Match

match compares a value against a list of patterns and runs the code for the first one that fits.

match

Each arm is pattern => expression. Rust checks the arms from top to bottom and stops at the first match.

1enum Coin { Penny, Dime }23let c = Coin::Dime;4let cents = match c {5    Coin::Penny => 1,6    Coin::Dime => 10,7};

match is an expression, so it gives back a value. Every arm must give the same type.

Exhaustive matching

A match must cover every possible value. If you forget a variant, the code does not compile (error E0004). This means adding a new variant later shows you every match you need to update.

Binding in patterns

A pattern can pull data out of a variant into a new variable. The variable only exists inside that arm.

1let n: Option<i32> = Some(5);2match n {3    Some(x) => println!("got {x}"),4    None => println!("nothing"),5}

Wildcards, | and ranges

• _ matches anything and ignores it. Put it last, as a catch-all.
• | lets one arm match several patterns.
• 1..=5 matches any number from 1 to 5.

1let n = 7;2let msg = match n {3    0 => "zero",4    1 | 2 => "small",5    3..=9 => "medium",6    _ => "big",7};

Numbers have too many values to list, so a match on a number almost always needs _.

if let

When you only care about one pattern, if let is shorter than match.

1if let Some(x) = n {2    println!("got {x}");3}

You can add an else for everything else.

Remember

• The first arm that fits wins.
• Cover every case, or use _ (E0004 otherwise).
• Names in a pattern only live inside that arm.
• Use if let for one pattern.