Exceptions: Difference between revisions

=={{header|Common Lisp}}==

The Common Lisp condition system allows much more control over condition signaling and condition handling than many exception-based systems. The following example, however, simply defines a condition type, <code>unexpected-odd-number</code>, defines a function <code>get-number</code> which generates a random number, returning it if it is even, but signaling an <code>unexpected-odd-number</code> condition if it is odd. The function <code>get-even-number</code> uses <code>[http://www.lispworks.com/documentation/HyperSpec/Body/m_hand_1.htm handler-case]</code> to call <code>get-number</code> returning its result if no condition is signaled, and, in the case that an <code>unexpected-odd-number</code> condition is signaled, returning one plus the odd number.

<lang lisp>(define-condition unexpected-odd-number (error)

((number :reader number :initarg :number))

(:report (lambda (condition stream)

(format stream "Unexpected odd number: ~w."

(number condition)))))

(defun get-number (&aux (n (random 100)))

(if (not (oddp n)) n

(error 'unexpected-odd-number :number n)))

(defun get-even-number ()

(handler-case (get-number)

(unexpected-odd-number (condition)

(1+ (number condition)))))</lang>

A good introduction to Lisp's condition system is the chapter [http://gigamonkeys.com/book/beyond-exception-handling-conditions-and-restarts.html Beyond Exception Handling: Conditions and Restarts] from Peter Seibel's [http://gigamonkeys.com/book/ Practical Common Lisp].