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Deploy Haskell applications


Haskell is a purely functional language, especially suited for robust web applications.

There are many ways to write web applications in haskell, from raw WAI to full-stack frameworks like Yesod, simple libraries like scotty or type-safe solutions like servant.

Create an application on Clever Cloud

via the web console

  1. Create a new app by clicking on the Add an Application button, in the sidebar.
  2. Select a brand new instance (or a repository from GitHub if your account is linked).
  3. Then select Haskell in the platform list.
  4. Configure your scaling options.
  5. Enter your application’s name and description and click “Next”. You can also select the region you want (North America or Europe).

Refer to the getting started for more details on application creation via the console.

via the Clever Tools CLI

  1. Make sure you have clever-tools installed locally. Report to the getting started.
  2. In your code folder, do clever create --type <type> <app-name> --region <zone> --org <org> where type is the type of technology you rely on, app-name the name you want for your application, zone deployment zone (par for Paris and mtl for Montreal), and org the organization ID the application will be created under.

Refer to the documentation for more details on application creation with Clever Tools.

Setting up environment variables on Clever Cloud

With the Clever Cloud console

  1. Go to the Clever Cloud console, and find the app you want to fine tune under it’s organization.
  2. Find the Environment variables menu and select it.
  3. In this menu, you will see a form with VARIABLE_NAME and variable value fields. Fill them with the desired values then select Create.

With the Clever Tools CLI

  1. Make sure you have clever-tools installed locally. Report to the getting started
  2. In your code folder, do clever env set <variable-name> <variable-value>

Refer to environment variables reference for more details on available environment variables on Clever Cloud. You can of course create custom ones with the interface we just demonstrated, they will be available for your application.

Configure your Haskell application

Mandatory configuration

Be sure that:

  • you have pushed in master branch
  • you listen on port 8080
  • you have one and only one binary target in your .cabal file
  • your project has a stack.yaml file

The steps ran in order are:

  • stack setup
  • stack install --only-dependencies
  • stack install
  • ./<path>/my-exe (or the contents of CC_RUN_COMMAND)

The executable built by stack build (or the command you specify) must start a web server listening on

For instance, a minimal scotty application can look like this:

{-# LANGUAGE OverloadedStrings #-}

import Web.Scotty

main = scotty 8080 $ do get "/" $ do html $ "Hello world"


Make sure to list all your dependencies in your .cabal file. For the example above, you need:

  build-depends:       base
                     , scotty

Cached dependencies

Enabling dependencies caching

You can enable dependencies caching by adding the CC_CACHE_DEPENDENCIES=true environment variable in your application. It is enabled by default only for rust and haskell applications.

Disabling dependencies caching

You can disable dependencies caching completely by removing the CC_CACHE_DEPENDENCIES environment variable from the Clever Cloud console, in the Environment variables menu of your application. Or by setting it to CC_CACHE_DEPENDENCIES=false To fully remove cached dependencies, you have to rebuild your application from scratch. You can select “rebuild and restart” from the Clever Cloud console or launch clever restart --without-cache with the Clever Tools CLI.

Custom run command

If you need to run a custom command (or just pass options to the binary built by stack), you can specify it through the CC_RUN_COMMAND environment variable.

For instance, for a hakyll website, you can put

CC_RUN_COMMAND=~./local/bin/site server --host --port 8080

To access environment variables from your code, just get them from the environment with getEnv :: String -> IO String in System.Environment.

If some environment variables are critical to your application, here is an approach you can use:

{-# LANGUAGE OverloadedStrings #-}
module Main where

import Data.Text.Lazy (pack) import System.Environment (getEnv) import Web.Scotty (get, html, scotty)

envVar = getEnv "MY_VAR"

main = do myVar <- envVar scotty 8080 $ do get "/" $ do html . pack $ "Hello world " ++ myVar

This loads the environment variable in your main function and tests it. This way, the application will refuse to start with an helpful error message if MY_VAR is not defined.

Git Deployment on Clever Cloud

You need git on your computer to deploy via this tool. Here is the official website of Git to get more information: git-scm.com

Setting up your remotes

  1. The “Information” page of your app gives you your git deployment URL. It looks like this: git+ssh://[email protected]/<your_app_id>.git. Copy it in your clipboard.

  2. Locally, under your code folder, type in git init to set up a new git repository or skip this step if you already have one.

  3. Add the deploy URL with git remote add <name> <your-git-deployment-url>.

  4. Add your files via git add <files path> and commit them via git commit -m <your commit message>.

  5. Now push your application on Clever Cloud with git push <name> master

Refer to the quickstart for more details on git deployments.

Linking a database or any other add-on to your application

By linking an application to an add-on, the application has the add-on environment variables in its own environment by default.

On add-on creation

Many add-ons do exist on Clever Cloud, please refer to the full list and check add-ons dedicated pages for full instructions.

During add-on creation, you will see Applications screen, with a list of your applications. You can toggle the button to Link and click next. If you finish the process of add-on creation, the application will automatically be linked to it.

Add-on already exists

In the Clever Cloud console, under the Service Dependencies menu of your application, you can use the Link addons dropdown menu to select the name of the add-on you want to link and use the add button to finish the process.

You can also link another application from the same page in the Clever Cloud console, using the Link applications dropdown menu.

More configuration

Need more configuration? To run a script at the end of your deployment? To add your private SSH key to access private dependencies? Go check the Common configuration page.

You may want to have an advanced usage of your application, in which case we recommend you to read the Administrate documentation section.

If you can’t find something or have a specific need like using a non supported version of a particular software, please reach out to the support.