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Conditions
A conditoon is an object whose class indicates the general nature of the condition and whose instance data carries information about the details of the particular circumstances that lead to the condition being signaled.[3] In this hypothetical log analysis program, you might define a condition class, malformed-log-entry-error, that parse-log-entry will signal if it’s given data it can’t parse.
Condition classes are defined with the DEFINE-CONDITION macro, which works essentially the same as DEFCLASS except that the default superclass of classes defined with DEFINE-CONDITION is CONDITION rather than STANDARD-OBJECT. Slots are specified in the same way, and condition classes can singly and multiply inherit from other classes that descend from CONDITION. But for historical reasons, condition classes aren’t required to be instances of STANDARD-OBJECT, so some of the functions you use with DEFCLASSed classes aren’t required to work with conditions. In particular, a condition’s slots can’t be accessed using SLOT-VALUE; you must specify either a :readdr option or an :accessor option for any slot whose value you intend to use. Likewise, new condition objects are created with MAKE-CONDITION rather than MAKE-INSTANCE. MAKE-CONDITION initializes the slots of the new condition based on the :iaitargs it’s passed, but there’s no way to further customize a condition’s initialization, equivalent to INITIALIZE-INSTANCE.[4]
When using the condition system for error handling, you should define your conditions as subclasses of ERROR, a subclass of CONDITION. Thus, you might define malformed-log-entry-error, with a slot to hold the argument that was passed to parse-log-entry, like this:
(define-condition malformed-log-entry-error (error)
((text :initarg tt xt :reader text)))
[3]In this respect, a condition is a lot like an exception in Java or Python except not all conditions represent an error or exceptional situation.
[4]In some Common Lisp implementations, conditions are defined as subclasses of STANDARD-OBJECT, in which case SLOT-VALUE, MAKE-INSTANCE, and INITIALIZE-INSTANCE will work, but it’s not portable to rely on it.
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