What is a Nix Flake?

A Nix Flake is a new feature in Nix that aims to improve the reproducibility, composability, and discoverability of Nix projects. It’s a structured, purely functional build system that allows for easy dependency management and composition of packages and configurations.

The Batteries Included main repository is a Nix Flake, which contains several packages specified from docker images to static rust binaries. Each package in the repo has its dependencies and build instructions, all managed by Nix.

Manual

Nix takes the requested devshell specified in nix/shell.nix and the package version in flake.lock and builds every library and binary needed. To get all of those onto the PATH and usable, enter into the root of the main repository. Then run nix develop to check that all versions are built, build any missing ones, and drop you in a bash shell with all tools ready and installed.

The manual method is easy to use as there are no dependencies. However, it’s a pain to remember and only sometimes plays nicely with vscode that will spawn other processes that don’t always inherit all the paths, or update when doing a git pull.

nix develop

Direnv

Direnv is the alternative. It’s another tool that will watch a directory’s contents and then run a script, updating the shell environment for all other shells. The nice part of this is you install direnv once, install the vscode addon and you are done.

curl -sfL https://direnv.net/install.sh | bash

# Add the direnv binary to PATH
echo 'export PATH=$PATH:${HOME}/.bun/bin/' >> ~/.bashrc
echo 'export PATH=$PATH:${HOME}/.bun/bin/' >> ~/.zshrc

sudo apt install direnv

echo 'eval "$(direnv hook bash)"' >> ~/.bashrc
echo 'eval "$(direnv hook zsh)"' >> ~/.zshrc

Update Nix Dependencies

If you want to upgrade the tools that nix provides, there’s a simple nix command to run. Be sure to run tests and formatting as new versions often cause changes.

nix flake update
nix fmt
nix flake check

Update Mix Dependencies

Elixir is the only language that we can’t rely 100% on the language dependency resolution. Elixir’s mix deps.get reaches out to the network and isn’t determinitic; so we have to specify the expected hash value of all dependencies to nix.

That means when updating mix dependencies we need to change nix code as well.

mix hex.outdated
# Change any specified versions needed
# in mix.exs in the apps of umbrella

# Then change the .lock file
mix deps.unlock --all
mix deps.get

Next, in nix/platform.nix, change the sha256 for mixFodDeps and mixTestFodDeps to the fake value provided by lib like this:

      mixTestFodDeps = beamPackages.fetchMixDeps {
        pname = "mix-deps-platform-test";
        inherit src version LANG;
        mixEnv = "test";
        #sha256 = "TO BE RECOMPUTED";
        sha256 = lib.fakeSha256;
      };

      mixFodDeps = beamPackages.fetchMixDeps {
        pname = "mix-deps-platform";
        inherit src version LANG;
        #sha256 = "TO BE RECOMPUTED";
        sha256 = lib.fakeSha256;
      };

Then get nix to recompute the expected hash value, removing the fake value and replacing it with the expected value

First find the non-test hash

nix build -L ".#control-server"

Then the test hash

nix flake check -L

Finally ending up with something like this:

      mixTestFodDeps = beamPackages.fetchMixDeps {
        pname = "mix-deps-platform-test";
        inherit src version LANG;
        mixEnv = "test";
        sha256 = "sha256-a290WCJZetXA6NIoZNQj//dDO7Pj02PTSV0IOZzOEd8=";
        #sha256 = lib.fakeSha256;
      };

      mixFodDeps = beamPackages.fetchMixDeps {
        pname = "mix-deps-platform";
        inherit src version LANG;
        sha256 = "sha256-ddfioRONtL3nfO5wp6ocX3RzRWTLElZMH3GNnZS6PoI=";
        #sha256 = lib.fakeSha256;
      };