Updated travis script to test with multiple GHC versions.

.travis.yml

@@ -1,8 +1,45 @@
-language: haskell
+# NB: don't set `language: haskell` here
+
+# The following enables several GHC versions to be tested; often it's enough to test only against the last release in a major GHC version. Feel free to omit lines listings versions you don't need/want testing for.
+env:
+ - GHCVER=6.12.3
+ - GHCVER=7.4.2
+ - GHCVER=7.6.3
+ - GHCVER=7.8.1 # see note about Alex/Happy
+# - GHCVER=head  # see section about GHC HEAD snapshots
+
+# Note: the distinction between `before_install` and `install` is not important.
 before_install:
-  - cabal install cabal-dev
-  - 'git clone https://github.com/jgm/pandoc-types && cabal-dev add-source pandoc-types'
+ - travis_retry sudo add-apt-repository -y ppa:hvr/ghc
+ - travis_retry sudo apt-get update
+ - travis_retry sudo apt-get install cabal-install-1.18 ghc-$GHCVER happy
+ - export PATH=/opt/ghc/$GHCVER/bin:$PATH
+ - git clone https://github.com/jgm/pandoc-types && cd pandoc-types && cabal install && cd ..
+ - |
+   if [ $GHCVER = "head" ] || [ $GHCVER = "7.8.1" ]; then
+     $CABAL install happy alex
+     export PATH=$HOME/.cabal/bin:$PATH
+   fi
+
 install:
-  - cabal-dev install-deps --enable-tests
+ - cabal-1.18 update
+ - cabal-1.18 install --only-dependencies --enable-tests
+
+# Here starts the actual work to be performed for the package under test; any command which exits with a non-zero exit code causes the build to fail.
 script:
-  - 'cabal-dev configure --enable-tests && cabal-dev build --ghc-options=-Werror && cabal-dev test'
+ - cabal-1.18 configure --enable-tests -v2  # -v2 provides useful information for debugging
+ - cabal-1.18 build --ghc-options=-Werror  # this builds all libraries and executables (including tests/benchmarks)
+ - cabal-1.18 test
+ - cabal-1.18 check
+ - cabal-1.18 sdist   # tests that a source-distribution can be generated
+
+# The following scriptlet checks that the resulting source distribution can be built & installed
+ - export SRC_TGZ=$(cabal-1.18 info . | awk '{print $2 ".tar.gz";exit}') ;
+   cd dist/;
+   if [ -f "$SRC_TGZ" ]; then
+      cabal-1.18 install "$SRC_TGZ";
+   else
+      echo "expected '$SRC_TGZ' not found";
+      exit 1;
+   fi
+