yarn2nix

yarn2nix [path/to/yarn.lock]
yarn2nix --template [path/to/package.json]

Convert a `yarn.lock` into a synonymous nix expression.
If no path is given, search for `./yarn.lock`.
In the second invocation generate a template for your `package.json`.

Features

• Purely transform yarn.lock files into very minimal, line-diffable nix expressions.
• Nix is used to its fullest. Every package is a derivation, whole dependency subtrees are shared in an optimal way, even between projects.
• The ability to resolve git dependencies by prefetching their repos and including the hashes.
• Completely local transformation if there are no git dependencies (can be used inside nix-build, no large file check-in).
• Extremely fast.
• Nice code that can be easily extended, new repositories introduced, adapt to new versions of the yarn.lock format.
• Comes with a nix library that uses the power of overlays to make overriding dependencies possible.
• POWERED BY HNIX™ since before it was cool.

Probably a few more.

Example Output

The HackMD project is an elaborate npm package with hundreds of dependencies. yarn2nix flawlessly parses the current (2017-12) yarn.lock file distributed with the project, including resolving their manual git forks of multiple npm packages:

dist/build/yarn2nix/yarn2nix ~/tmp/hackmd/yarn.lock | wc
   5306   17068  280246
cat ~/tmp/hackmd/yarn.lock | wc
   7667   11307  266652

The output of this conversion can be seen here. Also note that git dependencies are resolved correctly.

Pushing it through the provided library of nix functions, we get a complete build of HackMD dependencies, using the project template (generated with --template), we also build HackMD. Included executables will be in node_modules/.bin as expected and correctly link to their respective library paths in the nix store, for example:

 ls /nix/store/2jc8b4q9i2cvx7pamv5r8md45prrvx4f-hackmd-0.5.1-0.5.1/node_modules/.bin/markdown-it --help
Usage: ls [OPTION]... [FILE]...
List information about the FILEs (the current directory by default).

Building yarn2nix

$ nix-build
$ result/bin/yarn2nix

Using the generated nix files to build a project

Note: This is a temporary interface. Ideally, the library will be in nixpkgs and yarn2nix will be callable from inside the build (so the resulting nix files don’t have to be checked in).

Once you have the yarn2nix binary, use it to generate nix files for the yarn.lock file and the package.json:

$ yarn2nix ./jsprotect/yarn.lock > npm-deps.nix
$ yarn2nix --template ./jsproject/package.json > npm-package.nix

Then use the library to assemble the generated files in a default.nix:

let
  nixpkgsPath = <nixpkgs>;
  pkgs = import nixpkgsPath {};
  nixLib = pkgs.callPackage /path/to/yarn2nix/nix-lib {
    # WARNING (TODO): for now you need to use this checked out yarn2nix
    # because the upstream package (in haskellPackages) might have
    # broken dependencies (yarn-lock and yarn2nix are not in stackage)
    yarn2nix = import /path/to/yarn2nix { inherit nixpkgsPath; };
  };

in
  nixLib.buildNodePackage
    ( { src = nixLib.removePrefixes [ "node_modules" ] ./.; } //
      nixLib.callTemplate ./npm-package.nix
        (nixLib.buildNodeDeps (pkgs.callPackage ./npm-deps.nix {}))

Finally, run nix-build, and voilà, in ./result/ you find the project with all its dependencies correctly linked to their corresponding node_modules folder, recursively.

Development

$ nix-shell
nix-shell> hpack
nix-shell> cabal build