ANSI Common Lisp 9 Conditions
9.1 Condition System Concepts
Common Lisp constructs are described not only in terms of their behavior in situations during which they are intended to be used (see the "Description" part of each operator specification), but in all other situations (see the "Exceptional Situations" part of each operator specification).
A situation is the evaluation of an expression in a specific context. A condition is an object that represents a specific situation that has been detected. Conditions are generalized instances of the class condition. A hierarchy of condition classes is defined in Common Lisp. A condition has slots that contain data relevant to the situation that the condition represents.
An error is a situation in which normal program execution cannot continue correctly without some form of intervention (either interactively by the user or under program control). Not all errors are detected. When an error goes undetected, the effects can be implementation-dependent, implementation-defined, unspecified, or undefined. See Section 1.4 Definitions. All detected errors can be represented by conditions, but not all conditions represent errors.
Signaling is the process by which a condition can alter the flow of control in a program by raising the condition which can then be handled. The functions error, cerror, signal, and warn are used to signal conditions.
The process of signaling involves the selection and invocation of a handler from a set of active handlers. A handler is a function of one argument (the condition) that is invoked to handle a condition. Each handler is associated with a condition type, and a handler will be invoked only on a condition of the handler's associated type.
Active handlers are established dynamically (see handler-bind or handler-case). Handlers are invoked in a dynamic environment equivalent to that of the signaler, except that the set of active handlers is bound in such a way as to include only those that were active at the time the handler being invoked was established. Signaling a condition has no side-effect on the condition, and there is no dynamic state contained in a condition.