Brady Ouren

Servant Patterns for Clients

Edit: Servant has changed its Client api, but this is a post about how to migrate to the new way of deriving clients.

Clients for Free

The unfortunate reality of many of our day jobs is that we are not able to replace existing services with haskell. However, don’t despair! We can still write services which consume our existing services quite easily with Servant.

I’ve picked up some patterns for writing clean haskell code from tfausak and they’ve all come together in a working user client.

data User = User {
    id             :: Int
  , email          :: Text
  , first_name     :: Text
  , last_name      :: Text
  , dob            :: Day
  } deriving (Show, Generic, ToJSON, FromJSON)

data UsersResponse = UsersResponse {
  users :: [User]
  } deriving (Show, Generic, FromJSON, ToJSON)

run action = do
  result <- Either.runEitherT action
  case result of
    Left message -> error (show message)
    Right x -> return x

type UserAPI =
  :<|> GetUser
  :<|> CreateUser
  :<|> UpdateUser
  :<|> DestroyUser

type GetUsers = "user" :> "users"
    :> QueryParam "id" Int
    :> QueryParam "email" Text
    :> Get '[JSON] UsersResponse

type GetUser = "user" :> "users"
    :> Capture "id" Integer
    :> Get '[JSON] User

type CreateUser =  "user" :> "users"
    :> ReqBody '[JSON] User
    :> Post '[JSON] User

type UpdateUser = "user" :> "users"
    :> Capture "id" Integer
    :> ReqBody '[JSON] User
    :> Put '[JSON] User

type DestroyUser = "user" :> "users"
    :> Capture "id" Integer
    :> Delete '[JSON] User

getUsers :<|> getUser :<|> createUser :<|> updateUser :<|> destroyUser =
  client (Proxy :: Proxy UserAPI) (BaseUrl Http "localhost" 5000)

With these client functions defined and assuming you have a user service instance running on port 5000 we can test this code out with stack ghci servant-server servant-client --resolver=lts-3.14

-- > :load Main.hs

-- to get a specific user
run $ getUser 10001035

-- to query on an attribute
run $ getUsers Nothing (Just "")

The only thing to add for production-ready status is a header with auth-tokens, but it really goes to show the ease of generating clients with Servant.