# Where parallels cross

Interesting bits of life

# Moldable Emacs: extending the Playground powers via hooks (to include Dired)

The Playground is at its best when a self variable is defined. Molds always define such a variable, but you can define one even for normal buffer. Here I show how to do that for Dired buffers.

## The problem

It happened again! I was in a Dired buffer, I selected some files, and I thought: "how cool would it be to find out how complex these files are?". And once more I scratched my head and let my thoughts pass by: too difficult! Sometimes I need just a simple mechanism to run an Elisp snippet through files I selected in Dired. I know I could use eshell, or first collect the files in a list and then write an Elisp snippet. But now that I use moldable-emacs's Playground, I really would like to do that more easily!

## It is a problem indeed

Well, Dired fantastic capabilities are only a part of the story. Loop a snippet through files selected with Dired is cool, but I expect to have similar use cases for other existing Emacs modes. For example, I expect to want to script my way through a list of commits selected in Magit. Or a list of snippets selected in grep-mode!

The molds of moldable-emacs set a variable named self to contain the useful contents of a buffer. The idea is to make easy for other molds (Playground in primis) to transform those contents. The issue is that self is only set by molds! Dired does not set that. So if I open a Playground from a Dired buffer, I have no self.

How could we set self for buffers of other amazing modes?

## And there is a solution

This took much more thought than needed. The other evening I realized I had all the pieces I needed to make that happen.

Let me show the result first!

This is just magic! I mark some (Nyxt source) files in Dired, then I carry the files in a Playground. Next I use my fabulous code-compass complexity analysis on their contents. I can produce an alist or even better a plist. And as you know, I can make an Org table from that and plot the results!

The beauty of this system is that I can compose at will. And when I am not sure how to compose, I have just to invoke me-mold. That will suggest me working molds to use next! For example, I could have used also the line plot! (Vega-Lite is still in the works by the way!)

You can see how simple is the code in the Playground.

(--map
(cons (file . ,it)
(c/calculate-complexity-stats (with-file it (buffer-string))))
self)

(--> (--map
(cons (file . ,it)
(c/calculate-complexity-stats (with-file it (buffer-string))))
self)
(--map (list :file (alist-get 'file it)
:complexity (alist-get 'total it))
it))


Dired is doing the hard work of letting me select files! Indeed, my aim is to leverage amazing software and integrating it with our newer system. I guess this is going towards software environmentalism in the words of Tudor Girba: we reuse the brightness of our predecessors. (And what a view from these giants shoulders!)

Anyway, if you wish, you can extend the Playground too. The trick for Dired is in the following code.

(defun me/set-dired-self-for-playground ()
"Set Playground self' to dired list of files."
(when (and
(equal me/last-used-mold "Playground")
(ignore-errors mold-data)
(eq (plist-get mold-data :old-mode) 'dired-mode))
(setq-local self (with-current-buffer (plist-get mold-data :old-buffer)
(or (progn (goto-char (point-min)) (dired-get-marked-files))
(progn (mark-whole-buffer)
(call-interactively #'dired-mark)
(let ((files (dired-get-marked-files)))
(call-interactively #'dired-unmark-all-files)
files)))))))
(add-hook 'me/mold-after-hook #'me/set-dired-self-for-playground) ;; the order is important: keep before me/set-self-mold-data


In short this code runs after a mold has run (i.e., me/mold-after-hook). If the mold was a Playground and the starting buffer was a Dired one, we set self. In this case, I exploit dired-get-marked-files to get the list of file names. Note that mold-data is also set via a hook. That is why I got the comment at the end: this must run after that data is set otherwise it would not work.

That is all! If you need to extend moldable-emacs's interpreter you have me/mold-after-hook and me/mold-before-hook and even me/mold-before-mold-runs-hook. So you are free to make me/mold` smarter!

## Conclusion

Let's jump on the shoulders of Emacs giants! By extending the Playground we acquire incredible superpowers, because we can make existing modes behave like molds. So grab a copy of moldable-emacs and start scripting your marked files away!

Happy Dired-ing!