Spotify Client Credentials Flow with Haskell

Lib.hs

{-# LANGUAGE OverloadedStrings #-}

module Lib
  ( someFunc
  ) where

import Network.HTTP.Client
import Network.HTTP.Client.TLS
import qualified Data.ByteString.Char8 as B8 (pack)
import qualified Data.ByteString.Lazy.Char8 as L8
import System.Environment (lookupEnv)
import Data.Aeson
import Text

data Auth = Auth {token :: Text} deriving (Show)

instance FromJSON Auth where
  parseJSON = withObject "auth" $ \o ->
    Auth <$> o .: "access_token"

instance ToJSON Auth where
  toJSON a = object ["access_token" .= token a]

someFunc :: IO ()
someFunc = do
  putStrLn "Search for an artist on Spotify."
  line <- getLine
  runSpotify

runSpotify :: IO ()
runSpotify = do
  secret <- lookupEnv "SPOTIFY_SECRET" 
  case secret of
    Just s -> do
      auth <- authenticate s
      case auth of
        Just a -> putStrLn $ show a
        Nothing -> putStrLn "No records"
    Nothing -> putStrLn "SPOTIFY_SECRET couldn't be retrieved from environmental variables."

authenticate :: String -> IO (Maybe Auth)
authenticate s = do
  manager <- newManager tlsManagerSettings
  initReq <- parseRequest "https://accounts.spotify.com/api/token"
  let requestObject = [("grant_type", "client_credentials")]
  let req = urlEncodedBody requestObject initReq
              { method = "POST"
              , requestHeaders =
                [ ("Authorization", B8.pack s) 
                , ("Content-Type", "application/x-www-form-urlencoded")
                ]
              }
  response <- httpLbs req manager
  return $ decode $ responseBody response