The interactive condition handler returns only through non-local transfer of control to specially defined restarts that can be set up either by the system or by user code. Transferring control to a restart is called "invoking" the restart. Like handlers, active restarts are established dynamically, and only active restarts can be invoked. An active restart can be invoked by the user from the debugger or by a program by using invoke-restart.
A restart contains a function to be called when the restart is invoked, an optional name that can be used to find or invoke the restart, and an optional set of interaction information for the debugger to use to enable the user to manually invoke a restart.
The name of a restart is used by invoke-restart. Restarts that can be invoked only within the debugger do not need names.
Restarts can be established by using restart-bind, restart-case, and with-simple-restart. A restart function can itself invoke any other restart that was active at the time of establishment of the restart of which the function is part.
The restarts established by a restart-bind form, a restart-case form, or a with-simple-restart form have dynamic extent which extends for the duration of that form’s execution.
Restarts of the same name can be ordered from least recent to most recent according to the following two rules:
Each restart in a set of active restarts R_1 is more recent than every restart in a set R_2 if the restarts in R_2 were active when the restarts in R_1 were established.
Let r_1 and r_2 be two active restarts with the same name established by the same form. Then r_1 is more recent than r_2 if r_1 was defined to the left of r_2 in the form that established them.
If a restart is invoked but does not transfer control, the values resulting from the restart function are returned by the function that invoked the restart, either invoke-restart or invoke-restart-interactively.