servant/servant-examples/tutorial/T5.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE OverloadedStrings #-}
module T5 where

import Control.Monad.IO.Class
import Control.Monad.Trans.Either
import Data.Aeson
import GHC.Generics
import Network.Wai
import Servant
import System.Directory

type IOAPI = "myfile.txt" :> Get '[JSON] FileContent

ioAPI :: Proxy IOAPI
ioAPI = Proxy

newtype FileContent = FileContent
  { content :: String }
  deriving Generic

instance ToJSON FileContent

server :: Server IOAPI
server = do
  exists <- liftIO (doesFileExist "myfile.txt")
  if exists
    then liftIO (readFile "myfile.txt") >>= return . FileContent
    else left custom404Err

  where custom404Err = err404 { errBody = "myfile.txt just isn't there, please leave this server alone." }

app :: Application
app = serve ioAPI server