A user-break polling handler is periodically invoked to determine if the user wants to escape from the current evaluation. (For example, a user-break polling handler might check to see if control-c was pressed.) A user-break polling handler takes no arguments and returns #f to continue the current evaluation.
User-break polling is initially disabled in some applications that use
MzScheme. (break-enabled) returns #t when user-break polling is
enabled or #f otherwise. (break-enabled v) disables
user-break polling if v is #f or enables polling otherwise.
(dynamic-enable-break thunk) invokes
thunk with breaking allowed; on exit from thunk, the enable state is
restored.
When a user break is detected, the exn:misc:user-break exception is raised.