Re: `unreadablep'

[Steingold]

> * 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.

> I wonder whether this could be efficiently implemented by, say, having
> some kind of special value for PRINTCHARFUN for prin1, but I haven't
> looked at the code yet.

Yeah, we could create a "/dev/null"-type object and pass it to prin1.
However, this would still make the above approach better in most cases.

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