Error handling

Errors in XMLang work similarly to errors in many other programming languages.

They can be thrown (either manually or by a built-in operation) and caught using the <try> and <catch> elements.

When an error is thrown, the program execution stops and control is transferred to the nearest <catch> element that can handle the error. If no <catch> element is found, the program execution stops and the error is printed to the standard output.

<throw>

The <throw> element is used to throw an error.

Attributes

• message (string, optional): The error message to throw.

Children

If the message attribute has been provided, <throw> does not accept any children. The provided message is used as the error message.

If the message attribute has not been provided, <throw> can optionally have children, which are evaluated, converted to strings and concatenated, just like in the <print> element.

• If at least 1 child are provided, the resulting string is used as the error message.
• If no children are provided or the constructed error message is empty, the error message is set to An error occurred, but no message was provided..

<try>

The <try> element is a statement that allows you to execute a block of code that may throw an error. If an error is thrown, control is transferred to the nearest <try> statement's <catch> element.

Children

This is a statement. The only elements that can be direct children of <try> are the <do> and <catch> elements, which aren't valid elements anywhere else.

<do>

The <do> element is used to define a block of fallible code that will be executed when the <try> statement is reached. It is a block.

If an error is thrown during the execution of the <do> block, control is transferred to the nearest <catch> element.

<catch>

The <catch> element is used to handle errors thrown by the <do> block. It is a block.

Specials

The <catch> element can access the error that was thrown by the <do> block using the <special> element with the name attribute set to error. This will return the error as a string.

Example

<program>
    <try>
        <do>
            <print>
                The meaning of life is
                <space />
                <div>
                    <int>42</int>
                    <int>0</int> <!-- Oh no! Division by zero! -->
                </div>
            </print>
        </do>
        <catch>
            <print>
                An error occurred:
                <space />
                <special name="error" />
            </print> <!-- Prints: "An error occurred: Division by zero is not allowed" -->
        </catch>
    </try>
</program>