Build on pull requeests

.github/workflows/haskell.yml

@@ -1,18 +1,28 @@
 name: Haskell CI
 
-on: [push]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
 
 jobs:
   build:
-
+    name: ghc ${{ matrix.ghc }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-16.04
+    strategy:
+      matrix:
+        ghc: ["8.8.1", "8.5.6"]
+        cabal: ["3.0"]
 
     steps:
     - uses: actions/checkout@v1
     - uses: actions/setup-haskell@v1
+      name: Setup Haskell
       with:
-        ghc-version: '8.6.5'
+        ghc-version: ${{ matrix.ghc }}
-        cabal-version: '3.0'
+        cabal-version: ${{ matrix.cabal }}
+
     - name: Install dependencies
       run: |
         cabal update