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servant-template: production-ready Haskell web services in 5 minutes

·3 mins

If you want to write a web API in Haskell, then you should start by using my new cookiecutter template at It’ll get you a production-ready web service in 5 minutes or less.

Whenever you start any new web service and you actually care about getting it working and available to users, it’s very useful to have:

  • logging
  • monitoring
  • continuous integration
  • tests
  • deployment
  • command-line parsing

These are largely boring, but nearly essential. Logs and monitoring give you visibility into the code’s behaviour in production, tests and continuous integration help you make sure you don’t break it, and, of course, you need some way of actually shipping code to users. As an engineer who cares deeply about running code in production, these are pretty much the bare minimum for me to be able to deploy something to my users.

The cookiecutter template at gh:jml/servant-template creates a simple Haskell web API service that does all of these things:

As the name suggests, all of this enables writing a servant server. Servant lets you declaring web APIs at the type-level and then using those API specifications to write servers. It’s hard to overstate just how useful it is for writing RESTful APIs.

Get started with:

$ cookiecutter gh:jml/servant-template
project_name [awesome-service]: awesome-service
$ cd awesome-service
$ stack test
$ make image
$ docker run awesome-service:latest --help
awesome-service - TODO fill this in

Usage: awesome-service --port PORT [--access-logs ARG] [--log-level ARG]
  One line description of project

Available options:
  -h,--help                Show this help text
  --port PORT              Port to listen on
  --access-logs ARG        How to log HTTP access
  --log-level ARG          Minimum severity for log messages
  --ghc-metrics            Export GHC metrics. Requires running with +RTS.
$ docker run -p 8080:80 awesome-service --port 80
[2016-10-16T20:50:07.983292987000] [Informational] Listening on :80

For this to work, you’ll need to have Docker installed on your system. I’ve tested it on my Mac with Docker Machine, but haven’t yet with Linux.

You might have to run stack docker pull before make image, if you haven’t already used stack to build things from within Docker.

Once it’s up and running, you can browse to http://localhost:8080/ (or http://$(docker-machine ip):8080/) if you’re on a Mac, and you’ll see a simple HTML page describing the API and giving you a link to the /metrics page, which is where all the Prometheus metrics are exported.

There you have it, a production-ready web service. At least for some values of “production-ready”.

Of course, the API it offers is really simple. You can make it your own by editing the API definition and the server implementation to make it really your own. Note these two are in separate libraries to make it easier to generate client code.

The template comes with a test suite that uses servant-quickcheck to guarantee that none of your endpoints return 500s, take longer than 100ms to serve, and that all the 201s include Location headers.

If you’re so inclined, you could push the created Docker image to a repository somewhere—it’s around 25MB when built. Then, people could use it and no one would have to know that it’s Haskell, they’d just notice a fast web service that works.

As the README says, I’ve made a few questionable decisions when building this. If you disagree, or think I could have done anything better I’d love to know. If you use this to build something cool, or even something silly, please let me know on Twitter.