[O] PDF-tools... and LaTeX

[AW]

Dear all,

I dare to ask this slightly off-topic question, because of the discussion 
about pdf-tools a day ago. 

I installed pdf-tools (https://github.com/politza/pdf-tools). It is a _lot_ 
faster than doc-view. Many thanks to Matt Price 
(http://matt.hackinghistory.ca/2015/11/11/note-taking-with-pdf-tools/) I got 
it working with orgmode. 

But I'd like to use it to display my PDFs I produce with LaTeX (Emacs, AUCTeX) 
as well.

The trouble is the automagic reloading of the PDF every time it get's changed 
on disk. Okular does this, but pdf-tools need some kind of invitation?!

>From here https://github.com/politza/pdf-tools/issues/128  
is this setup for .emacs:

============= 8< ==============


(pdf-tools-install) 
(load "pdf-tools")
(setq TeX-view-program-selection '((output-dvi "Okular") 
(output-pdf "PDF Tools") ;; 
))

(require 'subr-x)
(defun th/pdf-view-revert-buffer-maybe (file)
(when-let ((buf (find-buffer-visiting file)))
(with-current-buffer buf
(when (derived-mode-p 'pdf-view-mode)
(pdf-view-revert-buffer nil t)))))

(add-hook 'TeX-after-TeX-LaTeX-command-finished-hook
 #'th/pdf-view-revert-buffer-maybe)


================ >8 ============

If I compile a *.tex file again, I get the error 

"error in process sentinel: Symbol's function definition is void: when-let"

...and the PDF won't get updated.

Can anybody help me to get a working setup? Really, this libpoppler behind 
pdf-tools is such a lot faster, it reminds me on xpdf.

I'm on Linux, Emacs 24.5.1 and AUCTeX 11.89.

Thanks!

Alexander