Re: `unreadablep'

[Qiantan Hong]

> On Dec 15, 2021, at 7:32 AM, Steingold <sds@gnu.org> wrote:
> 
>> * Lars Ingebrigtsen <ynefv@tahf.bet> [2021-12-15 08:49:58 +0100]:
>> 
>> Do we have any primitive that can be used to check whether an object is
>> printable or not?  Code like this (from savehist.el) makes me believe
>> "not":
>> 
>>              ;; Print elements of VALUE one by one, carefully.
>>              (dolist (elt value)
>>                (let ((start (point)))
>>                  (insert " ")
>>                  ;; Try to print and then to read an element.
>>                  (condition-case nil
>>                      (progn
>>                        (prin1 elt (current-buffer))
>>                        (save-excursion
>>                          (goto-char start)
>>                          (read (current-buffer))))
>>                    (error
>>                     ;; If writing or reading gave an error, comment it out.
>>                     (goto-char start)
>>                     (insert "\n")
>>                     (while (not (eobp))
>>                       (insert ";;; ")
>>                       (forward-line 1))
>>                     (insert "\n")))
>>                  (goto-char (point-max))))
>> 
>> It would be nice to have such a function (i.e., that says whether it can
>> be read back after printing it).  The problem is, of course, complex
>> structures that require recursing (and then checking for loops), etc, so
>> you basically have to implement it the way printing is done if you want
>> it to be fast, I think?
>> 
>> Does anybody have any thoughts on this issue?
> 
> I think the above code is _perfect_.
> Why would one want to know in advance if the object is printable readably?
> Only to decide how to print it, right?
> Then the optimal approach is to _try_ to print readably and then take an
> alternative/remedial action on failure.
> This will usually save a structure traversal.
print already traverse it once, read traverse it twice.
If print support it natively we save in the best case one
half of the work.