2022-12-26

This is the third and final post in a series about stacklock2nix. The previous post is about building Dhall with stacklock2nix.

This post uses stacklock2nix to build Pandoc with Nix. Pandoc is a tool for converting from one markup format to another. It is one of the most widely-used tools written in Haskell. This post explains how to use stacklock2nix to build Pandoc, but with a twist that Pandoc will be fully statically-linked. This means that you can take the Pandoc binary built with Nix and use it on any Linux distribution.

The following commit adds a flake.nix to the Pandoc repo that can be used to build with Nix and statically-link:

https://github.com/cdepillabout/pandoc/commit/ae4d9ffd7f8fd593fe6caa980c33fccecd187539

This flake.nix is based on the easy example of stacklock2nix. This post explains how to use this flake.nix.

What can you do with this flake.nix?

First, clone the above repo locally with the following commands:

$ git clone git@github.com:cdepillabout/pandoc.git
$ cd pandoc/
$ git checkout build-with-stacklock2nix

This flake.nix is very similar to the one added for building PureScript. You can run all the same commands, like nix build, or nix develop and then cabal build all.

The big difference is that this flake.nix for Pandoc is setup to statically-link the binaries it produces. So if you build the binary, you can see that it has been statically-linked:

$ nix build
...
$ file result/bin/pandoc
result/bin/pandoc: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, stripped

You can see that it says "statically linked".

We can try running the pandoc binary on our own system just to confirm it works:

$ ./result/bin/pandoc --to=plain ./README.md
...

Here we're using Pandoc to convert the README.md file to a plain-text format.

Now, we can run the same binary in an Ubuntu Docker container and prove that it does actually work on another Linux distribution:

$ docker run -it \
    -v `pwd`/result/bin/pandoc:/pandoc \
    -v `pwd`/README.md:/README.md \
    ubuntu \
    /pandoc --to=plain /README.md
...

Great! You can see that it outputs the same thing as when we ran it locally. This is proof that our statically-linked pandoc binary is able to run both on the system it is built on, and on any other arbitrary Linux system. This is exactly what we were going for.

Conclusion

stacklock2nix can be an easy way to get your Stack-based project to compile with Nix. It reuses the Nixpkgs infrastructure, so even things like static-linking are possible.

If you've read through this series and have any problems with stacklock2nix, feel free to create an issue or send a PR.

tags: haskell, nixos