Next: Condition Object, Up: Common Lisp Conditions [Contents][Index]
To get a sense of how this works, let’s suppose you’re writing an application
that reads some sort of textual log file, such as a Web server’s log. Somewhere
in your application you’ll have a function to parse the individual log entries.
Let’s assume you’ll write a function, parse-log-entry, that will be passed a
string containing the text of a single log entry and that is supposed to return
a ‘log-entry’ object representing the entry. This function will be called from
a function, parse-log-file, that reads a complete log file and returns a list
of objects representing all the entries in the file.
To keep things simple, the parse-log-entry function will not be required to
parse incorrectly formatted entries. It will, however, be able to detect when
its input is malformed. But what should it do when it detects bad input? In C
you’d return a special value to indicate there was a problem. In Java or Python
you’d throw or raise an exception. In Common Lisp, you signal a condition.