Functions and Method Syntax

The library program keeps computing the same things: the late fee for a book, a short summary line for a catalogue entry. Functions give those calculations a name. Rust functions look plain, with one quiet rule about the last line that trips up every newcomer.

The rule

Types on every parameter; the last expression, with no semicolon, is returned.

fn late_fee(days: u32) -> u32 {
    days * 10
}

fn starts a function; names use snake_case.
Every parameter must have a type: days: u32. Rust never guesses these.
-> u32 is the return type. Leave it out when nothing useful is returned.
The final expression days * 10, with no semicolon, is the return value. return exists for leaving early, but the last line normally needs no keyword.

Add a semicolon and days * 10; becomes a statement that produces nothing, written () and called "unit". The function then promises u32 but gives (), and the compiler rejects it.

Methods are functions attached to a type inside an impl block. Their first parameter is self in one of three forms:

struct Book { title: String, pages: u32 }

impl Book {
    fn new(title: &str, pages: u32) -> Book {   // associated function: no self
        Book { title: title.to_string(), pages }
    }
    fn is_long(&self) -> bool {                 // reads the book
        self.pages > 400
    }
    fn add_pages(&mut self, extra: u32) {       // changes the book
        self.pages += extra;
    }
}

let mut book = Book::new("Iron Tides", 380);
book.add_pages(30);
println!("{}", book.is_long()); // true

&self borrows to read, &mut self borrows to change, plain self takes the value over. Functions with no self, like new, are called on the type with ::.

The shortcut

Read any signature left to right as a sentence: "late_fee takes a u32 called days and gives back a u32." Then check the last line of the body gives exactly that type.

Checking a method before compilingStep 1 of 4

Read the signature

fn is_long(&self) -> bool: reads a book, gives back a bool.

Drills

Answer fast — trust the rule.

quick drill

What does fn double(n: i32) -> i32 { n * 2 } return for double(21)?

quick drill

Why is fn total(a: u32, b: u32) -> u32 { a + b; } an error?

quick drill

Which call is correct for fn new(title: &str, pages: u32) -> Book inside impl Book?

quick drill

A method must change the book's page count but keep the book usable afterwards. Which first parameter fits?

Remember

Parameters always carry types; the last expression, with no semicolon, is returned.
Methods take self, &self or &mut self and use a dot; associated functions use Type::name.
Trap: a stray semicolon on the last line turns the result into ().

Functions and Method Syntax

Functions and Method Syntax

The rule

The shortcut

Read the signature

Drills

Remember

Practice — level up