ZuriHac 2021 - Building Haskell with Bazel

Workshop on porting a Haskell and Elm project to be built with Bazel.

• Slides
• Recording
• Completed build

Setup

The project contains a Dockerfile that installs all required dependencies. Use as follows:

1. Build the Docker image

   $ docker build -t zurihac2021 docker
   

2. Start a Docker container as a daemon

   $ docker run -d -v "$PWD:/work" --network host -it zurihac2021
   cbc8a033b90dec3df4b470b157903fc23cc4c1a956a1370c8c6ca55b9aa63ef2
   

3. Open a shell session in the Docker container (can be executed multiple times)

   $ docker exec -it -w /work --detach-keys="ctrl-@" cbc8a033b90d bash
   

You can query for the Docker container's id using

$ docker ps
CONTAINER ID   IMAGE         COMMAND   ...
cbc8a033b90d   zurihac2021   "bash"    ...

You can then stop that container using

$ docker stop cbc8a033b90d

Generate Elm Repositories

To generate an elm_repository for each dependency in client/elm.json use

$ bazel run //scripts:generate_elm_repositories -- client/elm.json -o elm_repositories.bzl

Build and Test

To build a particular target or all targets

$ bazel build //server/src:server
$ bazel build //...

To run a particular test or all tests

$ bazel test //server/test:spec
$ bazel test //...

Run the Server

To run the Haskell server

$ bazel run //server/src:server
listening on port 3000...

Then browse to http://localhost:3000.

Interactive Development

You can use bazel-watcher to continuously rebuild the assets whenever any of its sources change, like so.

$ ibazel build //assets

You can use ghcid to continuously reload the backend whenever any of its sources change, like so.

$ bazel build @stackage-exe//ghcid
$ ASSETS_DIR=bazel-bin/assets \
    ghcid --command="bazel run //server/src:serve@ghci" \
      --test=Main.main \
      --reload=bazel-bin/assets

This will load the Haskell code into a GHCi session and run the server in the interpreter. It will reload the session and restart the server whenever any of the Haskell source files or the assets change.

Note, if the dependency graph changes, e.g. when you edit a BUILD.bazel file, then you will have to restart ghcid itself.

Haskell Servant and Elm Example

Based on haskell-servant/example-servant-elm.

This example project illustrates how to set up a project that:

• Uses haskell and servant as a backend exposing a JSON api (and serving some files).
• Uses elm to write the frontend.
• Uses servant-elm to generate client functions in elm for the JSON api. This means that mismatches regarding the JSON api will be detected statically.
• Allows a very fast development cycle: You can type-check the server and client code in a very short amount of time.

More examples

This repository demonstrates a servant backend and an elm frontend. More code generation examples can be found in haskell-servant/servant-elm.