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](https://github.com/mietek/haskell-on-heroku) which detects your project dependencies and fetches a custom pre-built sandbox.
Example: deploying a Snap app
# 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 $PORT' > Procfile # Create a git repo and deploy! git init . echo "dist\n.cabal-sandbox\ncabal.sandbox.config" > .gitignore git add * git commit -m 'Initial commit' heroku create --stack=cedar-14 --buildpack https://github.com/begriffs/heroku-buildpack-ghc.git git push heroku master
The first deploy is slowest as the environment downloads and bootstraps. Subsequent deploys use cached binaries and cached cabal packages to go faster.
Beating the Fifteen-Minute Build Limit
The first time you try to deploy a big framework like Yesod the compilation can take so long that Heroku cuts it off. If this happens fear not, you can build your app with an Anvil server.
# Enable Anvil builds heroku plugins:install https://github.com/ddollar/heroku-anvil # Move big build artifacts out of the way or else the upload # to Anvil will be very slow mkdir -p /tmp/deploy-stash ; mv .cabal-sandbox /tmp/deploy-stash ; mv dist /tmp/deploy-stash # Build your slug and cache without any time limits heroku build -r -b https://github.com/begriffs/heroku-buildpack-ghc.git # Use Anvil-generated cache next time we do a regular git push to Heroku heroku config:set EXTERNAL_CACHE=$(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.
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Cabal sometimes gets confused on Heroku and tries installing outdated packages.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||188.8.131.52|
|PREBUILT||Base url for the prebuilt binary cache||https://s3.amazonaws.com/heroku-ghc|
|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 184.108.40.206
- 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
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
/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.
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.
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 "https://s3.amazonaws.com/aws-cli/awscli-bundle.zip" -o "awscli-bundle.zip" unzip awscli-bundle.zip awscli-bundle/install ~/.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] (http://blog.begriffs.com/2013/08/haskell-on-heroku-omg-lets-get-this.html).