errors

lib/Hazard/UserAPI.hs:65:18:
    Couldn't match expected type ‘b0 :~> EitherT ServantErr IO’
                with actual type ‘(forall a. m0 a -> n0 a) -> m0 :~> n0’
    Probable cause: ‘Nat’ is applied to too few arguments
    In the first argument of ‘(.)’, namely ‘Nat’
    In the expression: Nat . readerToEither'

lib/Hazard/UserAPI.hs:65:24:
    Couldn't match expected type ‘Reader String :~> b0’
                with actual type ‘Reader String a0 -> EitherT ServantErr IO a0’
    Probable cause: ‘readerToEither'’ is applied to too few arguments
    In the second argument of ‘(.)’, namely ‘readerToEither'’
    In the expression: Nat . readerToEither'

UserAPI.hs

-- Copyright (c) 2015 Jonathan M. Lange <jml@mumak.net>
--
-- Licensed under the Apache License, Version 2.0 (the "License");
-- you may not use this file except in compliance with the License.
-- You may obtain a copy of the License at
--
--     http://www.apache.org/licenses/LICENSE-2.0
--
-- Unless required by applicable law or agreed to in writing, software
-- distributed under the License is distributed on an "AS IS" BASIS,
-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-- See the License for the specific language governing permissions and
-- limitations under the License.

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeOperators #-}

module Hazard.UserAPI (userAPI, server) where

import BasicPrelude

import Control.Monad.Reader
import Control.Monad.Trans.Either

import Servant
import Servant.HTML.Blaze (HTML)

import Hazard.Users (UserCreationRequest, User, UserID)


type UserAPI = "users" :> Get '[JSON, HTML] [(UserID, Text)]
               :<|> "users" :> ReqBody '[JSON] UserCreationRequest :> Post '[JSON] User
               :<|> "user" :> Capture "userID" UserID :> Get '[JSON, HTML] User


userAPI :: Proxy UserAPI
userAPI = Proxy


serverT :: ServerT UserAPI (Reader String)
serverT = allUsers :<|> addUser :<|> oneUser


server :: Server UserAPI
server = enter readerToEither serverT


allUsers :: m [(UserID, Text)]
allUsers = undefined


addUser :: UserCreationRequest -> m User
addUser = undefined


oneUser :: UserID -> m User
oneUser = undefined


readerToEither :: Reader String :~> EitherT ServantErr IO
readerToEither = Nat . readerToEither'

readerToEither' :: forall a. Reader String a -> EitherT ServantErr IO a
readerToEither' r = return (runReader r "hi")