Simplify matrix config, update comments

Author: andrjohns

.github/workflows/main.yml

@@ -1,3 +1,4 @@
+# Note that the workflows below use Powershell (pwsh) as they were originally written for Windows runners.
 name: Rtools & ARM64 CI
 
 on:
@@ -20,22 +21,19 @@ concurrency:
 
 jobs:
   prim-rev:
-    name: ${{ matrix.config.label }} tests (${{ matrix.config.os }})
-    runs-on: ${{ matrix.config.os }}
+    name: ${{ matrix.config.label }} tests (${{ matrix.os }})
+    runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
       matrix:
-        config:
-          - { os: windows-latest, label: misc, tests: 'test/unit/*_test.cpp test/unit/math/*_test.cpp test/unit/math/memory' }
-          - { os: ubuntu-24.04-arm, label: misc, tests: 'test/unit/*_test.cpp test/unit/math/*_test.cpp test/unit/math/memory' }
-          - { os: windows-latest, label: prim, tests: 'test/unit/math/prim' }
-          - { os: ubuntu-24.04-arm, label: prim, tests: 'test/unit/math/prim' }
-          - { os: windows-latest, label: rev, tests: 'test/unit/math/rev' }
-          - { os: ubuntu-24.04-arm, label: rev, tests: 'test/unit/math/rev' }
-          - { os: windows-latest, label: fwd, tests: 'test/unit/math/fwd' }
-          - { os: ubuntu-24.04-arm, label: fwd, tests: 'test/unit/math/fwd' }
-          - { os: windows-latest, label: 'non-fun mix', tests: 'test/unit/math/mix/core test/unit/math/mix/meta test/unit/math/mix/*_test.cpp' }
-          - { os: ubuntu-24.04-arm, label: 'non-fun mix', tests: 'test/unit/math/mix/core test/unit/math/mix/meta test/unit/math/mix/*_test.cpp' }
+        os: [windows-latest, ubuntu-24.04-arm]
+        config: [
+          { label: misc, tests: 'test/unit/*_test.cpp test/unit/math/*_test.cpp test/unit/math/memory' },
+          { label: prim, tests: 'test/unit/math/prim' },
+          { label: rev, tests: 'test/unit/math/rev' },
+          { label: fwd, tests: 'test/unit/math/fwd' },
+          { label: 'non-fun mix', tests: 'test/unit/math/mix/core test/unit/math/mix/meta test/unit/math/mix/*_test.cpp' }
+        ]
 
     steps:
     - uses: actions/checkout@v4
@@ -77,22 +75,13 @@ jobs:
         path: '**/*_test.xml'
 
   mix-fun:
-    name: mix fun and prob tests ${{ matrix.config.group }} (${{ matrix.config.os }})
-    runs-on: ${{ matrix.config.os }}
+    name: mix fun and prob tests ${{ matrix.group }} (${{ matrix.os }})
+    runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
       matrix:
-        config:
-          - { os: windows-latest, group: 1 }
-          - { os: ubuntu-24.04-arm, group: 1 }
-          - { os: windows-latest, group: 2 }
-          - { os: ubuntu-24.04-arm, group: 2 }
-          - { os: windows-latest, group: 3 }
-          - { os: ubuntu-24.04-arm, group: 3 }
-          - { os: windows-latest, group: 4 }
-          - { os: ubuntu-24.04-arm, group: 4 }
-          - { os: windows-latest, group: 5 }
-          - { os: ubuntu-24.04-arm, group: 5 }
+        os: [windows-latest, ubuntu-24.04-arm]
+        group: [1, 2, 3, 4, 5]
 
     steps:
     - uses: actions/checkout@v4
@@ -128,14 +117,15 @@ jobs:
         $NumberTests = $MixFunProbTests.Length
         $FifthNumberTests = [math]::Floor($NumberTests / 5)
 
-        $MixFunProbTests1 = $MixFunProbTests[0..($FifthNumberTests - 1)]
-        $MixFunProbTests2 = $MixFunProbTests[$FifthNumberTests..(2 * $FifthNumberTests - 1)]
-        $MixFunProbTests3 = $MixFunProbTests[(2 * $FifthNumberTests)..(3 * $FifthNumberTests - 1)]
-        $MixFunProbTests4 = $MixFunProbTests[(3 * $FifthNumberTests)..(4 * $FifthNumberTests - 1)]
-        $MixFunProbTests5 = $MixFunProbTests[(4 * $FifthNumberTests)..($NumberTests - 1)]
-        $MixFunProbTestsArray = @($MixFunProbTests1, $MixFunProbTests2, $MixFunProbTests3, $MixFunProbTests4, $MixFunProbTests5)
+        $MixFunProbTestsArray = @( `
+          $MixFunProbTests[0..($FifthNumberTests - 1)], `
+          $MixFunProbTests[$FifthNumberTests..(2 * $FifthNumberTests - 1)], `
+          $MixFunProbTests[(2 * $FifthNumberTests)..(3 * $FifthNumberTests - 1)], `
+          $MixFunProbTests[(3 * $FifthNumberTests)..(4 * $FifthNumberTests - 1)], `
+          $MixFunProbTests[(4 * $FifthNumberTests)..($NumberTests - 1)] `
+        )
 
-        python runTests.py -j2 $MixFunProbTestsArray[(${{ matrix.config.group }} - 1)]
+        python runTests.py -j2 $MixFunProbTestsArray[(${{ matrix.group }} - 1)]
 
     - name: Upload gtest_output xml
       uses: actions/upload-artifact@v4