HTTP Fibonacci service in Haskell

http_wai_fib.hs

{-# LANGUAGE OverloadedStrings #-}
import Char
import Control.Monad (join)
import Network.Wai
import Network.HTTP.Types
import Network.Wai.Handler.Warp (run)
import Data.ByteString.Lazy.Char8 ()  -- Just for an orphan instance
import Control.Monad.IO.Class (liftIO)
import Data.Conduit
import Data.String.Utils ()
import Data.ByteString as BS (ByteString, putStrLn)
import Data.ByteString.Char8 as B (pack, unpack)
import Data.ByteString.Lazy.Char8 as LBS (pack, unpack)
import Data.Text as T (intercalate, pack, unpack)

headers :: ResponseHeaders
headers = [("Content-Type", "text/plain")]

app :: Application
app req
    | rawPathInfo req == "/fib" = fibHandler req
    | otherwise = notFoundHandler

fibHandler :: Request -> ResourceT IO Response
fibHandler req = do
    case getFibIndex (queryString req) of
        Nothing -> return $ responseLBS
            badRequest400
            headers
            "bad request"
        Just n -> return $ responseLBS
            ok200
            headers
            (LBS.pack $ show $ fib n)

fib :: Int -> Int
fib n = fibs !! n

fibs :: [Int]
fibs = 1 : 2 : zipWith (+) fibs (tail fibs)

getFibIndex :: Query -> Maybe Int
getFibIndex query = Control.Monad.join (lookup "n" query) >>=  extractInt

isInteger :: BS.ByteString -> Bool
isInteger bs = isInteger' $ B.unpack bs
    where isInteger' xs = all isDigit xs

extractInt :: BS.ByteString -> Maybe Int
extractInt s = case isInteger s of
    True -> Just $ read $ B.unpack s
    False -> Nothing

notFoundHandler :: ResourceT IO Response
notFoundHandler = return $ responseLBS
    status404
    headers
    "Not found"

main :: IO ()
main = do
    BS.putStrLn $ "http://localhost:8080/"
    run 8080 $ app