes if you put a semicolon after open + closed?
Show a suggested answer
The function body no longer returns the u32 expression; the declared return type conflicts with (). Remove the semicolon or use return explicitly.
Sources and scope
Documentation and examples reviewed 2026-10-18. Provider screens, package versions and output wording can differ. Examples show expected outcomes; they are not a claim that OpenViST connected to your server or verified your setup.