Servant CSV download sample

servant_download_csv.hs

{-# LANGUAGE DataKinds                  #-}
{-# LANGUAGE DeriveGeneric              #-}
{-# LANGUAGE FlexibleInstances          #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses      #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE ScopedTypeVariables        #-}
{-# LANGUAGE TypeOperators              #-}

import Control.Monad.IO.Class             -- liftIO
import Control.Monad.Trans.Except         -- Handler
import Data.Aeson.Types                   -- JSON
import Data.Aeson.Parser                  -- JSON
import GHC.Generics
import Network.Wai.Handler.Warp
import Servant
import qualified Network.HTTP.Media          as M
import Text.CSV
import qualified Data.ByteString.Lazy.Char8  as C

type MyAPI =  "csv" :> Capture "file" String :> Get '[CSV'] CSV

data CSV'

instance Accept CSV' where
  contentType _ = "application" M.// "csv"

instance MimeRender CSV' CSV where
  mimeRender _ val = C.pack $ printCSV val 

csv' :: String -> Handler CSV
csv' f = do 
  ret <- liftIO $ parseCSVFromFile ("./data/" ++ f ++ ".csv")
  case ret of 
    Left  l -> throwError err400 {errBody="parse error"} 
    Right r -> return r 

myServer :: Server MyAPI
myServer = csv'

myProxy :: Proxy MyAPI
myProxy = Proxy

myApp :: Application
myApp = serve myProxy myServer

main :: IO ()
main = run 8081 myApp