Restarts are a general mechanism for establishing a protocol between
condition-signalling and condition-handling code. The Scheme error
system provides “packaging” for a number of common protocols. It also
provides lower-level hooks that are intended for implementing customized
protocols. The mechanism used by signalling code (
with-simple-restart) is used for both purposes.
Four additional operations are provided for the use of
condition-handling code. Two operations (
find-restart) allow condition-handling code to locate active
restarts. The other two operations (
invoke-restart-interactively) allow restart effectors to be
invoked once the restart object has been located.
In addition, there is a data abstraction that provides access to the information encapsulated in restart objects.
Returns a list of all currently active restart objects, most recently
bound-restarts should be used with caution by
condition-handling code, since it reveals all restarts that are active
at the time it is called, rather than at the time the condition was
signalled. It is useful, however, for collecting the list of restarts
for inclusion in newly generated condition objects or for inspecting the
current state of the system.
Returns the first restart object named name in the list of
restarts (permissible values for restarts are described
above in Restarts). When used in a condition handler,
find-restart is usually passed the name of a particular restart
and the condition object that has been signalled. In this way
the handler finds only restarts that were available when the condition
was created (usually the same as when it was signalled). If
restarts is omitted, the currently active restarts would be used,
and these often include restarts added after the condition ocurred.
Calls the restart effector encapsulated in restart, passing the
specified arguments to it.
invoke-restart is intended for
use by condition-handling code that understands the protocol implemented
by restart, and can therefore calculate and pass an appropriate
set of arguments.
If a condition handler needs to interact with a user to gather the
arguments for an effector (e.g. if it does not understand the protocol
implemented by restart)
be used instead of
First calls the interactor encapsulated in restart to interactively gather the arguments needed for restart’s effector. It then calls the effector, passing these arguments to it.
invoke-restart-interactively is intended for calling interactive
restarts (those for which
restart/interactor is not
invoke-restart-interactively will call the
restart’s effector with no arguments if the restart has no interactor;
this behavior may change in the future.