ANSI Common Lisp   9 Conditions   9.1 Condition System Concepts

9.1.4 Signaling and Handling Conditions

Each handler is associated with a type specifier that must designate a subtype of type condition. A handler is said to be applicable to a condition if that condition is of the type designated by the associated type specifier.

Active handlers are established by using handler-bind (or an abstraction based on handler-bind, such as handler-case or ignore-errors).

Active handlers can be established within the dynamic scope of other active handlers. At any point during program execution, there is a set of active handlers. When a condition is signaled, the most recent active applicable handler for that condition is selected from this set. Given a condition, the order of recentness of active applicable handlers is defined by the following two rules:

• 1. Each handler in a set of active handlers H₁ is more recent than every handler in a set H₂ if the handlers in H₂ were active when the handlers in H₁ were established.

• 2. Let h₁ and h₂ be two applicable active handlers established by the same form. Then h₁ is more recent than h₂ if h₁ was defined to the left of h₂ in the form that established them.

Once a handler in a handler binding form (such as handler-bind or handler-case) has been selected, all handlers in that form become inactive for the remainder of the signaling process. While the selected handler runs, no other handler established by that form is active. That is, if the handler declines, no other handler established by that form will be considered for possible invocation.

The next figure shows operators relating to the handling of conditions.

9.1.4.1  Signaling
9.1.4.2  Restarts