Guard

Add a condition to a pattern with if guard.

Guards

Conditions that a pattern can't express (like a relation between two bound variables) are added to the end of the arm with if:

fn iliski(c: (i32, i32)) -> &'static str {
    match c {
        (a, b) if a == b => "eşit",
        (a, b) if a + b == 0 => "zıt",
        _ => "farklı",
    }
}

Guards can also reach outer variables: Some(x) if x == hedef. Writing an outer variable's name inside the pattern, however, means a new binding, not a comparison; this is a common mistake.

Guards and exhaustiveness

The compiler doesn't evaluate guard conditions, so guarded arms don't count toward the exhaustiveness check and an unconditional arm is usually needed at the end:

match n {
    x if x < 0 => "negatif",
    x if x >= 0 => "pozitif ya da sıfır",
    _ => unreachable!(),   // the compiler doesn't know the two conditions cover everything
}

In such cases it's better to write the patterns as ranges: i32::MIN..=-1 and 0...

Exercise Relation

Write the iliski function above with guards. Print the results for (2, 2), (3, -3) and (1, 5) on one line separated by spaces.

Show example solution

fn iliski(c: (i32, i32)) -> &'static str {
    match c {
        (a, b) if a == b => "eşit",
        (a, b) if a + b == 0 => "zıt",
        _ => "farklı",
    }
}

fn main() {
    println!("{} {} {}", iliski((2, 2)), iliski((3, -3)), iliski((1, 5)));
}

Test yourself

What does <code>let hedef = 5; match v { Some(hedef) => .., _ => .. }</code> do?
- Matches every Some value and creates a new hedef variable (shadowing)
- Matches only Some(5)
- It doesn't compile
- Panic
Show answer

Matches every Some value and creates a new hedef variable (shadowing) — A name in a pattern is always a new binding; for comparison write Some(x) if x == hedef.

Conditional arm

match c {
    (a, b) ____ a == b => "eşit",
    _ => "farklı",
}

Show answer

match c {
    (a, b) if a == b => "eşit",
    _ => "farklı",
}

Pair and result

(2, 2)

eşit
(3, -3)

zıt
(1, 5)

farklı
(0, 0)

eşit (first arm)

Next lesson: Binding modes

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