The Stack build tool guide contains a section on Travis configuration with a sample configuration based on Stack. After iterating through a few different designs over multiple projects, I've just pushed a significant update to the configuration.

The configuration itself includes inline comments to explain what it's doing and how it can be extended, so I won't dwell on those details. I would however like to give a quick summary of the features of this configuration and why I'd recommend switching projects over to it:

The implementation for cabal-install was heavily inspired by/stolen from Herber'ts multi-ghc-travis, which provides a cabal-install-only implementation for Travis. Versus that implementation, this Travis configuration provides OS X support and automaticaly LTS Haskell/Stackage Nightly testing.

Feel free to take this script and modify it in whatever way you want, such as removing older GHCs, modifying command line arguments, or testing with different stack.yaml files. If there is any feedback on ways to improve this configuration, please let me know!

For the lazy who don't want to click through to the Stack docs, here's the current version of the .travis.yml file at time of writing:

# Copy these contents into the root directory of your Github project in a file
# named .travis.yml

# Use new container infrastructure to enable caching
sudo: false

# Choose a lightweight base image; we provide our own build tools.
language: c

# Caching so the next build will be fast too.
cache:
  directories:
  - $HOME/.ghc
  - $HOME/.cabal
  - $HOME/.stack

# The different configurations we want to test. We have BUILD=cabal which uses
# cabal-install, and BUILD=stack which uses Stack. More documentation on each
# of those below.
#
# We set the compiler values here to tell Travis to use a different
# cache file per set of arguments.
#
# If you need to have different apt packages for each combination in the
# matrix, you can use a line such as:
#     addons: {apt: {packages: [libfcgi-dev,libgmp-dev]}}
matrix:
  include:
  # We grab the appropriate GHC and cabal-install versions from hvr's PPA. See:
  # https://github.com/hvr/multi-ghc-travis
  - env: BUILD=cabal GHCVER=7.0.4 CABALVER=1.16
    compiler: ": #GHC 7.0.4"
    addons: {apt: {packages: [cabal-install-1.16,ghc-7.0.4], sources: [hvr-ghc]}}
  - env: BUILD=cabal GHCVER=7.2.2 CABALVER=1.16
    compiler: ": #GHC 7.2.2"
    addons: {apt: {packages: [cabal-install-1.16,ghc-7.2.2], sources: [hvr-ghc]}}
  - env: BUILD=cabal GHCVER=7.4.2 CABALVER=1.16
    compiler: ": #GHC 7.4.2"
    addons: {apt: {packages: [cabal-install-1.16,ghc-7.4.2], sources: [hvr-ghc]}}
  - env: BUILD=cabal GHCVER=7.6.3 CABALVER=1.16
    compiler: ": #GHC 7.6.3"
    addons: {apt: {packages: [cabal-install-1.16,ghc-7.6.3], sources: [hvr-ghc]}}
  - env: BUILD=cabal GHCVER=7.8.4 CABALVER=1.18
    compiler: ": #GHC 7.8.4"
    addons: {apt: {packages: [cabal-install-1.18,ghc-7.8.4], sources: [hvr-ghc]}}
  - env: BUILD=cabal GHCVER=7.10.3 CABALVER=1.22
    compiler: ": #GHC 7.10.3"
    addons: {apt: {packages: [cabal-install-1.22,ghc-7.10.3], sources: [hvr-ghc]}}

  # Build with the newest GHC and cabal-install. This is an accepted failure,
  # see below.
  - env: BUILD=cabal GHCVER=head  CABALVER=head
    compiler: ": #GHC HEAD"
    addons: {apt: {packages: [cabal-install-head,ghc-head], sources: [hvr-ghc]}}

  # The Stack builds. We can pass in arbitrary Stack arguments via the ARGS
  # variable, such as using --stack-yaml to point to a different file.
  - env: BUILD=stack ARGS="--resolver lts-2"
    compiler: ": #stack 7.8.4"
    addons: {apt: {packages: [ghc-7.8.4], sources: [hvr-ghc]}}

  - env: BUILD=stack ARGS="--resolver lts-3"
    compiler: ": #stack 7.10.2"
    addons: {apt: {packages: [ghc-7.10.2], sources: [hvr-ghc]}}

  - env: BUILD=stack ARGS="--resolver lts-5"
    compiler: ": #stack 7.10.3"
    addons: {apt: {packages: [ghc-7.10.3], sources: [hvr-ghc]}}

  # Nightly builds are allowed to fail
  - env: BUILD=stack ARGS="--resolver nightly"
    compiler: ": #stack nightly"
    addons: {apt: {packages: [libgmp-dev]}}

  # Build on OS X in addition to Linux
  - env: BUILD=stack ARGS="--resolver lts-2"
    compiler: ": #stack 7.8.4 osx"
    os: osx

  - env: BUILD=stack ARGS="--resolver lts-3"
    compiler: ": #stack 7.10.2 osx"
    os: osx

  - env: BUILD=stack ARGS="--resolver lts-5"
    compiler: ": #stack 7.10.3 osx"
    os: osx

  - env: BUILD=stack ARGS="--resolver nightly"
    compiler: ": #stack nightly osx"
    os: osx

  allow_failures:
  - env: BUILD=cabal GHCVER=head  CABALVER=head
  - env: BUILD=stack ARGS="--resolver nightly"

before_install:
# Using compiler above sets CC to an invalid value, so unset it
- unset CC

# We want to always allow newer versions of packages when building on GHC HEAD
- CABALARGS=""
- if [ "x$GHCVER" = "xhead" ]; then CABALARGS=--allow-newer; fi

# Download and unpack the stack executable
- export PATH=/opt/ghc/$GHCVER/bin:/opt/cabal/$CABALVER/bin:$HOME/.local/bin:$PATH
- mkdir -p ~/.local/bin
- |
  if [ `uname` = "Darwin" ]
  then
    curl --insecure -L https://www.stackage.org/stack/osx-x86_64 | tar xz --strip-components=1 --include '*/stack' -C ~/.local/bin
  else
    curl -L https://www.stackage.org/stack/linux-x86_64 | tar xz --wildcards --strip-components=1 -C ~/.local/bin '*/stack'
  fi

install:
- echo "$(ghc --version) [$(ghc --print-project-git-commit-id 2> /dev/null || echo '?')]"
- if [ -f configure.ac ]; then autoreconf -i; fi
- |
  case "$BUILD" in
    stack)
      stack --no-terminal --install-ghc $ARGS test --only-dependencies
      ;;
    cabal)
      cabal --version
      travis_retry cabal update
      cabal install --only-dependencies --enable-tests --enable-benchmarks --force-reinstalls --ghc-options=-O0 --reorder-goals --max-backjumps=-1 $CABALARGS
      ;;
  esac

script:
- |
  case "$BUILD" in
    stack)
      stack --no-terminal $ARGS test --haddock --no-haddock-deps
      ;;
    cabal)
      cabal configure --enable-tests --enable-benchmarks -v2 --ghc-options="-O0 -Werror"
      cabal build
      cabal check || [ "$CABALVER" == "1.16" ]
      cabal test
      cabal sdist
      cabal copy
      SRC_TGZ=$(cabal info . | awk '{print $2;exit}').tar.gz && \
        (cd dist && cabal install --force-reinstalls "$SRC_TGZ")
      ;;
  esac

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