a quick example of a haskell web service

HaskellTodo.hs

{-# LANGUAGE OverloadedStrings #-}

module HaskellTodo where

import Network.Wai
import Network.HTTP.Types.Status
import Network.Wai.Handler.Warp (run)
import System.IO
import qualified Data.ByteString.Lazy as B
import Data.ByteString.Lazy.Char8 (pack)
import Debug.Trace (trace)

class VisitorCountRepository a where
  addOne :: a -> IO ()
  getCount :: a -> IO (Int)


todoApp :: (VisitorCountRepository r) => r -> Application
todoApp store request respond = trace (show r) $ case r of 
  ("POST", ["add"]) -> do
    addOne store
    respond $ responseLBS status200 [("Content-Type", "text/plain")] "Added One"

  ("GET", []) -> do
    counter <- getCount store
    let msg = "Hello Visitor Number " `B.append` (pack . show $ counter)
    respond $ responseLBS status200 [("Content-Type", "text/plain")] msg

  _ -> respond $ responseLBS status404 [("Content-Type", "text/plain")] "Not Found"

  where r = (requestMethod request, pathInfo request)


main :: IO ()
main = do
  putStrLn "Listening on port 8020"
  run 8020 (todoApp (FileVisitorCountRepository "counts.log"))


data FileVisitorCountRepository = FileVisitorCountRepository String

countOneVisitor :: FileVisitorCountRepository -> IO ()
countOneVisitor (FileVisitorCountRepository path) = withFile path AppendMode (`hPutChar` 'x')

getVisitorCount :: FileVisitorCountRepository -> IO (Int)
getVisitorCount (FileVisitorCountRepository path) = withFile path ReadWriteMode hGetLine >>= (return . length)

instance VisitorCountRepository FileVisitorCountRepository where
  addOne = countOneVisitor
  getCount = getVisitorCount