144 lines
5 KiB
Text
144 lines
5 KiB
Text
# Querying an API
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While defining handlers that [serve an API](Server.lhs) 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.
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**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`
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and friends (see [the tutorial introduction](ApiType.lhs) 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.
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The source for this tutorial section is a literate Haskell file, so first we need to have some language extensions and imports:
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``` haskell
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{-# LANGUAGE DataKinds #-}
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{-# LANGUAGE DeriveGeneric #-}
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{-# LANGUAGE TypeOperators #-}
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module Client where
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import Data.Aeson
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import Data.Proxy
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import GHC.Generics
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import Network.HTTP.Client (newManager, defaultManagerSettings)
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import Servant.API
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import Servant.Client
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```
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Also, we need examples for some domain specific data types:
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``` haskell
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data Position = Position
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{ xCoord :: Int
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, yCoord :: Int
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} deriving (Show, Generic)
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instance FromJSON Position
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newtype HelloMessage = HelloMessage { msg :: String }
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deriving (Show, Generic)
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instance FromJSON HelloMessage
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data ClientInfo = ClientInfo
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{ clientName :: String
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, clientEmail :: String
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, clientAge :: Int
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, clientInterestedIn :: [String]
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} deriving Generic
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instance ToJSON ClientInfo
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data Email = Email
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{ from :: String
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, to :: String
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, subject :: String
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, body :: String
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} deriving (Show, Generic)
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instance FromJSON Email
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```
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Enough chitchat, let's see an example. Consider the following API type from the previous section:
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``` haskell
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type API = "position" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] Position
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:<|> "hello" :> QueryParam "name" String :> Get '[JSON] HelloMessage
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:<|> "marketing" :> ReqBody '[JSON] ClientInfo :> Post '[JSON] Email
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:<|> EmptyAPI
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```
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What we are going to get with **servant-client** here is three functions, one to query each endpoint:
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``` haskell
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position :: Int -- ^ value for "x"
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-> Int -- ^ value for "y"
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-> ClientM Position
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hello :: Maybe String -- ^ an optional value for "name"
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-> ClientM HelloMessage
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marketing :: ClientInfo -- ^ value for the request body
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-> ClientM Email
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```
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Each function makes available as an argument any value that the response may
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depend on, as evidenced in the API type. How do we get these functions? By calling
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the function `client`. It takes one argument:
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- a `Proxy` to your API,
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``` haskell
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api :: Proxy API
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api = Proxy
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position :<|> hello :<|> marketing :<|> EmptyClient = client api
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```
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`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.
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``` haskell ignore
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-- | URI scheme to use
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data Scheme =
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Http -- ^ http://
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| Https -- ^ https://
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deriving
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-- | Simple data type to represent the target of HTTP requests
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-- for servant's automatically-generated clients.
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data BaseUrl = BaseUrl
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{ baseUrlScheme :: Scheme -- ^ URI scheme to use
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, baseUrlHost :: String -- ^ host (eg "haskell.org")
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, baseUrlPort :: Int -- ^ port (eg 80)
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, baseUrlPath :: String -- ^ path (eg "/a/b/c")
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}
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```
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That's it. Let's now write some code that uses our client functions.
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``` haskell
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queries :: ClientM (Position, HelloMessage, Email)
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queries = do
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pos <- position 10 10
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message <- hello (Just "servant")
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em <- marketing (ClientInfo "Alp" "alp@foo.com" 26 ["haskell", "mathematics"])
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return (pos, message, em)
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run :: IO ()
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run = do
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manager <- newManager defaultManagerSettings
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res <- runClientM queries (ClientEnv manager (BaseUrl Http "localhost" 8081 ""))
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case res of
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Left err -> putStrLn $ "Error: " ++ show err
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Right (pos, message, em) -> do
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print pos
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print message
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print em
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```
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Here's the output of the above code running against the appropriate server:
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```
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Position {xCoord = 10, yCoord = 10}
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HelloMessage {msg = "Hello, servant"}
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Email {from = "great@company.com", to = "alp@foo.com", 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!"}
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```
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The types of the arguments for the functions are the same as for (server-side) request handlers. You now know how to use **servant-client**!
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