Subject: Re: bug#20343: csv-mode fails if mode line is customized
From: Stefan Monnier <[email-masked]>
To: Peter Eisentraut <[email-masked]>
Cc: [email-masked]
Date: Wed, 15 Apr 2015 21:29:59 -0400
List-Id: <debbugs-submit.debbugs.gnu.org>

> The reason is that in the definition of csv-mode-line-format, the
> arguments of the `last' function are flipped:

> (defconst csv-mode-line-format
>   ;; See bindings.el for details of `mode-line-format' construction.
>   (let* ((ml (copy-sequence (default-value 'mode-line-format)))
>          (x (or (memq 'mode-line-position ml) (last 3 ml))))  ;; wrong

Oops, indeed.

> (I doubt that messing around with the mode line like that is current
> practice, but that might be a separate discussion.)

I've changed the code to make fewer assumptions.  See patch below,

        Stefan

diff --git a/packages/csv-mode/csv-mode.el b/packages/csv-mode/csv-mode.el
index a8ae4e4..e727027 100644
--- a/packages/csv-mode/csv-mode.el
+++ b/packages/csv-mode/csv-mode.el
@@ -249,16 +249,7 @@ Number of spaces used by `csv-align-fields' after separators."

 (defconst csv-mode-line-format
-  ;; See bindings.el for details of `mode-line-format' construction.
-  (let* ((ml (copy-sequence (default-value 'mode-line-format)))
-         (x (or (memq 'mode-line-position ml) (last 3 ml))))
-    (when x
-      (setcdr x (cons
-                 `(csv-field-index-string
-                   ("" csv-field-index-string
-                    ))
-                 (cdr x))))
-    ml)
+  '(csv-field-index-string ("" csv-field-index-string))
   "Mode line format string for CSV mode.")
 
 (defvar csv-mode-map
@@ -322,9 +313,13 @@ CSV mode provides the following specific keyboard key bindings:
   (setq
    ;; Font locking -- separator plus syntactic:
    font-lock-defaults '(csv-font-lock-keywords)
-   buffer-invisibility-spec csv-invisibility-default
-   ;; Mode line to support `csv-field-index-mode':
-   mode-line-format csv-mode-line-format)
+   buffer-invisibility-spec csv-invisibility-default)
+  ;; Mode line to support `csv-field-index-mode':
+  (set (make-local-variable 'mode-line-position)
+       (pcase mode-line-position
+         (`(,(or (pred consp) (pred stringp)) . ,_)
+          `(,@mode-line-position ,csv-mode-line-format))
+         (_ `("" ,mode-line-position ,csv-mode-line-format))))
   (set (make-local-variable 'truncate-lines) t)
   ;; Enable or disable `csv-field-index-mode' (could probably do this
   ;; a bit more efficiently):