Rust Notes: Functions

Intro

Rust is a functional language, it may or may not be shocking to discover that functions are a big part of the language.

A function is defined with the fn keyword. fn is pronounced: fun 🥳

// Function with no parameters or return value
fn stuff() {
    println!("stuff"); // => stuff
}

// Function with parameters that returns an i32
fn add(i: i32, j: i32) -> i32 {
    // return i + j; // supported, but not preferred.
    i + j // bare tail expression is perferred.
}

// The `main` function is the programs entry point.
fn main() {
    stuff(); // call a function.
    println!("{}", add(1, 2)); // => 3
}

Considerations

• Functions are defined in snake_case by convention.
• Type annotations are required for any parameters.
• If the function returns something, the return type must be specified.
• A functions name, its parameters and return type make up a functions signature .
• The main function is the entrypoint to the program.
• The last statement in a function is automatically returned, this is known as a tail expression .
• The return keyword is not required unless exiting before the tail expression .
• The bare tail expression format is the preferred Rust convention.
• The terminating semicolon ; is not required for a returned value.
• Functions do not support default parameter assignment.
• Functions do not support named parameters, all parameters are positional.
• Functions must be called with all the parameters in the correct order.