Implement a way to customize "default" values

* doc/lispref/minibuf.texi (Text from Minibuffer): Document them.

* lisp/minibuffer.el (format-prompt): New function (bug#12443).
(minibuffer-default-prompt-format): New variable.

doc/lispref/minibuf.texi

@@ -411,6 +411,34 @@ following bindings, in addition to those of @code{minibuffer-local-map}:
 @end table
 @end defvar
 
+@vindex minibuffer-default-prompt-format
+@defun format-prompt prompt default &rest format-args
+Format @var{prompt} with default value @var{default} according to the
+@code{minibuffer-default-prompt-format} variable.
+
+@code{minibuffer-default-prompt-format} is a format string (defaulting
+to @samp{" (default %s)"} that says how the ``default'' bit in prompts
+like @samp{"Local filename (default somefile): "} are to be formatted.
+
+To allow the users to customize how this is displayed, code that
+prompts the user for a value (and has a default) should look something
+along the lines of this code snippet:
+
+@lisp
+(read-file-name
+ (format-prompt "Local filename" file)
+ nil file)
+@end lisp
+
+If @var{format-args} is @code{nil}, @var{prompt} is used as a literal
+string.  If @var{format-args} is non-@code{nil}, @var{prompt} is used
+as a format control string, and @var{prompt} and @var{format-args} are
+passed to @code{format} (@pxref{Formatting Strings}).
+
+@code{minibuffer-default-prompt-format} can be @samp{""}, in which
+case no default values are displayed.
+@end defun
+
 @node Object from Minibuffer
 @section Reading Lisp Objects with the Minibuffer
 @cindex minibuffer input, reading lisp objects

etc/NEWS

@@ -886,6 +886,16 @@ window after starting).  This variable defaults to nil.
 
 ** Miscellaneous
 
++++
+*** The user can now customize how \"default\" values are prompted for.
+The new utility function 'format-prompt' has been added which uses the
+new 'minibuffer-default-prompt-format' variable to format \"default\"
+prompts.  This means that prompts that look like "Enter a number
+(default 10)" can be customized to look like, for instance, "Enter a
+number [10]", or not have the default displayed at all, like "Enter a
+number".  (This requires that all callers are altered to user
+'format-prompt', though.)
+
 ---
 *** New 'diff-mode' font locking face 'diff-error'.
 This face is used for error messages from diff.

lisp/minibuffer.el

@@ -3028,6 +3028,19 @@ the commands start with a \"-\" or a SPC."
   :version "24.1"
   :type 'boolean)
 
+(defcustom minibuffer-default-prompt-format " (default %s)"
+  "Format string used to output \"default\" values.
+When prompting for input, there will often be a default value,
+leading to prompts like \"Number of articles (default 50): \".
+The \"default\" part of that prompt is controlled by this
+variable, and can be set to, for instance, \" [%s]\" if you want
+a shorter displayed prompt, or \"\", if you don't want to display
+the default at all.
+
+This variable is used by the `format-prompt' function."
+  :version "28.1"
+  :type 'string)
+
 (defun completion-pcm--pattern-trivial-p (pattern)
   (and (stringp (car pattern))
        ;; It can be followed by `point' and "" and still be trivial.
@@ -3845,6 +3858,19 @@ the minibuffer was activated, and execute the forms."
   (with-minibuffer-selected-window
     (scroll-other-window-down arg)))
 
+(defun format-prompt (prompt default &rest format-args)
+  "Format PROMPT with DEFAULT according to `minibuffer-default-prompt-format'.
+If FORMAT-ARGS is nil, PROMPT is used as a plain string.  If
+FORMAT-ARGS is non-nil, PROMPT is used as a format control
+string, and FORMAT-ARGS are the arguments to be substituted into
+it.  See `format' for details."
+  (concat
+   (if (null format-args)
+       prompt
+     (apply #'format prompt format-args))
+   (format minibuffer-default-prompt-format default)
+   ": "))
+
 (provide 'minibuffer)
 
 ;;; minibuffer.el ends here