Course of Raku / Objects, I/O, and exceptions / Exceptions / Custom exceptions / Exercises / A negative error

Solution: A negative error

Here is a possible solution to the task.

Code

class Negative is Exception {
    has $.n;

    method message {
        "$.n is negative";
    }
}

sub check($n) {
    Negative.new(n => $n).throw if $n < 0;
    return $n;
}

{
    say check(-5);

    CATCH {
        when Negative {
            say .message;
        }
    }
}

Negative.new(n => -10).throw;

🦋 You can find the source code in the file negative-error.raku.

Output

-5 is negative
-10 is negative
  in block <unit> at negative-error.raku line 24

Comments

1. Negative is Exception makes the class throwable, and its message method uses the n attribute to build the text.

2. check validates its input and throws the custom exception for a negative number. The exception travels out of check to the CATCH in the calling block, where when Negative matches it by type and prints the message. Validating input and signalling bad values with a typed exception is a very common, real use of custom exceptions.

3. The { … } around the call is there because a CATCH phaser handles the exceptions thrown in its own enclosing block. The block groups the risky check(-5) together with the CATCH that guards it, so the thrown exception is caught right here, and execution resumes just after the block. Without wrapping them in a block, the CATCH would guard the whole program instead — and once it caught the exception the program would simply end, with no natural place to carry on.

4. The final Negative.new(n => -10).throw shows the other side of this. It sits outside the block, so nothing catches it: the exception propagates all the way to the top, and the program dies, printing the message and a backtrace to standard error and exiting with a non-zero status. That is the default fate of any exception you do not catch — and exactly why the first throw needed a CATCH to survive.

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