lang::fn

Function declaration.

fn name(params) -> Ret { body }. The last expression is the return value (no return needed); -> Ret may be omitted when the function returns (). A parameter takes name: Type, &Type (read-shared), or &mut Type (writes through to the caller).

Mutable-reference arguments must be explicit at the call site. A writable binding is passed as &mut value, while an existing &mut Type reference can be forwarded as-is. Passing a bare value never grants a callee mutable access.

fn area(w: i64, h: i64) -> i64 { w * h }
fn greet(name: &String) { println!("hi, {name}") }

Parameter pattern destructuring

A parameter may be a pattern instead of a plain name - tuple or struct - and binds its components directly:

struct Point { x: i64, y: i64 }
struct Pair { left: i64, right: i64 }

fn dot((a, b): (i64, i64)) -> i64 { a * b }
fn sum(Point { x, y }: Point) -> i64 { x + y }
fn diff(Pair { left, right }: Pair) -> i64 { left - right }

Generics

A function may take type parameters (fn id<T>(x: T) -> T), a trait bound (fn report<T: Shape>(s: &T)), or a const-generic array length (fn sum<const N: usize>(xs: [i64; N])). Each instantiation is monomorphised - see generics.