Defining a formatter for Cabal files

For Haskell code I can use lsp-format-buffer and lsp-format-region to keep my file looking nice, but I've never found a function for doing the same for Cabal files. There's a nice command line tool, cabal-fmt, for doing it, but it means having to jump to a terminal. It would of course be nicer to satisfy my needs for aesthetics directly from Emacs. A few times I've thought of writing the function myself, I mean how hard can it be? But then I've forgotten about it until then next time I'm editing a Cabal file.

A few days ago I noticed emacs-reformatter popping up in my feeds. That removed all reasons to procrastinate. It turned out to be very easy to set up.

The package doesn't have a recipe for straight.el so it needs a :straight section. Also, the naming of the file in the package doesn't fit the package name, hence the slightly different name in the use-package declaration:¹

(use-package reformatter
  :straight (:host github
             :repo "purcell/emacs-reformatter"))

Now the formatter can be defined

(reformatter-define cabal-format
  :program "cabal-fmt"
  :args '("/dev/stdin"))

in order to create functions for formatting, cabal-format-buffer and cabal-format-region, as well as a minor mode for formatting on saving a Cabal file.