bug#36324: 27.0.50; Error in savehist-save: (wrong-type-argument listp t)

[Lars Ingebrigtsen <larsi@gnus.org>]

Michael Heerdegen <michael_heerdegen@web.de> writes:

>> The doc string for `read-from-minibuffer' doesn't document the t value
>> for HISTORY at all -- perhaps that should also be fixed?
>
> It's also not hardcoded AFAIK.  t is non-nil so Qminibuffer_history
> isn't substituted in `read-from-minibuffer', and it's not listp so
> `add-to-history' ignores it silently.  AFAIU other values V with these
> properties would also work this way but still provoke the
> (wrong-type-argument listp V) error.
>
> Having some input not recorded in a history is a useful feature.  IMHO
> this should be doable in an official way, and be documented.

Yes, it's a useful feature, and it should be documented, but before
doing that, I wonder how this feature developed.

It was developed in 2012, apparently, when `read-passwd' changed from
this ad-hoc way to the `t':

-            (read-string prompt nil
-                         (let ((sym (make-symbol "forget-history")))
-                           (set sym nil)
-                           sym)
-                         default)
+            (read-string prompt nil t default) ; t = "no history"

This apparently came from an XEmacs convention, which explains this 2005
change to savehist.el:

-  (add-to-list 'savehist-minibuffer-history-variables
-	       minibuffer-history-variable))
+  ;; XEmacs sets minibuffer-history-variable to t to mean "no history
+  ;; is being recorded".
+  (unless (eq minibuffer-history-variable t)
+    (add-to-list 'savehist-minibuffer-history-variables
+		 minibuffer-history-variable)))

So the `t' being the "no history" signal isn't just some random
artefact, but is the design, so I'm just going to go ahead and document
that in `read-from-minibuffer', which explains all the details about
HISTORY.  I'll add it to `minibuffer-history-variable' for completeness'
sake, too.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no