Add Nix

I am not a Nix expert, but the change should be simple enough for me to
be able to maintain it.

.github/workflows/mdbook.yml

@@ -35,24 +35,21 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: classabbyamp/treeless-checkout-action@v1
-      - name: Download latest mdbook, mdbook-toc and mdbook-admonish
-        run: |
-          MDBOOK_VERSION="$(basename $(curl -Ls -o /dev/null -w %{url_effective} https://github.com/rust-lang/mdBook/releases/latest))"
-
-          wget -q -O- "https://github.com/rust-lang/mdBook/releases/download/${MDBOOK_VERSION}/mdbook-${MDBOOK_VERSION}-x86_64-unknown-linux-gnu.tar.gz" | tar -xzf - -C .
-
-          MDBOOK_TOC_VERSION="$(basename $(curl -Ls -o /dev/null -w %{url_effective} https://github.com/badboy/mdbook-toc/releases/latest))"
-
-          wget -q -O- "https://github.com/badboy/mdbook-toc/releases/download/${MDBOOK_TOC_VERSION}/mdbook-toc-${MDBOOK_TOC_VERSION}-x86_64-unknown-linux-gnu.tar.gz" | tar -xzf - -C .
-
-          MDBOOK_ADMONISH_VERSION="$(basename $(curl -Ls -o /dev/null -w %{url_effective} https://github.com/tommilligan/mdbook-admonish/releases/latest))"
-
-          wget -q -O- "https://github.com/tommilligan/mdbook-admonish/releases/download/${MDBOOK_ADMONISH_VERSION}/mdbook-admonish-${MDBOOK_ADMONISH_VERSION}-x86_64-unknown-linux-gnu.tar.gz" | tar -xzf - -C .
+      - name: Setup Nix
+        uses: nixbuild/nix-quick-install-action@v28
+      - name: Restore and cache Nix store
+        uses: nix-community/cache-nix-action@v5
+        # See https://github.com/nix-community/cache-nix-action/blob/main/README.md#example-steps
+        # Cache purging is delegated to GitHub (this means a maximum 10G of data accessed
+        # less than a week ago).
+        with:
+          primary-key: nix-${{ hashFiles('**/*.nix') }}
+          restore-prefixes-first-match: nix-
       - name: Setup Pages
         id: pages
         uses: actions/configure-pages@v4
       - name: Build with mdBook
-        run: PATH="$GITHUB_WORKSPACE:$PATH" ./book.sh build
+        run: ./nix-book.sh build
       - name: Upload artifact
         uses: actions/upload-pages-artifact@v3
         with:

README.md

@@ -20,6 +20,14 @@ currently packaged on Void Linux. You can download its executable in
 extract the appropriate archive and put the executable into a directory in
 `$PATH`.
 
+#### Building with Nix
+Alternatively, [nix](https://github.com/NixOS/nix) can be used to automatically
+(and reproducibly) download dependencies. This can be done with `nix-book.sh`,
+which is a wrapper around `book.sh` (described below); it behaves the same,
+and it accepts the same arguments.
+
+`nix-book.sh` is used in the GitHub workflow which builds the website.
+
 ### Build process
 You mustn't build this book with raw `mdbook`. This repo provides a `book.sh`
 script which handles book building. See `book.sh -h` for more info about the

nix-book.sh

@@ -0,0 +1,4 @@
+#!/usr/bin/env nix-shell
+#! nix-shell -i bash --pure
+
+exec ./book.sh "$@"

shell.nix

@@ -0,0 +1,13 @@
+let
+  nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/nixos-24.05";
+  pkgs = import nixpkgs {};
+in
+
+pkgs.mkShellNoCC {
+  packages = with pkgs; [
+    bash
+    mdbook
+    mdbook-toc
+    mdbook-admonish
+  ];
+}