Add basic GH CI

Author: tmcgilchrist

.github/workflows/haskell.yml

@@ -0,0 +1,49 @@
+name: CI
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    types:
+      - opened
+      - synchronize
+jobs:
+  build:
+    strategy:
+      fail-fast: false
+      matrix:
+        os:    [macos-latest, ubuntu-latest]
+        cabal: ["3.4"]
+        ghc:   ["8.10.7"]
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: haskell/actions/setup@v1
+        id: setup-haskell-cabal
+        with:
+          ghc-version: ${{ matrix.ghc }}
+          cabal-version: ${{ matrix.cabal }}
+      - name: Cabal Update
+        run: |
+          cabal v2-update
+          cabal v2-freeze $CONFIG
+      - uses: actions/[email protected]
+        with:
+          path: |
+            ${{ steps.setup-haskell-cabal.outputs.cabal-store }}
+            dist-newstyle
+          key: ${{ runner.os }}-${{ matrix.ghc }}-${{ hashFiles('cabal.project.freeze') }}
+          restore-keys: |
+            ${{ runner.os }}-${{ matrix.ghc }}-${{ matrix.cabal }}-
+      - name: Build all
+        run: |
+          cabal build all
+          cabal sdist all
+      - name: Run tests
+        run: |
+           cabal test all
+      - name: Build haddock
+        run: |
+          cabal haddock all