Functions

Declare a function with fn, typed parameters, and an optional return annotation. A final expression can provide the return value. The return type is written either : R or -> R; both spellings parse.

fn multiply(a: i32, b: i32): i32 {
    a * b
}

fn square(a: i32) -> i32 {
    a * a
}

Overloading

Functions overload on parameter count and on parameter types. Overload resolution picks the matching declaration at the call site; an unresolvable call reports E2007, and an equally good pair reports E2008.

fn area(w: i32) -> i32 {
    w * w
}

fn area(w: i32, h: i32) -> i32 {
    w * h
}

Because overloads share a source name, linkage names are qualified as <module>.<Owner>.<name>(<params>). The two exceptions are extern fn and main, which keep their plain C-visible names.

Function kinds

fn, flow fn, raw fn, and extern fn parse and lower through HIR to code generation. const fn is still a parse error: const is a binding keyword, so const fn f() is read as a const binding named fn rather than a compile-time function.

See Implementation Status before relying on the other forms, and Concurrency for the status of the legacy async fn spelling.