22.4.12 Printing in Edebug

If the result of an expression in your program contains a circular reference, you may get an error when Edebug attempts to print it. You can set print-length to a non-zero value to limit the print length of lists (the number of cdrs), and in Emacs 19, set print-level to a non-zero value to limit the print depth of lists. But you can print such circular structures and structures that share elements more informatively by using the cust-print package.

To load cust-print and activate custom printing only for Edebug, simply use the command M-x edebug-install-custom-print. To restore the standard print functions, use M-x edebug-uninstall-custom-print. You can also activate custom printing for printing in any Lisp code; see the package for details.

Here is an example of code that creates a circular structure:

(progn
  (edebug-install-custom-print)
  (setq a '(x y))
  (setcar a a))

Edebug will print the result of the setcar as ‘Result: #1=(#1# y)’. The ‘#1=’ notation names the structure that follows it, and the ‘#1#’ notation references the previously named structure. This notation is used for any shared elements of lists or vectors.

Independent of whether cust-print is active, while printing results Edebug binds print-length, print-level, and print-circle to edebug-print-length (50), edebug-print-level (50), and edebug-print-circle (t) respectively, if these values are non-nil. Also, print-readably is bound to nil since some objects simply cannot be printed readably.