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servant - A Type-Level Web DSL


Getting Started

We have a tutorial that introduces the core features of servant. After this article, you should be able to write your first servant webservices, learning the rest from the haddocks' examples.

The core documentation can be found here. Other blog posts, videos and slides can be found on the website.

If you need help, drop by the IRC channel (#servant on freenode) or mailing list.



Release process outline (by phadej)

  • Update changelog and bump versions in master
    • git log --oneline v0.12.. | grep 'Merge pull request' is a good starting point (use correct previous release tag)
  • Create a release branch, e.g. release-0.13
    • Release branch is useful for backporting fixes from master
  • Smoke test in servant-universe
    • git submodule foreach git checkout master and git submodule foreach git pull to get newest of everything.
    • cabal new-build --enable-tests all to verify that everything builds, and cabal new-test all to run tests
      • It's a good idea to separate these steps, as tests often pass, if they compile :)
    • See cabal.project to selectively allow-newer
    • If some packages are broken, on your discretisation there are two options:
      • Fix them and make PRs: it's a good idea to test against older servant version too.
      • Temporarily comment out broken package
    • If you make a commit for servant-universe, you can use it as submodule in private projects to test even more
  • When ripples are cleared out:
    • git tag -s the release
    • git push --tags
    • cabal sdist and cabal upload


.travis.yml is generated using make-travis-yml tool, in multi-ghc-travis repository.

To regenerate the script use (note: atm you need to comment doc/cookbook/ packages).

runghc ~/Documents/other-haskell/multi-ghc-travis/make_travis_yml_2.hs regenerate

In case Travis jobs fail due to a dependency failing to build, you can temporarily add constraints to the cabal.project file, and regenerate the .travis.yml. For example, the following will disallow a single troublemaker-13.37 package version:

  troublemaker <13.37 && > 13.37

TechEmpower framework benchmarks

We develop and maintain the servant TFB entry in

To verify (i.e. compile and test that it works)

./tfb --mode verify --test servant servant-beam servant-psql-simple --type json plaintext db fortune

To compare with warp

./tfb --mode benchmark --test warp servant servant-beam servant-psql-simple --type json plaintext db fortune

To compare with reitit (Clojure framework)

./tfb --mode benchmark --test reitit reitit-async reitit-jdbc servant servant-beam servant-psql-simple --type json plaintext db fortune

You can see the visualised results at


Main repository for the servant libraries -- DSL for describing, serving, querying, mocking, documenting web applications and more!



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