Hello World - Haskell

A simple web app written in Haskell that you can use for testing. It reads in an env variable TARGET and prints “Hello ${TARGET}!”. If TARGET is not specified, it will use “World” as the TARGET.


  • A Kubernetes cluster with Knative installed and DNS configured. Follow the installation instructions if you need to create one.
  • Docker installed and running on your local machine, and a Docker Hub account configured (we’ll use it for a container registry).

Recreating the sample code

While you can clone all of the code from this directory, hello world apps are generally more useful if you build them step-by-step. The following instructions recreate the source files from this folder.

  1. Create a new file named stack.yaml and paste the following code:

    flags: {}
      - .
    extra-deps: []
    resolver: lts-10.7
  2. Create a new file named package.yaml and paste the following code

    name: helloworld-haskell
      - base >= 4.7 && < 5
      - scotty
      - text
        main: Main.hs
        source-dirs: app
          - -threaded
          - -rtsopts
          - -with-rtsopts=-N
  3. Create a app folder, then create a new file named Main.hs in that folder and paste the following code. This code creates a basic web server which listens on port 8080:

    {-# LANGUAGE OverloadedStrings #-}
    import           Data.Maybe
    import           Data.Monoid        ((<>))
    import           Data.Text.Lazy     (Text)
    import           Data.Text.Lazy
    import           System.Environment (lookupEnv)
    import           Web.Scotty         (ActionM, ScottyM, scotty)
    import           Web.Scotty.Trans
    main :: IO ()
    main = do
      t <- fromMaybe "World" <$> lookupEnv "TARGET"
      pStr <- fromMaybe "8080" <$> lookupEnv "PORT"
      let p = read pStr :: Int
      scotty p (route t)
    route :: String -> ScottyM()
    route t = get "/" $ hello t
    hello :: String -> ActionM()
    hello t = text $ pack ("Hello " ++ t)
  4. In your project directory, create a file named Dockerfile and copy the code block below into it.

    # Use the official Haskell image to create a build artifact.
    # https://hub.docker.com/_/haskell/
    FROM haskell:8.2.2 as builder
    # Copy local code to the container image.
    WORKDIR /app
    COPY . .
    # Build and test our code, then build the “helloworld-haskell-exe” executable.
    RUN stack setup
    RUN stack build --copy-bins
    # Use a Docker multi-stage build to create a lean production image.
    # https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
    FROM fpco/haskell-scratch:integer-gmp
    # Copy the "helloworld-haskell-exe" executable from the builder stage to the production image.
    WORKDIR /root/
    COPY --from=builder /root/.local/bin/helloworld-haskell-exe .
    # Run the web service on container startup.
    CMD ["./helloworld-haskell-exe"]
  5. Create a new file, service.yaml and copy the following service definition into the file. Make sure to replace {username} with your Docker Hub username.

    apiVersion: serving.knative.dev/v1
    kind: Service
      name: helloworld-haskell
      namespace: default
            - image: docker.io/{username}/helloworld-haskell
                - name: TARGET
                  value: "Haskell Sample v1"

Build and deploy this sample

Once you have recreated the sample code files (or used the files in the sample folder) you’re ready to build and deploy the sample app.

  1. Use Docker to build the sample code into a container. To build and push with Docker Hub, enter these commands replacing {username} with your Docker Hub username:

    # Build the container on your local machine
    docker build -t {username}/helloworld-haskell .
    # Push the container to docker registry
    docker push {username}/helloworld-haskell
  2. After the build has completed and the container is pushed to Docker Hub, you can deploy the app into your cluster. Ensure that the container image value in service.yaml matches the container you built in the previous step. Apply the configuration using kubectl:

    kubectl apply --filename service.yaml
  3. Now that your service is created, Knative will perform the following steps:

    • Create a new immutable revision for this version of the app.
    • Network programming to create a route, ingress, service, and load balance for your app.
    • Automatically scale your pods up and down (including to zero active pods).
  4. To find the URL for your service, enter:

    kubectl get ksvc helloworld-haskell  --output=custom-columns=NAME:.metadata.name,URL:.status.url
    NAME                   URL
    helloworld-haskell     http://helloworld-haskell.default.
  5. Now you can make a request to your app and see the result. Replace the URL below with the URL returned in the previous command.

    curl http://helloworld-haskell.default.
    Hello world: Haskell Sample v1

Removing the sample app deployment

To remove the sample app from your cluster, delete the service record:

kubectl delete --filename service.yaml