One Magic Line of Code that fixes Nix syntax highlighting in Vim

1299 words, around 4 minutes to read

Hello, friends!

Say you use Vim, and say you’ve decided to finally look into this Nix thing you keep hearing about. You open up some package definition files, and you immediately notice that there’s no syntax highlighting.

That makes sense: Nix is a relatively new language, so why would Vim ship with a plugin for it? You’re up for trying out bleeding edge stuff, so you know what to do: ask your preferred search engine what plugin the community uses.

If that’s what brought you here: the best Vim plugin for looking at Nix code is vim-nix.

So you install vim-nix through your favorite plugin manager (or at least your current favorite), and you start looking through nixpkgs, and writing your own packages, and it’s great. The isolation makes environment-specific build errors a thing of the past, the caching and binary substitution speeds up builds and deploys more than you could have hoped. You want to run your laptop on it, and your company. You want it everywhere, and to tell everyone about it. Your productivity and peace of mind skyrocket, you become a leader in your field, and all of your conference talk proposals are accepted everywhere, forever.

But at some point in this glorious trajectory, you started installing Vim via Nix, and soon afterwards, you begin to notice a few problems with the syntax highlighting. Small ones, yes, and superficial…but noticeable, and eventually even bothersome.

Identifiers with - characters in them are painted two wrong colors rather than the single correct one.
Indented strings are unpredictably painted in inverted colors, as if everything outside them was the string, and everything inside them was the Nix code.
Double-single-quotes inside of double-quoted strings are painted as if they were the beginning of an indented string, which either appears to run the full length of the file, or to stop at the beginning of an actual indented string, triggering the inverted indented string situation mentioned in the previous bullet point.
Escaped antiquotes look exactly like unescaped ones.

“Huh,” you mutter with strained charity, “I guess vim-nix still needs some work.”

NO. WRONG. FALSE. It does not need work. You need the Magic Line of Code. Without it, you aren’t even using vim-nix.

Here it is:

vim.ftNix = false;

That’s it. That’s all that you were missing. Add those 18 characters to your ~/.nixpkgs/config.nix and it will fix every single one of the above problems.

Here’s what happened: at some point you realized that you could manage your Vim plugins and configuration by writing them into a Nix package. Every source on the matter told you the same thing: the function vim_configurable.customize will return a package with your own unique, customized Vim setup.

Here’s the simplest example of what that looks like:

vim_configurable.customize {
  name = "myVim";
  vimrcConfig = {};
};

customize requires two arguments: the name of the executable for the new package, and a set describing the vimrcConfig that will make this Vim package different from the default vim_configurable one. The example here just uses an empty set ({}), but yours will be full of plugins and vimrc lines.

(The vimrcConfig argument should probably be optional, but no one’s gotten around to making that change yet. Maybe that could be your first contribution to the nixpkgs repository?)

Having learned how to create this package, you wanted to install it with nix-env --install. To make environment. To make new packages available to the nix-env command no matter where you invoke it, you need to have a file at ~/.nixpkgs/config.nix which evaluates to a set that has a packageOverrides attribute, the value of which is a function that takes your system’s nixpkgs as an argument and returns a set of packages to add or change.

Here’s what that looks like with our simplest-possible-custom-vim package.

# ~/.nixpkgs/config.nix
{
  packageOverrides = super: {
    myVim = super.vim_configurable.customize {
      name = "myVim";
      vimrcConfig = {};
    };
  };
}

So with that, nix-env --install myVim will add the mvVim executable to your environment, and you’re off to the races.

NO. STOP. WAIT.

Any package you create from the vim_configurable.customize function will by default have some basic Nix syntax highlighting built in. This is the imperfect highlighting behind all those annoying glitches I listed before. It’s convenient for occasional edits, but you deserve the best; you deserve vim-nix.

So you add it as one of your plugins. Here’s the absolute simplest way to do that, just changing a single line:

# ~/.nixpkgs/config.nix
{
  packageOverrides = super: {
    myVim = super.vim_configurable.customize {
      name = "myVim";
      vimrcConfig.packages.thisPackage.start = [ vimPlugins.vim-nix ];
    };
  };
}

That changed line means that myVim will load vim-nix as soon as it starts, which is what we want.

(More precisely, it means that myVim will have a line in its vimrc which sets its packpath to be a directory containing pack/thisPackage/start/vim-nix, but for those kinds of details you can read the source of the customize function, and/or call :help packages from inside Vim. Or just contact me, because I wrote this one small part of nixpkgs. )

But it’s not all of what we want, because that basic syntax higlighting? The one I mentioned that vim_configurable includes by default, and which has all those problems? Guess what, you’re still using it, not vim-nix. It takes precedence over any other plugin defining a Nix filetype.

So by now you get it: the Magic Line of Code disables the basic built-in syntax highlighting, allowing the deluxe and flawless syntax highlighting of vim-nix to take over.

Nix is a purely functional language, which means that only arguments to a function can effect its output. Every Nix package is a function which takes all of its dependencies as arguments. Some packages also take the contents of your config.nix as an argument, and vary their behavior accordingly.

vim_configurable is one of these packages, and vim.ftNix determines whether or not its derivations include the filetype for Nix.

So, here, at the end of all mysteries, is the absolute minimum config.nix that will let you install your own special Vim which has the Ultimate Syntax-Highlighting Experience when looking at Nix files:

# ~/.nixpkgs/config.nix
{
  vim.ftNix = false;
  packageOverrides = super: {
    myVim = super.vim_configurable.customize {
      name = "myVim";
      vimrcConfig.packages.thisPackage.start = [ vimPlugins.vim-nix ];
    };
  };
}

I’d like to thank Daiderd both for their contributions to vim-nix, and for pointing out the real problem when I came barging into his github repository with a bogus issue. Hopefully this can save you all from such embarrasment.

Good luck,
nick