Run CI on more machines

.github/workflows/ci.yaml

@@ -6,19 +6,25 @@ on:
 
 jobs:
   build:
-    name: build (${{ matrix.os }})
+    name: ${{ matrix.os }}
     runs-on: ${{ matrix.os }}
     strategy:
+      # Run each build to completion, regardless of if any have failed
       fail-fast: false
+      # macOS-11.10 is available on GitHub Actions, but the build fails with "can't load framework: Cocoa (not found)"
       matrix:
-        os: [ubuntu-latest] # , macOS-latest]
+        os: [ubuntu-20.04, ubuntu-18.04, ubuntu-16.04, macOS-10.15]
     steps:
       - uses: actions/checkout@v2
+
+      # Cache ~/.stack, keyed by the contents of 'stack.yaml'.
       - uses: actions/cache@v2
         name: cache ~/.stack
         with:
           path: ~/.stack
-          key: x-${{runner.os}}-stack-${{hashFiles('stack.yaml')}}
+          key: stack_${{matrix.os}}-${{hashFiles('stack.yaml')}}
+
+      # Cache each local package's ~/.stack-work for fast incremental builds in CI.
       - uses: actions/cache@v2
         name: cache .stack-work
         with:
@@ -27,19 +33,36 @@ jobs:
             parser-typechecker/.stack-work
             unison-core/.stack-work
             yaks/easytest/.stack-work
-          key: x-${{runner.os}}-stack_work-${{hashFiles('stack.yaml')}}-${{github.sha}}
-          restore-keys: x-${{runner.os}}-stack_work-${{hashFiles('stack.yaml')}}-
-      - name: install stack
+          # Main hash key: the branch + commit hash. This should always result in a cache miss...
+          key: stack-work-${{matrix.os}}-${{github.ref}}-${{github.sha}}
+          # ...but then fall back on the latest cache stored on that branch.
+          restore-keys: stack-work_${{matrix.os}}-${{github.ref}}-
+
+      # Install stack by downloading the binary from GitHub. The installation process is different for Linux and macOS,
+      # so this is split into two steps, only one of which will run on any particular build.
+      - name: install stack (Linux)
+        if: runner.os == 'Linux'
         run: |
-          echo $PATH
           curl -L https://github.com/commercialhaskell/stack/releases/download/v2.5.1/stack-2.5.1-linux-x86_64.tar.gz | tar -xz
           echo "$HOME/stack-2.5.1-linux-x86_64/" >> $GITHUB_PATH
+      - name: install stack (macOS)
+        if: runner.os == 'macOS'
+        run: |
+          curl -L https://github.com/commercialhaskell/stack/releases/download/v2.5.1/stack-2.5.1-osx-x86_64.tar.gz | tar -xz
+          echo "$HOME/stack-2.5.1-osx-x86_64/" >> $GITHUB_PATH
+
+      # One of the transcripts fails if the user's git name hasn't been set.
       - name: set git username
         run: git config --global user.name "GitHub Actions"
+
+      # Build deps, then build local code. Splitting it into two steps just allows us to see how much time each step
+      # takes.
       - name: build dependencies
         run: stack --no-terminal --system-ghc build --fast --only-dependencies
       - name: build
         run: stack --no-terminal --system-ghc build --fast
+
+      # Run each test suite (tests and transcripts) on each runtime
       - name: tests
         run: stack --no-terminal --system-ghc exec tests
       - name: tests (new runtime)