Signaling and Handling Conditions

The operation of the condition system depends on the ordering of active applicable handlers from most recent to least recent.

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 H_1 is more recent than every handler in a set H_2 if the handlers in H_2 were active when the handlers in H_1 were established.

2. Let h_1 and h_2 be two applicable active handlers established by the same form. Then h_1 is more recent than h_2 if h_1 was defined to the left of h_2 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.

Figure 9–4 shows operators relating to the handling of conditions.

  handler-bind  handler-case  ignore-errors