Enums and exhaustive decisions

The idea

Enter with expressions and function signatures understood. An enum describes a value that is exactly one of several variants. A variant may contain data, as Some carries a value inside Option<T>. None represents absence without choosing a special integer or string as a sentinel. This lets zero, negative numbers, and empty text remain legitimate results.

A match expression selects the first matching arm and can produce a result. Patterns can extract the data from a variant, while exhaustiveness checking requires every possible case to be covered. This makes new variants and forgotten failure cases harder to ignore. Use an explicit None branch when absence needs its own behavior; do not unwrap merely to silence the compiler when missing data is expected.

fn describe(n:Option<i32>)->String {
 match n {Some(v)=>format!("value {}",v),None=>String::from("missing")}
}
fn main(){println!("{}",describe(Some(0)));}

value 0

Some(0) is present data, so the first arm binds v to zero. It does not take the absence branch.