CI stack: save stack already after building dependencies

Author: andreasabel

.github/workflows/stack.yml

@@ -58,16 +58,31 @@ jobs:
 
     - name: Set environment variables based on Haskell setup
       run: |
-        export STACK_VER=$(stack --numeric-version)
-        echo "STACK_VER=${STACK_VER}" >> ${GITHUB_ENV}
+        STACK_VER=$(stack --numeric-version)
+        echo "STACK_VER=${STACK_VER}" >> "${GITHUB_ENV}"
 
-    - uses: actions/cache@v3
-      name: Cache dependencies
+    - name: Cache dependencies (restore)
+      uses: actions/cache/restore@v3
       id: cache
       with:
         path: ${{ steps.haskell-setup.outputs.stack-root }}
-        # A unique cache is used for each stack.yaml.
-        key: ${{ runner.os }}-stack-${{ env.STACK_VER }}-${{ hashFiles(format('stack-{0}.yaml', matrix.ghc-ver)) }}
+        key:          ${{ runner.os }}-stack-${{ env.STACK_VER }}-ghc-${{ matrix.ghc-ver }}-${{ hashFiles(format('stack-{0}.yaml', matrix.ghc-ver)) }}
+        restore-keys: ${{ runner.os }}-stack-${{ env.STACK_VER }}-ghc-${{ matrix.ghc-ver }}-
+
+    - name: Install dependencies
+      if: ${{ !steps.cache.outputs.cache-hit }}
+      run: |
+        stack build ${{ env.ARGS }} --test --only-dependencies
+
+    - name: Cache dependencies (save)
+      uses: actions/cache/save@v3
+      env:
+        key: ${{ steps.cache.outputs.cache-primary-key }}
+      # Will fail if we already have a cache with this key.
+      if:   ${{ !(steps.cache.outputs.cache-hit && env.key == steps.cache.outputs.cache-matched-key) }}
+      with:
+        path: ${{ steps.haskell-setup.outputs.stack-root }}
+        key:  ${{ env.key }}
 
     - name: Build hackage-cli
       run: |