ci: Build/test with stack against LTS as well

Checks that:

* builds cleanly using stack
* builds fine with whatever deps are in LTS

Author: liskin

.github/workflows/stack.yml

@@ -0,0 +1,46 @@
+name: Stack
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  build:
+    name: Stack CI - Linux - ${{ matrix.resolver }}
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - resolver: lts-12
+          - resolver: lts-14
+          - resolver: lts-16
+          - resolver: lts-17
+
+    steps:
+      - name: Clone project
+        uses: actions/checkout@v2
+
+      - name: Install C dependencies
+        run: |
+          set -ex
+          sudo apt install -y \
+            libx11-dev \
+            libxext-dev \
+            libxrandr-dev \
+            libxss-dev \
+            #
+
+      - name: Cache Haskell dependencies
+        uses: actions/cache@v2
+        with:
+          path: ~/.stack
+          key: ${{ runner.os }}-${{ matrix.resolver }}-${{ hashFiles('stack.yaml') }}-${{ hashFiles('*.cabal') }}
+          restore-keys: |
+            ${{ runner.os }}-${{ matrix.resolver }}-${{ hashFiles('stack.yaml') }}-
+            ${{ runner.os }}-${{ matrix.resolver }}-
+
+      - name: Build and test
+        run: |
+          set -ex
+          stack test --fast --no-terminal --resolver=${{ matrix.resolver }}

stack.yaml

@@ -0,0 +1,12 @@
+resolver: lts-16.31
+
+packages:
+- ./
+
+nix:
+  packages:
+    - pkg-config
+    - xorg.libX11
+    - xorg.libXScrnSaver
+    - xorg.libXext
+    - xorg.libXrandr