Using two programming languages with the same file extension

Question

I am just starting to set up a perl environment in emacs but the problem is that I already program with prolog and both languages use the .pl file extension. I have two directory ~/src/prolog and ~/src/perl where I store code for the respective languages. Currently I only have this line in my init:

(setq auto-mode-alist (append '(("\\.pl$" . prolog-mode))
                               auto-mode-alist))

How can I tell emacs that .pl files in the ~/src/prolog directory should use prolog-mode while the .pl files in ~/src/perl should use cperl-mode?

Answer 1

You're lucky that the files for the different languages are segregated by directory like that.

You can define directory-local values for variable auto-mode-alist.

See the Elisp manual, node Directory Local Variables.

As the intro text of that node says:

This is useful when the files in the directory belong to some “project” and therefore share the same local variables.