Use the fused-effects Travis.

.travis.yml

@@ -1,27 +1,48 @@
-sudo: true
+language: c
-language: haskell
-
-git:
-  depth: 5
-
-cabal: "2.4"
 
 cache:
   directories:
-  - "$HOME/.cabal/store"
+    - $HOME/.cabal/packages
-  
+    - $HOME/.cabal/store
+    - $TRAVIS_BUILD_DIR/dist-newstyle
+
+before_cache:
+  - rm -fv $HOME/.cabal/packages/hackage.haskell.org/build-reports.log
+  # remove files that are regenerated by 'cabal update'
+  - rm -fv $HOME/.cabal/packages/hackage.haskell.org/00-index.*
+  - rm -fv $HOME/.cabal/packages/hackage.haskell.org/*.json
+  - rm -fv $HOME/.cabal/packages/hackage.haskell.org/01-index.cache
+  - rm -fv $HOME/.cabal/packages/hackage.haskell.org/01-index.tar
+  - rm -fv $HOME/.cabal/packages/hackage.haskell.org/01-index.tar.idx
+
+  - rm -rfv $HOME/.cabal/packages/head.hackage
 
 matrix:
   include:
-  - ghc: 8.6.4
+    - compiler: "ghc-8.2.2"
-  
+      addons: {apt: {packages: [cabal-install-2.4,ghc-8.2.2], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.4.4"
+      addons: {apt: {packages: [cabal-install-2.4,ghc-8.4.4], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.6.5"
+      addons: {apt: {packages: [cabal-install-2.4,ghc-8.6.5], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.8.1"
+      addons: {apt: {packages: [cabal-install-3.0,ghc-8.8.1], sources: [hvr-ghc]}}
+
+before_install:
+- mkdir -p $HOME/.local/bin
+- "PATH=/opt/ghc/bin:$HOME/local/bin:$PATH"
+- ghc --version
+- cabal --version
 
 install:
-  - cabal new-update
+- cabal v2-update -v
-  - cabal new-build --enable-tests --enable-benchmarks
+- cabal v2-configure --enable-benchmarks --enable-tests --write-ghc-environment-files=always -j2
+- cabal v2-build --only-dependencies
 
 script:
-  - cabal new-test --enable-tests
+- cabal v2-build
-
+- cabal v2-run test
-notifications:
+- cabal v2-run examples
-  email: false
+- cabal v2-haddock
+- cabal v2-sdist
+- cabal check