Deploy Haskell apps to Heroku
Latest commit 4f00643 Aug 20, 2016 @begriffs begriffs Merge pull request #76 from DougBurke/add-post-script
Add support for running a script at the end of the compile (via POST_SCRIPT env var)

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Deploy Haskell apps to Heroku

This buildpack supports frameworks like Yesod, Snap, and Happstack with the latest stable GHC binaries. Putting Haskell web applications online should be easy, and now it is. Try it for yourself.

**Note**, this buildpack does a basic build from scratch. For a faster build, try [mietek/haskell-on-heroku]( which detects your project dependencies and fetches a custom pre-built sandbox.

Example: deploying a Snap app

Here's how to go from zero to "hello world" on Heroku. You'll need to install the Haskell Platform and the Heroku Toolbelt on your local machine, then do this:

# Generate a barebones snap app called snapdemo

mkdir snapdemo && cd $_
cabal sandbox init
cabal install snap
cabal exec snap init barebones

# Tell Heroku how to start the server

echo 'web: cabal run -- -p $PORT' > Procfile

# Create a git repo and deploy!

git init .
echo "dist\n.cabal-sandbox\ncabal.sandbox.config" > .gitignore
git add *
git commit -m 'Initial commit'

heroku create --stack=cedar-14 --buildpack
git push heroku master

The first deploy is slowest as the environment downloads and bootstraps. Subsequent deploys use cached binaries and cached cabal packages to go faster.

Beating the Fifteen-Minute Build Limit

The first time you try to deploy a big framework like Yesod the compilation can take so long that Heroku cuts it off. If this happens fear not, you can build your app with an Anvil server.

# Enable Anvil builds
heroku plugins:install

# Move big build artifacts out of the way or else the upload
# to Anvil will be very slow
mkdir -p /tmp/deploy-stash ; mv .cabal-sandbox /tmp/deploy-stash  ; mv dist /tmp/deploy-stash

# Build your slug and cache without any time limits
heroku build -r -b

# Use Anvil-generated cache next time we do a regular git push to Heroku
heroku config:set EXTERNAL_CACHE=$(cat .anvil/cache)

# Bring your sandbox etc back
mv /tmp/deploy-stash/.cabal-sandbox . ; mv /tmp/deploy-stash/dist .

After the first deploy using Anvil you can go back to the regular deploy process. This is because the cabal sandbox etc are cached by Anvil and will be retrieved, making future builds incremental and fast.

Remember to heroku config:unset EXTERNAL_CACHE after your first successful regular (post-Anvil) git push.

Locking Package Versions

Cabal sometimes gets confused on Heroku and tries installing outdated packages. If you have your app working locally you can constrain the remote package versions to match your local environment. Just do this:

cabal freeze
git add cabal.config

# commit and push to fix remote build

Configuring the Build

You can change build settings through Heroku environment variables.

# set the variable of your choice
heroku config:set VARIABLE=value

Here are the options

CLEAR_CACHE Force everything to reinstall from scratch by setting to 1. 0
EXTERNAL_CACHE Url of replacement buildpack cache tarball. Useful for Anvil.
GHC_VER GHC version to download or build 7.10.3
CABAL_VER Version of cabal-install
PREBUILT Base url for the prebuilt binary cache
PRE_SCRIPT Path to an executable, relative to checkout root. If present, this will be chmodded (+x) and ran before anything else.
POST_SCRIPT Path to an executable, relative to checkout root. If present, this will be chmodded (+x) and ran after anything else.

Interacting with a running app

heroku run bash         # shell access
heroku run cabal repl   # Haskell repl with all app modules loaded

Benefits of this buildpack

  • Latest binaries: GHC 7.10.3, cabal-install
  • Uses cabal >=1.20 features to run the app and repl
  • Exposes Haskell platform binaries to your app and scripts
  • Uses prebuilt binaries for speed but...
  • ...can fall back to building the standard GHC distribution


There are a number of ways to improve this buildpack. Please see the Github issues for ideas.

In order to contribute to the build script it will help to understand how Heroku's deployment process works, and how that affects GHC. Heroku provides three areas for storing files during build: a cache directory, a working directory, and a build directory.

The cache, called $CACHE_DIR in the script, persists between deployments. We use it to avoid building binaries more than once. The working directory, called $WORKING_HOME, does not persist between builds, or even after the build script is done. Seems we should avoid this area, right? Well GHC has some idiosyncracies that make this area quite useful as you will see. Finally the build destination directory, $BUILD_DIR, holds the git repo the user pushes and gets copied into what will be /app in the deployed application. During build it lives in a weird nonce filename.

We want to use GHC binaries at build time to compile the app, and we would also like those binaries to be available in the app environment after deployment (so people can use runhaskell or cabal repl). Seems like we should install directly to $BUILD_DIR. There's one problem: GHC breaks if you move it to a new path after installation because many of its binaries are just scripts with hard-coded full paths in them to other GHC files. And as you remember, Heroku is going to move things in $BUILD_DIR to /app. So the trick will be to install to /app in the working directory, use GHC there, then copy that installation to the build directory which will be renamed in the deployed application and not notice it has been moved.

GHC is also sensitive to having libgmp named just right. We don't have privileges to adjust /usr/lib in the deployed app so we create a symbolic link in a place we are permitted and set linker variables in the shell so that everything can build.

External dependencies

Some packages have external dependencies (i.e. non-Haskell dependencies which cannot be satisfied by cabal). If you come across such a package, check in contribs to see if someone has already created patches for the dependencies. If they have, you should be able to

patch -p0 < contribs/DEP.patch

to patch the buildpack and proceed. If not, you'll need to modify bin/compile yourself. Contributing these changes back as patches is appreciated.

Building new binaries for Heroku

As new versions of GHC and Cabal are released we should build them for Heroku and put them on S3 to speed up future deploys for everyone. Luckily the buildpack can do the building too.

Adjust the GHC_VER and CABAL_VER environment vars and then deploy. It will build the new binaries from the standard GHC distribution. Then copy the results to S3 like this:

heroku run bash
# now SSH'd into the server

cd /app/vendor

url "" -o ""

~/.local/lib/aws/bin/aws configure
# ^^^ answer the configuration questions

tar zcf heroku-ghc-[VERSION].tar.gz ghc-[VERSION]/
tar zcf heroku-cabal-install-[VERSION].tar.gz cabal-install-[VERSION]/

~/.local/lib/aws/bin/aws s3 cp heroku-ghc-[VERSION].tar.gz s3://[BUCKET]
~/.local/lib/aws/bin/aws s3 cp heroku-cabal-install-[VERSION].tar.gz s3://[BUCKET]


Thanks to Brian McKenna and others for their work on heroku-buildpack-haskell which inspired and informed this buildpack. For a history of that project's contributions and ideas see [this article] (