Automatic derivation of querying functions for servant

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LTS Haskell 22.29:0.20@rev:6
Stackage Nightly 2024-07-16:0.20@rev:6
Latest on Hackage:0.20.1@rev:1

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BSD-3-Clause licensed by Servant Contributors
Maintained by [email protected]
This version can be pinned in stack with:servant-client-,4376

Module documentation for



This library lets you automatically derive Haskell functions that let you query each endpoint of a servant webservice.


{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}

import Data.Proxy
import Data.Text
import Network.HTTP.Client (newManager, defaultManagerSettings)
import Servant.API
import Servant.Client

type Book = Text

type MyApi = "books" :> Get '[JSON] [Book] -- GET /books
        :<|> "books" :> ReqBody '[JSON] Book :> Post '[JSON] Book -- POST /books

myApi :: Proxy MyApi
myApi = Proxy

-- 'client' allows you to produce operations to query an API from a client.
postNewBook :: Book -> ClientM Book
getAllBooks :: ClientM [Book]
(getAllBooks :<|> postNewBook) = client myApi

main :: IO ()
main = do
  manager' <- newManager defaultManagerSettings
  res <- runClientM getAllBooks (mkClientEnv manager' (BaseUrl Http "localhost" 8081 ""))
  case res of
    Left err -> putStrLn $ "Error: " ++ show err
    Right books -> print books


The latest version of this document is on GitHub. Changelog for servant package contains significant entries for all core packages.

  • Allow base-compat-0.11


  • Rename ServantError to ClientError, ServantErr to ServerError #1131

  • servant-client-core Rearrange modules. No more Internal modules, whole API is versioned. #1130

  • servant-client-core RequestBody is now

    = RequestBodyLBS LBS.ByteString
    | RequestBodyBS BS.ByteString
    | RequestBodySource (SourceIO LBS.ByteString)

    i.e. no more replicates http-clients API. #1117

  • servant-client-core Keep structured exceptions in ConnectionError constructor of ClientError #1115

    -| ConnectionError Text
    +| ConnectionError SomeException
  • servant-client-core Preserve failing request in FailureResponse constructor of ClientError #1114

    -FailureResponse Response
    +-- | The server returned an error response including the
    +-- failing request. 'requestPath' includes the 'BaseUrl' and the
    +-- path of the request.
    +FailureResponse (RequestF () (BaseUrl, BS.ByteString)) Response
  • servant-client Fix (implement) StreamBody instance #1110

  • servant-client Update CookieJar with intermediate request/responses (redirects) #1104


  • Streaming refactoring. #991 #1076 #1077

    The streaming functionality (Servant.API.Stream) is refactored to use servant’s own SourceIO type (see Servant.Types.SourceT documentation), which replaces both StreamGenerator and ResultStream types.

    New conversion type-classes are ToSourceIO and FromSourceIO (replacing ToStreamGenerator and BuildFromStream). There are instances for conduit, pipes and machines in new packages: servant-conduit servant-pipes and servant-machines respectively.

    Writing new framing strategies is simpler. Check existing strategies for examples.

    This change shouldn’t affect you, if you don’t use streaming endpoints.

  • servant-client Separate streaming client. #1066

    We now have two http-client based clients, in Servant.Client and Servant.Client.Streaming.

    Their API is the same, except for

    • Servant.Client cannot request Stream endpoints.
    • Servant.Client is run by direct runClientM :: ClientM a -> ClientEnv -> IO (Either ServantError a)
    • Servant.Client.Streaming can request Stream endpoints.
    • Servant.Client.Streaming is used by CPSised withClientM :: ClientM a -> ClientEnv -> (Either ServantError a -> IO b) -> IO b

    To access Stream endpoints use Servant.Client.Streaming with withClientM; otherwise you can continue using Servant.Client with runClientM. You can use both too, ClientEnv and BaseUrl types are same for both.

    Note: Servant.Client.Streaming doesn’t stream non-Stream endpoints. Requesting ordinary Verb endpoints (e.g. Get) will block until the whole response is received.

    There is Servant.Client.Streaming.runClientM function, but it has restricted type. NFData a constraint prevents using it with SourceT, Conduit etc. response types.

    runClientM :: NFData a => ClientM a -> ClientEnv -> IO (Either ServantError a)

    This change shouldn’t affect you, if you don’t use streaming endpoints.

  • Drop support for GHC older than 8.0 #1008 #1009

  • servant-client-core Add NFData (GenResponse a) and NFData ServantError instances. #1076

servant-client-core Add aeson and Lift BaseUrl instances #1037


  • Stream takes a status code argument

    -Stream method        framing ctype a
    +Stream method status framing ctype a

    (#966 #972)

  • ToStreamGenerator definition changed, so it’s possible to write an instance for conduits.

    -class ToStreamGenerator f a where
    -   toStreamGenerator :: f a -> StreamGenerator a
    +class ToStreamGenerator a b | a -> b where
    +   toStreamGenerator :: a -> StreamGenerator b


  • Added NoFraming streaming strategy (#959)

  • servant-client-core Add hoistClient to HasClient. Just like hoistServer allows us to change the monad in which request handlers of a web application live in, we also have hoistClient for changing the monad in which client functions live. Read tutorial section for more information. (#936)

  • servant-client Add more constructors to RequestBody, including RequestBodyStream. Note: we are looking for http-library agnostic API, so the might change again soon. Tell us which constructors are useful for you! (#913)

  • Support base-compat-0.10


  • Streaming endpoint support. (#836)
  • servant Add Servant.API.Modifiers (#873)
  • servant-client Support http-client’s CookieJar (#897 #883)

  • Send Accept header. (#858)


  • Factored out into servant-client-core all the functionality that was independent of the http-client backend.


Other changes

  • Path components are escaped (#696)
  • Req reqPath field changed from String to BS.Builder (#696)
  • Include Req in failure errors (#740)


Breaking changes

There shouldn’t be breaking changes. Released as a part of servant suite.

Other changes

  • Add MonadBase and MonadBaseControl instances for ClientM (#663)

  • client asks for any content-type in Accept contentTypes non-empty list (#615)

  • Add ClientLike class that matches client functions generated using client with client data structure. (#640)

  • Allow direct use of ‘RequestBody’ (#661)

  • Add MonadThrow and MonadCatch instances for ClientM


  • BACKWARDS INCOMPATIBLE: client now returns a ClientM which is a Reader for BasicEnv. BasicEnv comprises the HttpManager and BaseUrl that have had to be passed to each method returned by client.


  • Support GHC 8.0
  • ServantError has an Eq instance now.


  • client no longer takes BaseUrl and Manager arguments. Instead, each function returned by client requires these two arguments.


  • Use the text package instead of String.
  • Support for the HttpVersion, IsSecure, RemoteHost and Vault combinators
  • Added support for path on BaseUrl.
  • client now takes an explicit Manager argument.
  • Use http-api-data instead of Servant.Common.Text
  • Client functions now consider any 2xx successful.
  • Remove matrix params.
  • Added support for Basic authentication
  • Add generalized authentication support via the AuthClientData type family and AuthenticateReq data type


  • The HasClient instance for Delete cts () now does not care at all about content types provided.


  • Delete now is like Get, Post, Put, and Patch and returns a response body
  • Support content-type aware combinators and Accept/Content-type headers
  • Added a lot of tests
  • Support multiple concurrent threads
  • Use ServantError to report Errors instead of String
  • Make the clients for Raw endpoints return the whole Response value (to be able to access response headers for example)
  • Support for PATCH
  • Make () instances expect No Content status code, and not try to decode body.
  • Add support for response headers


  • Add TLS support
  • Add matrix parameter support