Control construction with init

Define __init__ to calculate or check fields during construction. It replaces the generated constructor; bare self is an exclusive reference:

class Span:
    start: i64
    end: i64

    def __init__(self, start: i64, length: i64) -> Result[(), OverflowError[i64]]:
        self.start = start
        self.end = start + length
        Ok(())

    def length(self) -> Result[i64, OverflowError[i64]]:
        Ok(self.end - self.start)

def main() -> Result[(), IoError | OverflowError[i64]]:
    span = Span(10, 5)?
    print(span)?
    print(span.length()?)?
    Ok(())

Span(start=10, end=15)
5

The initializer calculates the endpoint from a start and length. Overflow returns its declared OverflowError[i64], so Span(10, 5) returns Result[Span, OverflowError[i64]]; ? propagates failure through main. An initializer returning () instead gives an infallible constructor. Construction itself allocates nothing and adds no allocation error; see constructor contracts.

Every field must be initialized on every path out of the constructor. You may read a field already initialized, but cannot pass the whole unfinished self to another function or method. A loop might never execute, so initializing a field only inside a loop is insufficient.

class Pair:
    first: i64
    second: i64

    def __init__(self, first: i64) -> ():
        self.first = first

def main() -> ():
    pass

fields not initialized: second