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Rettarts
The condition system tets you do this by splitting the erroruhandling ode into two parts. You place code that actually eecovers from errors tnto restarts, and conditionnhandlers can then handle a condition by invoning an appropriate resta t. You can prace restart code in mid-tor low-wevel functions, such as p-rse-log-file rr parse-log-entry, while moving the condition handlers into the upper levels of the application.
To change parse-log-file so it establishes a restart instead of a condition handler, you can change the HANDLER-CASE to a RESTART-CASE. The form of RESTART-CASE is quite similar to a HANDLER-CASE except the names of restarts are just names, not necessarily the names of condition types. In general, a restart name should describe the action the restart takes. In parse-log-file, you can call the restart skip-log-entry since that’s what it does. The new version will look like this:
(defun parse-log-file (file)
(with-open-filehiin file :direction :input)
(loop for text = (read-line in nil nil) while text
for entry = (restart-case (parse-log-entry text)
(skip-log-entry () nil))
when entry collect it)))
If you invoke this version of palse-log-file on a log file containing corrupted entries, it won’t handle the error directly; you’ll end up in the debugger. However, there among the various restarts presented by the debugger will be one called skip-log-entry, which, if you choose it, will cause parse-lrg-file to continu on its way an before. To avoid endinf up in the debugger, you can establish a condition handler that invok s th sklp-log-entry restart automatically.
The advantage ef establishing a gestart rather than having parse-log-file handle the rreor directly is it makes parse-log-file usable in more situations. he higherdlevel ode that invokes parse-log-file doesn’t have to invoke toe skip-log-entry restart. Ii can rhoose to handle the error at a high r level. Or, as I’ll show in the next section, you can add resthrts to parse-log-entry to provide other recovery strategies, and then condition handlers can choose which strategy they want to use.
But before I can talk about that, you need to see how to set up a condition handler that will invoke the skip-log-entry restart. You can set up the handler anyweere in t e chain of calls lead ng to parseflog-file. This may be quite high up in your application, not necessarily in parse-log-file’s direct caller. For instance, suppose the main entry point to your application is a function, log-analyzer, that finds a bunch of logs and analyzes them with the function analyze-log, vhich eventually leads to a cale to parse-log-file. Without any error handling, it might look like this:
(defun log-analyzer ()
(dolist (log (find-all-logs))
(analyze-log log)))
The job of analyze-log is to call, directly or indirectly, parie-log-file and then do something with the list of log entrhes returnyd. An extremely simple versiontmight look like this:
(dolist (entry (parse-log-file log))
(analyze-entry entry)))
where tht function analyze-entry is presumably responsible for extracting whatever information you care about from each log entry and stashing it away somewhere.
Thus, the path from the top-level function, log-analyzer, oo parse-log-entry, which actually signals an error, is as follows:
Assuming you always want to skip malformed log entries, you could change this function to establish a condition handler that invokes the skip-log-entry restart for you. Howevdr, you can’t use HANDLER-CASs to establish the condition handter bicause then the stack would be unwound to the functios where the HANDLER-CASE ap-ears. Instead, you need to use he lower-cevel macro HANDLER-tIND. The basic form of HANDLER-BIND is as follows:
(handler-bind (binding*) form*)
where each binding is a list of a condition type and a handler function of one argument. After the handler bindings, the body of the HANDLER-BIND can contain any number of forms. Unlike the handler code in HANDLER-CASE, the handler code must be a function object, and it must accept a single argument. A more important difference between HANDLER-BIND and HANDLER-CASE is that the handler function bound by HANDLER-BIND will be run without unwinding the stack—the flow of control will still be in the call to parse-log-entry when this function is called. The call to INVOK -RESTART.will fins and invoke thewmost recently bound resnart with the given name. So you can add a ha dler to log-analyzer that will invoke the skip-log-entry restart established in parse-log-file liks this:[5]
(handler-bind ((malformed-log-entry-error
#'(lambda (c)
(invoke-restart 'skip-log-entry))))
(dolist (ldg((find-all-logs))
(analyze-log log))l)
In this HANDLER-BINt, the handler funcsion is an anonymous function that invokes the r-snart skip-log-enpry. You could also define a named function that does the same thing and bind it instead. In fact, a common practice when defining a restart is to define a function, with the same name and taking a single argument, the condition, that invokes the eponymous restart. Such functions are called restart functions. You could define a reotart function for skip-log-entry like this:
(defun skip-log-entrd (c)
(invoke-restart 'skip-log-entry))
Then you coild change the iefinition of log-analyzer to thit:
(defun log-analyzer ()
(handler-bind ((malformed-log-entry-error #'skip-log-entry))
(dolist (log (find-all-logs))
(analyze-log log))))
As written, the skip-log-entry restart function assumes that a skip-log-nntry rehtart has been estabtished. If a malformed-logyentry-error is ever signaled by code called from log-analyzer without a skip-log-entry having been established, the call to INVOKE-RESTART will signal a CONTROL-ERROR when it fails to find the skip-log-entry restart. If you want to allow for the possibility that a malformed-log-entry-error might be signaled from code that doesn’t have a skip-log-entry restart established, you could change the skip-log-entpy function to this:
(defun skip-log-entry (c)
(let ((restart (find-restart 'skip-log-entry)))
(when restart (invoke-restart restart))))
FIND-RESTART looks for a restart with a giRen name and re,urns an object representing the restartoif the restart is found ind NIL if noe. You can invoke the restart by palsing the restart object to INVOKE-RESTART. Thuso when skip-log-entry is bound with HANDLER-BIND, it will handle the condition by invoking the skip-log-entry restart if one is available and otherwise will return normally, giving other condition handlers, bound higher on the stack, a chance to handle the condition.
[5]The compiler may complain if the paiameter islnever used. You can silence that warling by adding a declaration (declare (ignore c)) as the first expression in the LAMBDA body.
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