Update CI to latest GHC and action versions, modernize caching

- separate caching into restore and save step
- always save cache even if build failed (speeds up next attempt)
- use build plan rather than freeze file as cache key

Author: andreasabel

.github/workflows/ci.yml

@@ -22,11 +22,11 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]   # macOS-latest, windows-latest
-        cabal: [3.10.3]
-        deploy-ghc: [9.6.6]
-        ghc: [9.4.8, 9.6.6, 9.8.2, 9.10.1]
+        cabal: [3.14.2.0]
+        deploy-ghc: [9.6.7]
+        ghc: [9.6.7, 9.8.4, 9.10.3, 9.12.2]
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v6
       if: github.event.action == 'opened' || github.event.action == 'synchronize' || github.event.ref == 'refs/heads/master'
 
     # Takes care of ghc and cabal. See https://github.com/haskell/actions.
@@ -37,11 +37,20 @@ jobs:
         ghc-version: ${{ matrix.ghc }}
         cabal-version: ${{ matrix.cabal }}
 
-    # Generate a cabal.project.freeze file with all dependencies. We use the
-    # hash of this as the cache key, so when a dependency changes we upload a
-    # new cache.
-    - name: Freeze
-      run: cabal freeze
+    # Andreas, 2025-11-28: Since the freeze file contains a timestamp of the Hackage index
+    # and the index changes several times a day,
+    # the freeze file is not very useful as a cache key.
+    # Instead, use the plan.json file (see below).
+    # # Generate a cabal.project.freeze file with all dependencies. We use the
+    # # hash of this as the cache key, so when a dependency changes we upload a
+    # # new cache.
+    # - name: Freeze
+    #   run: cabal freeze
+
+    - name: Configure the build plan
+      run: |
+        cabal build --dry-run
+      # cabal build --dry-run creates dist-newstyle/cache/plan.json
 
     # Install fix-whitespace since we need it for tests.
     # This is done after the freeze to make sure we re-use the packages
@@ -54,11 +63,14 @@ jobs:
     # every build. `restore-keys` makes it use the latest cache even if the
     # fingerprint doesn't match, so we don't need to start from scratch every
     # time a dependency changes.
-    - uses: actions/cache@v3
-      name: Cache ~/.cabal/store
+    - uses: actions/cache/restore@v4
+      name: Restore cached ~/.cabal/store
+      id: cache
       with:
         path:         ${{ steps.setup-haskell-cabal.outputs.cabal-store }}
-        key:          ${{ runner.os }}-${{ matrix.ghc }}-${{ hashFiles('cabal.project.freeze') }}
+        # The freeze file is a bad key, use dist-newstyle/cache/plan.json instead.
+        # key:          ${{ runner.os }}-${{ matrix.ghc }}-${{ hashFiles('cabal.project.freeze') }}
+        key:          ${{ runner.os }}-${{ matrix.ghc }}-${{ hashFiles('**/plan.json') }}
         restore-keys: ${{ runner.os }}-${{ matrix.ghc }}-
 
     - name: Run test suite
@@ -70,9 +82,15 @@ jobs:
 
     - name: Deploy Prelude HTML
       if: ${{ (matrix.ghc == matrix.deploy-ghc) && (github.ref == 'refs/heads/master') }}
-      uses: peaceiris/actions-gh-pages@v3
+      uses: peaceiris/actions-gh-pages@v4
       with:
         github_token: ${{ secrets.GITHUB_TOKEN }}
         publish_dir: html
         destination_dir: lib
 
+    - uses: actions/cache/save@v4
+      name: Cache ~/.cabal/store
+      if: always() && steps.cache.outputs.cache-hit != 'true'
+      with:
+        path: ${{ steps.setup-haskell-cabal.outputs.cabal-store }}
+        key:  ${{ steps.cache.outputs.cache-primary-key }}

.github/workflows/docs.yml

@@ -19,7 +19,7 @@ jobs:
   deploy:
     strategy:
       matrix:
-        python-version: ["3.10.8"]
+        python-version: ["3.14"]
 
     env:
       DOCS_DIR: docs
@@ -28,21 +28,26 @@ jobs:
 
     runs-on: ubuntu-latest
     steps:
+
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v6
+
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v4
+        uses: actions/setup-python@v6
         with:
           python-version: ${{ matrix.python-version }}
+
       - name: Install dependencies
         run: |
           pip install -r ${{ env.DOCS_DIR }}/requirements.txt
+
       - name: Build User Manual in HTML
         run: |
           cd ${{ env.DOCS_DIR }}
           make html
+
       - name: Deploy to GitHub Pages
-        uses: peaceiris/actions-gh-pages@v3
+        uses: peaceiris/actions-gh-pages@v4
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: ${{ env.DOCS_BUILD_HTML_DIR }}

.github/workflows/nix-ci.yml

@@ -33,8 +33,8 @@ jobs:
           - pretty: "Typecheck with Agda"
             derivation: containers-lib
     steps:
-    - uses: actions/checkout@v3
-    - uses: nixbuild/nix-quick-install-action@v30
+    - uses: actions/checkout@v6
+    - uses: nixbuild/nix-quick-install-action@v34
       with:
         nix_conf: |
           keep-env-derivations = true
@@ -50,8 +50,8 @@ jobs:
     name: "Test building inside a nix shell"
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
-    - uses: nixbuild/nix-quick-install-action@v30
+    - uses: actions/checkout@v6
+    - uses: nixbuild/nix-quick-install-action@v34
       with:
         nix_conf: |
           keep-env-derivations = true