bug#6017: 23.1.96; savehist-printable for hash tables

bug#6017: 23.1.96; savehist-printable for hash tables

[Drew Adams]

IIUC, starting with Emacs 23.2, we can prin1 and read hash tables. If
that's correct, then `savehist-printable' could presumably be updated to
add this sexp to the cond: (hash-table-p value). IOW:
 
(defun savehist-printable (value)
  "Return non-nil if VALUE is printable."
  (cond
   ((stringp value))
   ((numberp value))
   ((symbolp value))
   ((hash-table-p value))
   (t
    (with-temp-buffer
      (condition-case nil
   (let ((print-readably t) (print-level nil))
   (prin1 value (current-buffer))
   (read (point-min-marker))
   t)
 (error nil))))))

In GNU Emacs 23.1.96.1 (i386-mingw-nt5.1.2600)
 of 2010-04-20 on G41R2F1
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (3.4) --no-opt --cflags -Ic:/xpm/include'
 

bug#6017: 23.1.96; savehist-printable for hash tables

[Stefan Monnier]

> IIUC, starting with Emacs 23.2, we can prin1 and read hash tables. If
> that's correct, then `savehist-printable' could presumably be updated to
> add this sexp to the cond: (hash-table-p value). IOW:
 
> (defun savehist-printable (value)
>   "Return non-nil if VALUE is printable."
>   (cond
>    ((stringp value))
>    ((numberp value))
>    ((symbolp value))
>    ((hash-table-p value))
>    (t
>     (with-temp-buffer
>       (condition-case nil
>    (let ((print-readably t) (print-level nil))
>    (prin1 value (current-buffer))
>    (read (point-min-marker))
>    t)
>  (error nil))))))

That won't check whether the contents of the has-table are
themsevles printable.


        Stefan

bug#6017: 23.1.96; savehist-printable for hash tables

[Drew Adams]

> That won't check whether the contents of the has-table are
> themsevles printable.

Good point. I don't know how the support for printing & reading hash tables
works. If it just ensures that the top level is readable (the hash table
itself), then yes, there is no guarantee that its content is really readable.

Is this feature of hash-table print+read support specified anywhere, so we know
what the support really is? If not, could you please document it? 

It sounds like it is similar to printing a list - no guarantees about
readability of list members etc. If so, we should at least say that somewhere.

bug#6017: 23.1.96; savehist-printable for hash tables

[Chong Yidong]

"Drew Adams" <drew.adams@oracle.com> writes:

> Is this feature of hash-table print+read support specified anywhere,
> so we know what the support really is? If not, could you please
> document it?

See `Creating Hash Tables' in the Lisp manual.