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I want to set the variable truncate-lines to t in order to have truncated lines, but only in programming modes. So, following this thread, I put the following code in my configuration file:

(defun my-prog-mode-hook ()
  (setq truncate-lines t))
(add-hook 'prog-mode-hook #'my-prog-mode-hook)

My problem is this does not work, because truncate-lines is still nil when I start Emacs in fortran-mode (for example). The lambda version of the above code gives the same result.

What am I missing?

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    Inspect the value of prog-mode-hook with F1 v to make sure the function has been added to the hook. – wasamasa May 23 '17 at 17:20
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    And then make sure that some mode that derives from prog-mode does not, itself, change the value of truncate-lines to nil. You can also use M-x debug-on-entry my-prog-mode-hook to follow along and see what happens when it is invoked. – Drew May 23 '17 at 22:38
  • The variable prog-mode-hook contains my-prog-mode-hook, so I guess it is ok. Then when I M-x debug-on-entry my-prog-mode-hook, a list of functions that are run is displayed, but after a quick look, I haven't find any occurrence of truncate in their definition. But something came to my mind: I have (global-visual-line-mode 1) in my config file, could it be the culprit? – Giuseppe May 24 '17 at 9:30
  • (global-visual-line-mode 1) was most probably the problem. Thanks a lot to both of you anyway. – Giuseppe May 24 '17 at 9:57
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One possibility to achieve what I wanted is to have

    (add-hook 'text-mode-hook 'visual-line-mode)

in addition to

    (defun my-prog-mode-hook ()
      (setq truncate-lines t))
    (add-hook 'prog-mode-hook #'my-prog-mode-hook)

which wraps long lines in text-mode while truncating them in programming modes.

Beware not to have any (global-visual-line-mode 1) instruction elsewhere in the config file, otherwise the above hooks don't work (the lines are always wrapped).

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  • 3
    More precisely the hooks do work (my-prog-mode-hook successfully sets truncate-lines to t), but the globalized mode global-visual-line-mode subsequently triggers visual-line-mode (during after-change-major-mode-hook, after the mode hooks have run), which sets truncate-lines to nil, effectively undoing the earlier change. – phils May 24 '17 at 10:08
  • @phils Thank you for the precision. – Giuseppe May 24 '17 at 12:17

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