Querying an API

While defining handlers that serve an API has a lot to it, querying an API is simpler: we do not care about what happens inside the webserver, we just need to know how to talk to it and get a response back. That said, we usually have to write the querying functions by hand because the structure of the API isn’t a first class citizen and can’t be inspected to generate the client-side functions.

servant however has a way to inspect APIs, because APIs are just Haskell types and (GHC) Haskell lets us do quite a few things with types. In the same way that we look at an API type to deduce the types the handlers should have, we can inspect the structure of the API to derive Haskell functions that take one argument for each occurrence of Capture, ReqBody, QueryParam and friends (see the tutorial introduction for an overview). By derive, we mean that there’s no code generation involved - the functions are defined just by the structure of the API type.

The source for this tutorial section is a literate Haskell file, so first we need to have some language extensions and imports:

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

module Client where

import Data.Aeson
import Data.Proxy
import GHC.Generics
import Network.HTTP.Client (newManager, defaultManagerSettings)
import Servant.API
import Servant.Client

Also, we need examples for some domain specific data types:

data Position = Position
  { xCoord :: Int
  , yCoord :: Int
  } deriving (Show, Generic)

instance FromJSON Position

newtype HelloMessage = HelloMessage { msg :: String }
  deriving (Show, Generic)

instance FromJSON HelloMessage

data ClientInfo = ClientInfo
  { clientName :: String
  , clientEmail :: String
  , clientAge :: Int
  , clientInterestedIn :: [String]
  } deriving Generic

instance ToJSON ClientInfo

data Email = Email
  { from :: String
  , to :: String
  , subject :: String
  , body :: String
  } deriving (Show, Generic)

instance FromJSON Email

Enough chitchat, let’s see an example. Consider the following API type from the previous section:

type API = "position" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] Position
      :<|> "hello" :> QueryParam "name" String :> Get '[JSON] HelloMessage
      :<|> "marketing" :> ReqBody '[JSON] ClientInfo :> Post '[JSON] Email

What we are going to get with servant-client here is three functions, one to query each endpoint:

position :: Int -- ^ value for "x"
         -> Int -- ^ value for "y"
         -> ClientM Position

hello :: Maybe String -- ^ an optional value for "name"
      -> ClientM HelloMessage

marketing :: ClientInfo -- ^ value for the request body
          -> ClientM Email

Each function makes available as an argument any value that the response may depend on, as evidenced in the API type. How do we get these functions? By calling the function client. It takes one argument:

  • a Proxy to your API,
api :: Proxy API
api = Proxy

position :<|> hello :<|> marketing = client api

client api returns client functions for our entire API, combined with :<|>, which we can pattern match on as above. You could say client “calculates” the correct type and number of client functions for the API type it is given (via a Proxy), as well as their implementations.

If you have an EmptyAPI in your API, servant-client will hand you a value of type EmptyClient in the corresponding slot, where data EmptyClient = EmptyClient, as a way to indicate that you can’t do anything useful with it.

type API' = API :<|> EmptyAPI

api' :: Proxy API'
api' = Proxy

(position' :<|> hello' :<|> marketing') :<|> EmptyClient = client api'
-- | URI scheme to use
data Scheme =
    Http  -- ^ http://
  | Https -- ^ https://
  deriving

-- | Simple data type to represent the target of HTTP requests
--   for servant's automatically-generated clients.
data BaseUrl = BaseUrl
  { baseUrlScheme :: Scheme -- ^ URI scheme to use
  , baseUrlHost :: String   -- ^ host (eg "haskell.org")
  , baseUrlPort :: Int      -- ^ port (eg 80)
  , baseUrlPath :: String   -- ^ path (eg "/a/b/c")
  }

That’s it. Let’s now write some code that uses our client functions.

queries :: ClientM (Position, HelloMessage, Email)
queries = do
  pos <- position 10 10
  message <- hello (Just "servant")
  em  <- marketing (ClientInfo "Alp" "[email protected]" 26 ["haskell", "mathematics"])
  return (pos, message, em)

run :: IO ()
run = do
  manager' <- newManager defaultManagerSettings
  res <- runClientM queries (mkClientEnv manager' (BaseUrl Http "localhost" 8081 ""))
  case res of
    Left err -> putStrLn $ "Error: " ++ show err
    Right (pos, message, em) -> do
      print pos
      print message
      print em

Here’s the output of the above code running against the appropriate server:

Position {xCoord = 10, yCoord = 10}
HelloMessage {msg = "Hello, servant"}
Email {from = "[email protected]", to = "[email protected]", subject = "Hey Alp, we miss you!", body = "Hi Alp,\n\nSince you've recently turned 26, have you checked out our latest haskell, mathematics products? Give us a visit!"}

The types of the arguments for the functions are the same as for (server-side) request handlers.

Changing the monad the client functions live in

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. Consider the following trivial API:

type HoistClientAPI = Get '[JSON] Int :<|> Capture "n" Int :> Post '[JSON] Int

hoistClientAPI :: Proxy HoistClientAPI
hoistClientAPI = Proxy

We already know how to derive client functions for this API, and as we have seen above they all return results in the ClientM monad when using servant-client. However, ClientM rarely (or never) is the actual monad we need to use the client functions in. Sometimes we need to run them in IO, sometimes in a custom monad stack. hoistClient is a very simple solution to the problem of “changing” the monad the clients run in.

hoistClient
  :: HasClient ClientM api   -- we need a valid API
  => Proxy api               -- a Proxy to the API type
  -> (forall a. m a -> n a)  -- a "monad conversion function" (natural transformation)
  -> Client m api            -- clients in the source monad
  -> Client n api            -- result: clients in the target monad

The “conversion function” argument above, just like the ones given to hoistServer, must be able to turn an m a into an n a for any choice of type a.

Let’s see this in action on our example. We first derive our client functions as usual, with all of them returning a result in ClientM.

getIntClientM :: ClientM Int
postIntClientM :: Int -> ClientM Int
getIntClientM :<|> postIntClientM = client hoistClientAPI

And we finally decide that we want the handlers to run in IO instead, by “post-applying” runClientM to a fixed client environment.

-- our conversion function has type: forall a. ClientM a -> IO a
-- the result has type:
-- Client IO HoistClientAPI = IO Int :<|> (Int -> IO Int)
getClients :: ClientEnv -> Client IO HoistClientAPI
getClients clientEnv
  = hoistClient hoistClientAPI
                ( fmap (either (error . show) id)
                . flip runClientM clientEnv
                )
                (client hoistClientAPI)

Querying Streaming APIs.

Consider the following streaming API type:

type StreamAPI = "positionStream" :> StreamGet NewlineFraming JSON (ResultStream Position)

Note that when we declared an API to serve, we specified a StreamGenerator as a producer of streams. Now we specify our result type as a ResultStream. With types that can be used both ways, if appropriate adaptors are written (in the form of ToStreamGenerator and BuildFromStream instances), then this asymmetry isn’t necessary. Otherwise, if you want to share the same API across clients and servers, you can parameterize it like so:

type StreamAPI f = "positionStream" :> StreamGet NewlineFraming JSON (f Position)
type ServerStreamAPI = StreamAPI StreamGenerator
type ClientStreamAPI = StreamAPI ResultStream

In any case, here’s how we write a function to query our API:

streamAPI :: Proxy StreamAPI
streamAPI = Proxy

posStream :: ClientM (ResultStream Position)

posStream = client streamAPI

And here’s how to just print out all elements from a ResultStream, to give some idea of how to work with them.

printResultStream :: Show a => ResultStream a -> IO ()
printResultStream (ResultStream k) = k $ \getResult ->
       let loop = do
            r <- getResult
            case r of
                Nothing -> return ()
                Just x -> print x >> loop
       in loop

The stream is parsed and provided incrementally. So the above loop prints out each result as soon as it is received on the stream, rather than waiting until they are all available to print them at once.

You now know how to use servant-client!