Give constructors enough type information

A binding annotation, parameter, or return signature supplies missing type information. Some(42) infers Option[i64] from its payload. Nothing needs an option type from context; Ok and Err each need the other variant’s type:

def main() -> Result[(), IoError]:
    found = Some(42)
    missing: Option[i64] = Nothing
    success: Result[i64, str] = Ok(42)
    failure: Result[i64, str] = Err("not ready")
    print(found == Some(42))?
    print(missing == Option[i64].Nothing)?
    match failure:
        case Ok(value):
            print(value)?
        case Err(message):
            print(message)?
    Ok(())

True
True
not ready

Qualify a constructor, such as Option[i64].Nothing, to supply its type explicitly. Otherwise, insufficient context is an error:

def main() -> ():
    answer = Ok(42)

cannot infer `Ok`