Make decisions that produce values

A condition must have type bool. Comparisons such as ==, !=, <, <=, >, and >= produce booleans. There is no implicit conversion of zero, an empty string, or another value to False.

An if can supply a function’s result:

def maximum(first: i64, second: i64) -> i64:
    if first > second:
        first
    else:
        second

def main() -> Result[(), IoError]:
    print(maximum(7, 12))?
    Ok(())

12

Both continuing branches need the same type. Use elif for additional cases. For a short choice, use a conditional expression:

def main() -> Result[(), IoError]:
    age = 20
    category = "adult" if age >= 18 else "child"
    print(category)?
    Ok(())

adult

and and or short-circuit: the right side is evaluated only when needed. not negates a boolean. All three require boolean operands:

def main() -> Result[(), IoError | ZeroDivisionError | OverflowError[i64]]:
    divisor = 0
    safe = divisor != 0 and 10 // divisor > 1
    print(safe)?
    print(not safe)?
    Ok(())

False
True

The division is skipped here. Chained comparisons such as 0 < x < 10 are not supported yet; write 0 < x and x < 10.