ci: push haddocks to GH pages in ci.

Author: dmjio

.github/workflows/ci.yml

@@ -38,3 +38,30 @@ jobs:
       - name: Build and run tests (macOS, x86_64 via Rosetta)
         if: runner.os == 'macOS'
         run: nix build -L .#packages.x86_64-darwin.default
+
+  docs:
+    runs-on: ubuntu-latest
+    needs: build
+    if: github.event_name == 'push' && github.ref == 'refs/heads/master'
+    permissions:
+      contents: write
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: cachix/install-nix-action@v31
+        with:
+          nix_path: nixpkgs=channel:nixpkgs-unstable
+
+      - name: Build (produces Haddock docs via cabal)
+        run: nix build -L
+
+      - name: Collect Haddock output
+        run: |
+          html=$(find -L result/share/doc -type d -name html | head -1)
+          echo "HADDOCK_DIR=$html" >> "$GITHUB_ENV"
+
+      - name: Deploy to GitHub Pages
+        uses: peaceiris/actions-gh-pages@v4
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ${{ env.HADDOCK_DIR }}