(define-condition malformed-log-entry-error (error)
  ((text :initarg :text :reader text)))

;; this is totally unremarkable. Very similar to how an
;; exception type is defined in Java


(defun parse-log-entry (text)
  (if (well-formed-log-entry-p text)
    (make-instance 'log-entry ...)
    (error 'malformed-log-entry-error :text text)))

;; signal an error if malformed.
;; 
;; Default behavior--land in the debugger
;; 
;; * Providing multiple restarts

(defun parse-log-entry (text)
  (if (well-formed-log-entry-p text)
    (make-instance 'log-entry ...)
    (restart-case (error 'malformed-log-entry-error :text text)
      (use-value (value) value)
      (reparse-entry (fixed-text) (parse-log-entry fixed-text)))))

;; Here, the error can be handled in two ways--
;; 
;; use-value: have a handler provide a value
;; 
;; reparse-entry: have a handler provide a different text to
;; parse

(defun log-analyzer ()
  (handler-bind 
   ((malformed-log-entry-error
     #'(lambda (c)
         (use-value
          (make-instance 'malformed-log-entry 
                         :text (text c))))))
   (dolist (log (find-all-logs))
     (analyze-log log))))