This is going to be a short one. I recently added a development container definition to the repository that hosts this very website. The definition I’m using is taken from Microsoft’s devcontainer repository via the option integrated in Visual Studio Code¹.

For the most part, this development container works flawelessly. Except for this: the user’s cabal folder (located at ~/.cabal by default) is not persisted across container runs. This means I need to rebuild from scratch most of the dependencies that cabal saves in its user-wide package store (~/.cabal/store) every time I reopen the project (notably annoying with Hakyll’s dependency of big-chungus pandoc).

There are a couple of ways to solve this:

• By configuring cabal to use a package store located inside the repository’s folder (which is bind-mounted to the host’s file system).

  Problem: this would duplicate the work for any dependency shared across Haskell projects.

• Creating a ~/.cabal folder in my host computer and share it among devcontainers.

  Problem: this folder would be continuously modified by root, a permission hell waiting to happen.

• Mounting the package store to a Docker named volume.

  This is what I ended up doing, as it takes the least work and doesn’t clutter my host’s $HOME.

  The only changes needed are adding the following properties to .devcontainer/devcontainer.json:

  "mounts": [
      "source=cabal_store,target=/home/vscode/.cabal/store,type=volume"
    ],
  "postCreateCommand": "sudo chown vscode /home/vscode/.cabal/store"

  Where cabal_store is the name of the Docker volume to be created and vscode is the Linux user of this devcontainer. The postCreateCommand is there to make sure our container can access and modify the package store (only needed in case your devcontainer logs in to a user other than root). Repeat this for every Haskell devcontainer you want to share the cabal store with.

That’s it! Signing off.