Creating an emacs formatter the easy way

Some weeks ago I discovered a really nice package for emacs called reformatter.el. This package allows to define reformat functions in a easy way.

Most languages have a reformat tool. Elixir has mix format, Elm has elm format, python has black and so on. These formatters are convenient because they give uniformity to the code, but this is a topic for another post.

Having these formatters integrated within our favorite editor is great. These are enough(at least for me) reasons to use reformatter.el so let's get into the code:

reformatter.el has a simple macro that allows us to define a formatter with just a few lines.

For this example we'll create a formatter for haskell using hindent ¹.

(reformatter-define haskell-format
  :program "hindent")

We just need to define the command that will be used to format the code. In this case hindent. This will create some useful functions:

• haskell-format
• haskell-format-buffer
• haskell-format-region
• haskell-format-on-save-mode

These functions can be used with a key-binding:

(define-key haskell-mode-map (kbd "C-c C-f") 'haskell-format-buffer)

Also we can setup emacs to run the formatter when the file is saved, for example put this code in your .dir-locals.el and it will do the work.

(haskell-mode (mode . haskell-format-on-save))

The program used to format our code needs to be able to read from stdin and return the formatted code to stdout. In this case hindent does it by default.

In some cases the formatter doesn't do this by default. For those cases we can pass extra arguments to the command using :args key in the formatter macro. For example elixir format receive a file or a pattern by default but we can change that using mix format -, now it will read from stdin, so we need to pass these parameters to our formatter. The code should be:

(reformatter-define elixir-format
  :program "mix"
  :args '("format" "-"))

Now it will work properly.

This package is very useful if you don't want to install a external package just for formatting. I replaced hindent-mode (haskell formatter) and a custom elixir formatter with this package. Also this package is used in elm-mode package.