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Created Jun 14, 2018
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A mutating plus one function in rust
struct MyNumber {
n: i64,
// &mut MyNumber is the type for a mutable reference to MyNumber
fn plus_one(number: &mut MyNumber) {
number.n += 1;
// Rust doesn’t have nil-able pointers at all in most code (they do exist,
// but mainly for interoperation with C).
// Instead it has an Option type to signal missing values.
// Option<&mut MyNumber> is an enum that can be either:
// - None: Rust version of nil
// - Some(MyNumber): Some is just the name for the enum variant and MyNumber
// can be extracted from it as shown in the function
// Errors are also slightly different from go errors, since they use a Result
// type that is similar to the Option type.
// Result<(), &'static str> is an enum that can be either:
// - Ok(()): Indicates a success, without a return value, because () is
// an empty tuple
// - Err(&'static str): Indicates an error occured and contains a static
// string, i.e. one that is part of the source code
fn plus_one_nullable(nullabe_number: Option<&mut MyNumber>) -> Result<(), &'static str> {
// match is the Rust version of switch and => is the Rust version of case
match nullabe_number {
None => return Err("number shouldn't be None"),
Some(number) => plus_one(number),
return Ok(());
fn main() {
// we need to annotate that number is mut(able), so we can actually change it
let mut number = MyNumber { n: 5 };
plus_one(&mut number);
println!("{}", number.n); // 6
println!("{:?}", plus_one_nullable(Some(&mut number))); // Ok(())
println!("{}", number.n); // 7
println!("{:?}", plus_one_nullable(None)); // Err("number shouldn\'t be None")
println!("{}", number.n); // 7
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