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Re: Difficulties with `purecopy' discarding text properties from strings

From: Alan Mackenzie
Subject: Re: Difficulties with `purecopy' discarding text properties from strings
Date: Fri, 19 May 2017 19:54:58 +0000
User-agent: Mutt/1.5.24 (2015-08-30)

Hello, Stefan.

On Fri, May 19, 2017 at 15:22:50 -0400, Stefan Monnier wrote:
> > Presumably there are good reasons for discarding text properties when
> > purecopying strings.

> I don't think so.

> > Or, maybe, there was just no particular reason to amend the workings
> > of `purecopy' when text properties came into existence.

> I think that's what happens.

Yes, that's probably what happened.

> But wait a minute:
> > I naively constructed such a defcustom, with text properties on the
> > value option strings.

> How exactly did you do that?  Using #("..." ...)
> or using (propertize "..." ...)?

By using propertize:

(defun mode-line-%-propertize (str)
  "Add standard mode-line properties to STR, a string.
These enable certain mouse operations when the mouse pointer is
hovering over the display of STR in the mode line."
   'local-map mode-line-column-line-number-mode-map
   'mouse-face 'mode-line-highlight
   ;; XXX needs better description
   'help-echo "Size indication mode\n\
mouse-1: Display Line and Column Mode Menu"))

(defcustom mode-line-percent-position `(,(mode-line-%-propertize "%p"))
  "Type of \"percentage offset\" of window through buffer to display
This option specifies the type of offset displayed in
a component of the default `mode-line-format'."
  :type `(radio
          (const :tag "nil:  No offset is displayed" nil)
          (const :tag "\"%o\": Proportion of \"travel\" of the window through 
the buffer"
                 (,(mode-line-%-propertize "%o")))
          (const :tag "\"%p\": Offset of top of window through buffer"
                 (,(mode-line-%-propertize "%p")))
          (const :tag "\"%P\": Offset of bottom of window through buffer"
                 (,(mode-line-%-propertize "%P")))
          (const :tag "\"%q\": Offsets of both top and bottom of window"
                 (,(mode-line-%-propertize "%q"))))
  :version "26.1"
  :group 'mode-line)
(put 'mode-line-percent-position 'risky-local-variable t)

> > After some debugging, I discover that defcustom strips those text
> > properties off the strings.  It does this by using `purecopy' on these
> > strings (in the function `custom-declare-variable').

> But AFAICT custom-declare-variable only applies purecopy to its
> `default` argument which is supposed to be an expression (which
> evaluates to the intended value) rather than a value.  So if you use the
> (propertize "..." ...)  form, the properties should not be stripped.

> Or am I missing something?

purecopy is applied to the entire type: argument.

>         Stefan

Alan Mackenzie (Nuremberg, Germany).

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