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common-lispsbclslime

Limit length of strings in debug output

I use emacs, slime, and sbcl. When I'm thrown into debugger when condition occurs, how do I limit the size of the output? I have figured out *print-length* and *print-level*, but what to do about long strings or strings with many lines? Say,

(defun monkeys (length)
  "Generate a random string"
  (with-output-to-string (out)
    (dotimes (i length)
      (write-char
       (code-char
        (let ((c (random 27)))
          (if (zerop c)
              (if (zerop (random 5)) 10 32)
              (+ 95 c)))) out))))

(1+ (monkeys 10000)) ; drop into debugger
Solution

  • Long story short, on sbcl *print-vector-length* can be used. From the SBCL source code:

    (defparameter *print-vector-length* nil
  "Like *PRINT-LENGTH* but works on strings and bit-vectors.
Does not affect the cases that are already controlled by *PRINT-LENGTH*")

    Long story long, I somehow never thought to look at the source code. However, thanks to the answer from @tfb, I at least had a starting point. So I went on to read about the pretty printer's dispatch table, and, just to see how dispatch functions look, I checked what the default dispatch function for 'string is:

    (pprint-dispatch 'string)

    That gives #<FUNCTION SB-KERNEL:OUTPUT-UGLY-OBJECT>. I searched for it in the SBCL source code and found the necessary parameter along the way.