Add id to each item

* undone/main.scm (get-field-value): Allow specification of default
  return if no key is found.
  (next-id): New function.
  (add): Add `id' to the todo item.

undone/main.scm

@@ -12,8 +12,13 @@
 (define-syntax get-field-value
   (lambda (x)
     (syntax-case x ()
+      ((_ key alist default)
+       #'(let ((field (get-field key alist)))
+           (if field
+               (cdr field)
+               default)))
       ((_ key alist)
-       #'(cdr (get-field key alist))))))
+       #'(get-field-value key alist #f)))))
 
 (define-syntax define-view
   (lambda (x)
@@ -126,6 +131,7 @@
       (mkdir path)))
 
 (define (save)
+
   "Save the list."
   (mkdirs (dirname todo-list-file))
 
@@ -133,13 +139,20 @@
     (write todo-list port)
     (close-port port)))
 
+(define (next-id)
+  "Look through all known todo items and get the next id."
+  (1+ (apply max (map (lambda (elm)
+                        (get-field-value id elm 0))
+                      todo-list))))
+
 (define (add args)
   "Add item to the list."
   (set! todo-list
         (append todo-list
                 (list
                  (append
-                  `((content . ,(string-trim-right (read-delimited ""))))
+                  `((id . ,(next-id))
+                    (content . ,(string-trim-right (read-delimited ""))))
                   (map (lambda (arg)
                          (let* ((pair (string-split arg #\=))
                                 (var (string->symbol (car pair)))