A family of combinators for defining webservices APIs

Version on this page:0.2.2
LTS Haskell 11.0:0.13
Stackage Nightly 2018-03-12:0.13
Latest on Hackage:0.13

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BSD3 licensed by Servant Contributors
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Module documentation for 0.2.2

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A family of combinators for defining webservices APIs and serving them

You can learn about the basics in the tutorial.



The latest version of this document is on GitHub.


Significant changes

  • Streaming endpoint support. (#836)

    type StreamApi f = "streamGetNewline" :> StreamGet NewlineFraming JSON (f Person)

    See tutorial for more details - A web API as a type - StreamGet and StreamPost - Serving an API - streaming endpoints - Querying an API - Querying Streaming APIs

  • servant Add Servant.API.Modifiers (#873 #903)

    QueryParam, Header and ReqBody understand modifiers: - Required or Optional (resulting in a or Maybe a in handlers) - Strict or Lenient (resulting in a or Either String a in handlers)

    Also you can use Description as a modifier, but it doesn't yet work with servant-docs, only servant-swagger. There is an issue.

  • servant-client Support http-client’s CookieJar (#897 #883)

    ClientM preserves cookies between requests, if given initial CookieJar. To migrate from older code, change ClientEnv constructor to mkClientEnv which makes ClientEnv without CookieJar.

  • servant Mono-kind-ise modifiers, resulting in better error messages. (#887 #890)

  • servant Add TypeError ... => HasServers instances in GHC-8.2 for not saturated modifiers (Capture "foo" :> ...) or -> in place of :>. (#893)

  • Cookbook example projects at (#867 #892)

  • Experimental work servant-client-ghcjs (#818 #869)

Other changes


Bug fixes

  • Prevent double-escaping in link segments (#835 #878)


Significant changes

  • servant-client servant-client-core Factored out of servant-client all the functionality that was independent of the http-client backend. (#803 #821)

    If you have own combinators, you'll need to add an additional m argument in HasClient, Client and clientWithRoute:

    -class HasClient api
    -  type Client (api :: *) :: *
    -  clientWithRoute :: Proxy api -> Req -> Client api
    +class HasClient m api
    +  type Client (m :: * -> *) (api :: *) :: *
    +  clientWithRoute :: Proxy m -> Proxy api -> Request -> Client m api

    See for a real world example.

  • servant-server Added hoistServer member to the HasServer class, which is HasServer specific enter. (#804 #824)

    enter isn't exported from Servant module anymore. You can change enter to hoistServer in a straight forward way. Unwrap natural transformation and add a api type Proxy:

    -server = enter (NT nt) impl
    +server = hoistServer (Proxy :: Proxy MyApi) nt impl

    If you have own combinators, you'll need to define a new method of HasServer class, for example:

    type ServerT (MyCombinator :> api) m = MyValue -> ServerT api m
    hoistServerWithContext _ pc nt s = hoistServerWithContext (Proxy :: Proxy api) pc nt . s

    See for a real world example.

  • Add Description and Summary combinators (#767)

    It's possible to annotate endpoints with free form text. This information is used by e.g. by servant-swagger, see screenshot in

  • Lower :> and :<|> infix precedence to 4 and 3 respectively (#761)

    This shouldn't affect you, except if you define your own infix operators for Servant type-level DSL.

Other changes

  • servant-foreign Derive Data for all types (#809)
  • servant-docs Add authentication lenses (#787)
  • servant-docs Generated markdown improvements (#813 #767 #790 #788)
  • Add addLinks to generate all links for unnested APIs. (#851)
  • Allow newest dependencies

(#772 #842) - Documentation improvements and typo fixes (#757 #771 #775 #790 #791 #806) - Development process improvements (#764 #839)


Breaking changes

Other changes

  • Add a type representing an empty API (#753)
  • Add linkURI' and Link accessors (#745 , #717 , #715)
  • Prepare for GHC-8.2 (#722)
  • Add HasLink AuthProtect instance (#720)
  • AllCTRender [] () TypeError (use NoContent) (#671)
  • Documentation improvements and typo fixes (#702 , #709 , #716 , #725 , #727)


Breaking changes

  • Use NT from natural-transformation for Enter (#616)

  • Change to MkLink (Verb ...) = Link (previously URI). To consume Link use its ToHttpApiData instance or linkURI. (#527)

Other changes

  • Add Servant.API.TypeLevel module with type families to work with API types. (#345 , #305)

  • Default JSON content type change to application/json;charset=utf-8. (#263) Related browser bugs: Chromium and Firefox

  • Accept class may accept multiple content-types. MimeUnrender adopted as well. (#613 , #615)


  • Added 'noHeader' function for not adding response headers.


  • Added Eq, Show, Read, Generic and Ord instances to IsSecure
  • BACKWARDS INCOMPATIBLE: replace use of ToFromByteString with To/FromHttpApiData for GetHeaders/BuildHeadersTo
  • BACKWARDS INCOMPATIBLE: Moved From/ToFormUrlEncoded classes, which were renamed to From/ToForm to http-api-data


  • Add CaptureAll combinator. Captures all of the remaining segments in a URL.
  • Add Servant.API.TypeLevel module, with frequently used type-level functionaliy.


  • Minor fixes, documentation changes and cabal tweaks


  • Add module Servant.Utils.Enter (
  • Allow to set the same header multiple times in responses.


  • Add WithNamedConfig combinator.
  • Add HttpVersion, IsSecure, RemoteHost and Vault combinators
  • Fix safeLink, so Header is not in fact required.
  • Add more instances for (:<|>)
  • Use http-api-data instead of Servant.Common.Text
  • Remove matrix params.
  • Add PlainText String MimeRender and MimeUnrender instances.
  • Add new Verbs combinator, and make all existing and new verb combinators type synonyms of it.
  • Add BasicAuth combinator to support Basic authentication
  • Add generalized authentication support


  • Fix missing cases for Patch in safeLink


  • Allow whitespace after parsing JSON
  • Stricter matching for safeLink for Capture


  • Delete now is like Get, Post, Put, and Patch and returns a response body
  • Multiple content-type/accept support for all the relevant combinators
  • Provide JSON, PlainText, OctetStream and FormUrlEncoded content types out of the box
  • Type-safe link generation to API endpoints
  • Support for the PATCH HTTP method
  • Removed the home-made QuasiQuote for writing API types in a more human-friendly format until we come up with a better design for it
  • Make most if not all of the haddock code examples run through doctest
  • Some general code cleanup
  • Add response headers
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