In most languages, errors are handled by returning from a failing function and giving the caller the choice of either recovering or failing itself. Some languages use the normal function return mechanism, while languages with exceptions return control by throwing or raising an exception. Exceptions are a vast improvement over using normal function returns, but both schemes suffer from a common flaw: while searching for a function that can recover, the stack unwinds, which means code that might recover has to do so without the context of what the lower-level code was trying to do when the error actually occurred.
Consider the hypothetical call chain of high, medium, low. If low fails and medium can't recover, the ball is in high's court. For high to handle the error, it must either do its job without any help from medium or somehow change things so calling medium will work and call it again. The first option is theoretically clean but implies a lot of extra code—a whole extra implementation of whatever it was medium was supposed to do. And the further the stack unwinds, the more work that needs to be redone. The second option—patching things up and retrying—is tricky; for high to be able to change the state of the world so a second call into medium won't end up causing an error in low, it'd need an unseemly knowledge of the inner workings of both medium and low, contrary to the notion that each function is a black box.
The Lisp Way
Common Lisp's error handling system gives you a way out of this conundrum by letting you separate the code that actually recovers from an error from the code that decides how to recover. Thus, you can put recovery code in low-level functions without committing to actually using any particular recovery strategy, leaving that decision to code in high-level functions.
To get a sense of how this works, let's suppose you're writing an application that reads some sort of textual log file, such as a Web server's log. Somewhere in your application you'll have a function to parse the individual log entries. Let's assume you'll write a function, parse-log-entry, that will be passed a string containing the text of a single log entry and that is supposed to return a log-entry object representing the entry. This function will be called from a function, parse-log-file, that reads a complete log file and returns a list of objects representing all the entries in the file.
To keep things simple, the parse-log-entry function will not be required to parse incorrectly formatted entries. It will, however, be able to detect when its input is malformed. But what should it do when it detects bad input? In C you'd return a special value to indicate there was a problem. In Java or Python you'd throw or raise an exception. In Common Lisp, you signal a condition.
Conditions
A condition 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.[203] 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 :reader 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 :initargs it's passed, but there's no way to further customize a condition's initialization, equivalent to INITIALIZE-INSTANCE.[204]
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 :text :reader text)))
Condition Handlers
In parse-log-entry you'll signal a malformed-log-entry-error if you can't parse the log entry. You signal errors with the function ERROR, which calls the lower-level function SIGNAL and drops into the debugger if the condition isn't handled. You can call ERROR two ways: you can pass it an already instantiated condition object, or you can pass it the name of the condition class and any initargs needed to construct a new condition, and it will instantiate the condition for you. The former is occasionally useful for resignaling an existing condition object, but the latter is more concise. Thus, you could write parse-log-entry like this, eliding the details of actually parsing a log entry:
(defun parse-log-entry (text)
(if (well-formed-log-entry-p text)
(make-instance 'log-entry ...)
(error 'malformed-log-entry-error :text text)))
What happens when the error is signaled depends on the code above parse-log-entry on the call stack. To avoid landing in the debugger, you must establish a condition handler in one of the functions leading to the call to parse-log-entry. When a condition is signaled, the signaling machinery looks through a list of active condition handlers, looking for a handler that can handle the condition being signaled based on the condition's class. Each condition handler consists of a type specifier indicating what types of conditions it can handle and a function that takes a single argument, the condition. At any given moment there can be many active condition handlers established at various levels of the call stack. When a condition is signaled, the signaling machinery finds the most recently established handler whose type specifier is compatible with the condition being signaled and calls its function, passing it the condition object.
The handler function can then choose whether to handle the condition. The function can decline to handle the condition by simply returning normally, in which case control returns to the SIGNAL function, which will search for the next most recently established handler with a compatible type specifier. To handle the condition, the function must transfer control out of SIGNAL via a nonlocal exit. In the next section, you'll see how a handler can choose where to transfer control. However, many condition handlers simply want to unwind the stack to the place where they were established and then run some code. The macro HANDLER-CASE establishes this kind of condition handler. The basic form of a HANDLER-CASE is as follows:
203
In this respect, a condition is a lot like an exception in Java or Python except not all conditions represent an error or
204
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.