Using a predicate function in display-buffer-alist

Question

I want to add a rule to display-buffer-alist, that splits the frame to the right when opening a .pdf file if and only if there is a corresponding .tex file in the same folder.

According to the documentation of display-buffer-alist, I can specify a predicate function which takes a buffer name and an action. I can't get it to work however, and I don't understand what the action argument of the predicate function is supposed to do.

My code looks like this:

    (setq display-buffer-alist
          `((#'cst/latex-pdf-p . ((display-buffer-reuse-window display-buffer-in-side-window)
                                  (side . right)
                                  (slot . 1)
                                  (window-width . 0.33)
                                  (reusable-frames . nil)))))
    
    (defun cst/latex-pdf-p (buffer-name action)
      "Determine if there is a corresponding .tex file in the current folder."
      (when (string= "pdf" (file-name-extension buffer-name))
        (file-exists-p (concat (file-name-base buffer-name) ".tex"))))

The predicate function seems to work as expected when I evaluate it from a buffer split.pdf with split.tex in the same folder it returns t, on other pdfs it returns nil. When using the regexp \\.pdf$ the split works, but on all pdfs. Using the rule with the predicate function it does not work on any pdf.

I feel like I misunderstand how the predicate function should work.

Answer 1

try :

(setq display-buffer-alist
          '((cst/latex-pdf-p  
             (display-buffer-reuse-window display-buffer-in-side-window)
             (side . right)
             (slot . 1)
             (window-width . 0.33)
             (reusable-frames . nil))))

The predicate is evaluated by display-buffer-assq-regexp, which uses funcall on a variable with as value, your function. It won't work if you quote it. Also your alist was not well formed.