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16.5 Condition Instances

A condition, in addition to the information associated with its type, usually contains other information that is not shared with other conditions of the same type. For example, the condition type associated with “unbound variable” errors does not specify the name of the variable that was unbound. The additional information is captured in a condition object, also called a condition instance.

In addition to information that is specific to a given type of condition (such as the variable name for “unbound variable” conditions), every condition instance also contains a continuation that encapsulates the state of the computation in which the condition occurred. This continuation is used for analyzing the computation to learn more about the context in which the condition occurred. It is not intended to provide a mechanism for continuing the computation; that mechanism is provided by restarts.