Mastering Rust - Second Edition

Function types

Functions in Rust also have a concrete type and they differ in terms of their argument types and also in their arity, as in how many arguments they take, as in the example:

// function_types.rsfn add_two(a: u32, b: u32) -> u32 {    a + b}fn main() {    let my_func = add_two;    let res = my_func(3, 4);    println!("{:?}", res);}

Functions in Rust are first class citizens. This means they can be stored in variables or passed to other functions or returned from functions. The preceding code declares a function add_two, which we store in my_func and later invoke with 3 and 4.

Function types are not to be confused with Fn closures as they both have fn as their type signature prefix.