Use short symbols for query-replace-show-preview (bug#81583)

* lisp/replace.el (query-replace-show-preview): Accept the
symbols `replacement-only' and `both'.  Update the :type
accordingly, which also fixes the name of a function that does
not exist.
(query-replace-preview-match): Update the doc string.
(replace-preview-both, replace-preview-replacement-only): Say
which value of the option calls them.
(replace-preview--format): Dispatch on the new values, and still
call a function value.
(replace-preview-setup): Test the option for non-nil instead of
testing it with `functionp'.
* test/lisp/replace-tests.el (replace-tests--preview)
(replace-tests-preview-both, replace-tests-preview-cleanup)
(replace-tests-preview-disabled): Use the new values.
* doc/emacs/search.texi (Replace): Document the new values.

doc/emacs/search.texi

@@ -1576,14 +1576,15 @@ look after the replacement.  This tells you what back-references like
 replacement.  However, replacements that use @samp{\,} or @samp{\#} are
 not previewed.
 
-  The value can be @code{replace-preview-replacement-only}, to display
-the replacement alone, @code{replace-preview-both}, to display the match
-next to its replacement, separated by an arrow, or a function of your
-own, written by copying either of those two.  Such a function takes the
-match and the replacement and returns the string to display in place of
-the match.  If the replacement is empty,
-@code{replace-preview-replacement-only} would display nothing at all, so
-it displays a thin bar to mark the place of the match.
+  The value can be @code{replacement-only}, to display the replacement
+alone, @code{both}, to display the match next to its replacement,
+separated by an arrow, or a function of your own, written by copying
+@code{replace-preview-replacement-only} or @code{replace-preview-both},
+which are the functions those two symbols stand for.  Such a function
+takes the match and the replacement and returns the string to display in
+place of the match, or @code{nil} to leave that match alone.  If the
+replacement is empty, @code{replacement-only} would display nothing at
+all, so it displays a thin bar to mark the place of the match.
 
 @vindex query-replace-preview@r{ face}
 @vindex query-replace-preview-match@r{ face}

etc/NEWS

@@ -139,9 +139,10 @@ type it: the matches visible in the window are shown as they would look
 after the replacement.  Replacements that use '\,' or '\#' are not
 previewed.  The preview is off by default.
 
-The value is a function.  'replace-preview-replacement-only' shows the
-replacement alone, and 'replace-preview-both' shows the match and
-replacement side-by-side.  Or you can use a custom function.
+The value can be the symbol 'replacement-only', which shows the
+replacement alone, the symbol 'both', which shows the match and the
+replacement side-by-side, or a custom function of the match and the
+replacement, returning the string to show in place of the match.
 
 ** Electric Pair mode
 

lisp/replace.el

@@ -119,17 +119,17 @@ This variable affects only `query-replace-regexp'."
 The matches visible in the window are shown as they would look after the
 replacement.  Replacements using \\, or \\# are not previewed.
 
-The value can be nil, for no preview;
-`replace-preview-replacement-only', to show the replacement alone;
-`replace-preview-both', to show the match and the replacement side by
-side; or a function of two string arguments, the match and the
-replacement, returning the string to show in place of the match."
-  :type '(choice (const         :tag "No preview" nil)
-                 (function-item :tag "Show the replacement"
-                                replace-preview-only-replacement)
-                 (function-item :tag "Show both match and replacement"
-                                replace-preview-both)
-                 (function      :tag "Other function"))
+The value can be nil, for no preview; the symbol `replacement-only', to
+show the replacement alone; the symbol `both', to show the match and the
+replacement side by side; or a function of two string arguments, the
+match and the replacement, returning the string to show in place of the
+match, or nil to leave that match alone.  See the functions
+`replace-preview-replacement-only' and `replace-preview-both' for two
+examples of such a function."
+  :type '(choice (const    :tag "No preview" nil)
+                 (const    :tag "Show the replacement" replacement-only)
+                 (const    :tag "Show both match and replacement" both)
+                 (function :tag "Other function"))
   :group 'matching
   :version "32.1")
 
@@ -182,7 +182,7 @@ friends when `query-replace-show-preview' is non-nil."
   '((t (:inherit lazy-highlight)))
   "Face for the matched text in the preview.
 Used for the left half of the preview when `query-replace-show-preview'
-is `replace-preview-both'."
+is `both'."
   :group 'matching
   :version "32.1")
 
@@ -401,7 +401,8 @@ overlay string they would show something other than the preview."
 (defun replace-preview-both (match replacement)
   "Return MATCH and REPLACEMENT side by side, separated by an arrow.
 MATCH and REPLACEMENT are strings, shown in the faces
-`query-replace-preview-match' and `query-replace-preview'."
+`query-replace-preview-match' and `query-replace-preview'.  This is what
+`query-replace-show-preview' does when it is `both'."
   (let ((sep (if (char-displayable-p ?→) "→" "->")))
     (concat (replace-preview-propertize (concat match sep)
                                         'query-replace-preview-match)
@@ -411,7 +412,9 @@ MATCH and REPLACEMENT are strings, shown in the faces
 (defun replace-preview-replacement-only (_match replacement)
   "Return REPLACEMENT in the face `query-replace-preview'.
 REPLACEMENT is a string.  If it is empty, previewing it would show
-nothing at all, so return a thin bar to mark the place of the match."
+nothing at all, so return a thin bar to mark the place of the match.
+This is what `query-replace-show-preview' does when it is
+`replacement-only'."
   (let ((text (replace-preview-propertize replacement
                                           'query-replace-preview)))
     (if (equal text "")
@@ -421,12 +424,17 @@ nothing at all, so return a thin bar to mark the place of the match."
 (defun replace-preview--format (match replacement)
   "Return the string to show in place of MATCH, or nil for no preview.
 MATCH is the matched string and REPLACEMENT is the string that would
-replace it.  They are combined by the function
-`query-replace-show-preview'."
-  (when (functionp query-replace-show-preview)
-    (let ((s (save-match-data
-               (funcall query-replace-show-preview match replacement))))
-      (and (stringp s) s))))
+replace it.  They are combined as `query-replace-show-preview' says."
+  (let ((s (save-match-data
+             (pcase query-replace-show-preview
+               ('nil nil)
+               ('replacement-only
+                (replace-preview-replacement-only match replacement))
+               ('both
+                (replace-preview-both match replacement))
+               ((pred functionp)
+                (funcall query-replace-show-preview match replacement))))))
+    (and (stringp s) s)))
 
 (defun replace-preview-update (from to regexp-flag delimited-flag case-fold)
   "Preview the result of replacing FROM with TO in the current buffer.
@@ -472,7 +480,7 @@ on every change it shows, in the original window, how the visible
 matches of FROM would look after the replacement.
 REGEXP-FLAG and DELIMITED-FLAG say how to search for FROM, as in
 `replace-search'."
-  (if (or (not (functionp query-replace-show-preview)) (minibufferp))
+  (if (or (not query-replace-show-preview) (minibufferp))
       #'ignore
     (let ((unwind (make-symbol "replace-preview--unwind"))
 	  (after-change (make-symbol "replace-preview--after-change"))

test/lisp/replace-tests.el

@@ -709,9 +709,9 @@ bound to HIGHLIGHT-LOCUS."
   "Return the previews of replacing FROM with TO in a buffer holding TEXT.
 Each preview is a list (BEG END STRING).
 Unless the caller binds `query-replace-show-preview' to something else,
-the previews are those of `replace-preview-replacement-only'."
+the previews are those of `replacement-only'."
   (let ((query-replace-show-preview
-         (or query-replace-show-preview #'replace-preview-replacement-only)))
+         (or query-replace-show-preview 'replacement-only)))
     (with-temp-buffer
       (insert text)
       (set-window-buffer (selected-window) (current-buffer))
@@ -751,7 +751,7 @@ the previews are those of `replace-preview-replacement-only'."
                  '((1 4 " ")))))
 
 (ert-deftest replace-tests-preview-both ()
-  (let ((query-replace-show-preview #'replace-preview-both)
+  (let ((query-replace-show-preview 'both)
         (arrow (if (char-displayable-p ?→) "→" "->")))
     (should (equal (replace-tests--preview "foo\n" "foo" "bar" nil)
                    `((1 4 ,(concat "foo" arrow "bar")))))))
@@ -767,7 +767,7 @@ the previews are those of `replace-preview-replacement-only'."
   (with-temp-buffer
     (insert "foo foo\n")
     (set-window-buffer (selected-window) (current-buffer))
-    (let ((query-replace-show-preview #'replace-preview-replacement-only))
+    (let ((query-replace-show-preview 'replacement-only))
       (replace-preview-update "foo" "bar" nil nil nil))
     (should replace-preview-overlays)
     (replace-preview-cleanup)
@@ -777,7 +777,7 @@ the previews are those of `replace-preview-replacement-only'."
 (ert-deftest replace-tests-preview-disabled ()
   (let ((query-replace-show-preview nil))
     (should (eq (replace-preview-setup "foo" nil nil) #'ignore)))
-  (let ((query-replace-show-preview #'replace-preview-replacement-only))
+  (let ((query-replace-show-preview 'replacement-only))
     (should-not (eq (replace-preview-setup "foo" nil nil) #'ignore))))
 
 (ert-deftest test-count-matches ()