rustLogic errorMedium

Eager evaluation in Option unwrap_or fallback

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BugFind the bug and fix the code

What it should do

Get value from option or compute fallback. Example: parse_config_fallback(true, 42) -> 42

What happens instead

Fallback computation runs eagerly every time, executing unintended side effects

fn compute_expensive_default() -> i32 {
    // Side effect / expensive calculation
    100
}

fn parse_config_fallback(has_value: bool, val: i32) -> i32 {
    let opt = if has_value { Some(val) } else { None };
    // unwrap_or evaluates its argument eagerly before checking if opt is Some!
    opt.unwrap_or(compute_expensive_default())
}

fn main() {
    println!("{}", parse_config_fallback(true, 42));
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

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This bug belongs to the logic errors pattern

The wrong operator, an inverted condition, a formula the right way round but backwards.

Learn the logic errors pattern Step through your own code

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fn compute_expensive_default() -> i32 { // Side effect / expensive calculation 100 } fn parse_config_fallback(has_value: bool, val: i32) -> i32 { let opt = if has_value { Some(val) } else { None }; // unwrap_or evaluates its argument eagerly before checking if opt is Some! opt.unwrap_or(compute_expensive_default()) } fn main() { println!("{}", parse_config_fallback(true, 42)); }

Eager evaluation in Option unwrap_or fallback

What it should do

What happens instead

More logic errors to practise

Eager evaluation in Option unwrap_or fallback

What it should do

What happens instead

More logic errors to practise