What's wrong with this?

Lib.hs

{-# LANGUAGE DataKinds       #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeOperators   #-}
module Lib
    ( startApp
    , app
    ) where

import Data.Aeson
import Data.Aeson.TH
import Network.Wai
import Network.Wai.Handler.Warp
import Servant

data User = User
  { userId        :: Int
  , userFirstName :: String
  , userLastName  :: String
  } deriving (Eq, Show)

$(deriveJSON defaultOptions ''User)

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

startApp :: IO ()
startApp = run 8080 app

app :: Application
app = serve api server

api :: Proxy API
api = Proxy

server :: Server API
server = return users

users :: Integer -> Handler [User]
users id = return $ [ User 1 "Isaac" "Newton", User 2 "Albert" "Einstein" ]

    • Couldn't match type ‘Integer -> Handler [User]’
                     with ‘transformers-0.5.2.0:Control.Monad.Trans.Except.ExceptT
                             ServantErr IO [User]’
      Expected type: Server API
        Actual type: Integer -> Integer -> Handler [User]
    • Possible cause: ‘return’ is applied to too many arguments
      In the expression: return users
      In an equation for ‘server’: server = return users