PythonAnywhere already comes with the Haskell ghc already installed. For example, you could do runhaskell your-haskell-file.hs to compile and run your own Haskell file. Here is a great intro to Haskell.

Once you have gotten more familiar with Haskell, you may find that you want to setup automatic testing etc for your project. Below is a sample workflow to set up a Cabal environment where you can build your package and run tests:

# setup ~/.cabal and download latest package list from hackage.haskell.org
cabal update

# upgrade cabal-install, the CLI tool, to a shiny new version to get sandbox functionality
cabal install cabal-install --jobs=5

# the newly updated cabal will live in ~/.cabal/bin/cabal; make sure you are using it
echo "alias cabal=~/.cabal/bin/cabal" >> ~/.bashrc && source ~/.bashrc

mkdir a-cool-haskell-project
cd a-cool-haskell-project

# setup a "virtualenv"
cabal sandbox init
# create your cabal package!
cabal init

After you have all this set up, you will be able to start developing your cabal package! Some common commands include:

# after you making changes, rebuild and run test suite
cabal build && cabal test --test-options="colors" --show-details="always"

# if there are new build dependencies that you added to your .cabal file, need to configure and install packages before you build
cabal configure --enable-tests && cabal install --only-dependencies --enable-tests --jobs=5

You may also find these articles interesting and informative. Also, hspec has a useful automatic test discovery thing that may be pretty useful.