servant-cli

Parse command line arguments into a servant client, from a servant API, using optparse-applicative for parsing, displaying help, and auto-completion.

Hooks into the annotation system used by servant-docs to provide descriptions for parameters and captures.

See example/greet.hs for a sample program.

Getting started

We’re going to break down the example program in example/greet.hs.

Here’s a sample API revolving around greeting and some deep paths, with authentication.

type TestApi =
        Summary "Send a greeting"
           :> "hello"
           :> Capture "name" Text
           :> QueryParam "capital" Bool
           :> Get '[JSON] Text
   :<|> Summary "Greet utilities"
           :> "greet"
           :> ( Get  '[JSON] Int
           :<|> Post '[JSON] NoContent
              )
   :<|> Summary "Deep paths test"
           :> "dig"
           :> "down"
           :> "deep"
           :> Summary "Almost there"
           :> Capture "name" Text
           :> "more"
           :> Summary "We made it"
           :> Get '[JSON] Text

testApi :: Proxy TestApi
testApi = Proxy

To parse this, we can use parseClient, which generates a client action that we can run:

main :: IO ()
main = do
    c <- parseClient testApi (Proxy :: Proxy ClientM) $
            header "greet"
         <> progDesc "Greet API"

    manager' <- newManager defaultManagerSettings
    res      <- runClientM c $
        mkClientEnv manager' (BaseUrl Http "localhost" 8081 "")

    case res of
      Left e  -> throwIO e
      Right r -> putStrLn $ case r of
        Left g                 -> "Greeting: " ++ T.unpack g
        Right (Left (Left  i)) -> show i ++ " returned"
        Right (Left (Right _)) -> "Posted!"
        Right (Right s)        -> s

Note that parseClient and other functions all take InfoMods from optparse-applicative, to customize how the top-level --help is displayed.

The result will be a bunch of nested Eithers for each :<|> branch and endpoint. However, this can be somewhat tedious to handle.

With Handlers

The library also offers parseHandleClient, which accepts nested :<|>s with handlers for each endpoint, mirroring the structure of the API:

main :: IO ()
main = do
    c <- parseHandleClient testApi (Proxy :: Proxy ClientM)
      (header "greet" <> progDesc "Greet API") $
                (\g -> "Greeting: " ++ T.unpack g)
           :<|> ( (\i -> show i ++ " returned")
             :<|> (\_ -> "Posted!")
                )
           :<|> id

    manager' <- newManager defaultManagerSettings
    res      <- runClientM c $
        mkClientEnv manager' (BaseUrl Http "localhost" 8081 "")

    case res of
      Left e  -> throwIO e
      Right r -> putStrLn r

The handlers essentially let you specify how to sort each potential endpoint’s response into a single output value.

Clients that need context

Things get slightly more complicated when your client requires something that can’t be passed in through the command line, such as authentication information (username, password).

type TestApi =
        Summary "Send a greeting"
           :> "hello"
           :> Capture "name" Text
           :> QueryParam "capital" Bool
           :> Get '[JSON] Text
   :<|> Summary "Greet utilities"
           :> "greet"
           :> ( Get  '[JSON] Int
           :<|> BasicAuth "login" Int           -- ^ Adding 'BasicAuth'
             :> Post '[JSON] NoContent
              )
   :<|> Summary "Deep paths test"
           :> "dig"
           :> "down"
           :> "deep"
           :> Summary "Almost there"
           :> Capture "name" Text
           :> "more"
           :> Summary "We made it"
           :> Get '[JSON] Text

For this, you can pass in a context, using parseClientWithContext or parseHandleClientWithContext:

main :: IO ()
main = do
    c <- parseHandleClientWithContext
      testApi
      (Proxy :: Proxy ClientM)
      (getPwd :& RNil)
      (header "greet" <> progDesc "Greet API") $
                (\g -> "Greeting: " ++ T.unpack g)
           :<|> ( (\i -> show i ++ " returned")
             :<|> (\_ -> "Posted!")
                )
           :<|> id

    manager' <- newManager defaultManagerSettings
    res      <- runClientM c $
        mkClientEnv manager' (BaseUrl Http "localhost" 8081 "")

    case res of
      Left e  -> throwIO e
      Right r -> putStrLn r
  where
    getPwd :: ContextFor ClientM (BasicAuth "login" Int)
    getPwd = GenBasicAuthData . liftIO $ do
      putStrLn "Authentication needed for this action!"
      putStrLn "Enter username:"
      n <- BS.getLine
      putStrLn "Enter password:"
      p <- BS.getLine
      pure $ BasicAuthData n p