Haskell - Example using Happstack in Haskell

happstack.hs

module Main where

import Control.Monad    (msum)
import Data.Char        (toLower)
import Happstack.Server (FromReqURI(..), dirs, dir, path, seeOther, nullConf, simpleHTTP, toResponse, ok)

--main :: IO ()
--main = simpleHTTP nullConf $ ok (toResponse "Hello, World!")

data Subject = World | Haskell

sayHello :: Subject -> String
sayHello World      = "hello world!"
sayHello Haskell    = "helloooo haskell!"

instance FromReqURI Subject where
    fromReqURI sub =
        case map toLower sub of
            "haskell" -> Just Haskell
            "world"   -> Just World
            _         -> Nothing

main :: IO ()
main = simpleHTTP nullConf $
        msum [ dirs "hello/world"       $ ok "Hello, World!"
             , dirs "goodbye/moon"      $ ok "Goodbye, Moon!"
             , dir "hello" $ path $ \s -> ok $ "Hello, " ++ s 
         , dir "hoho" $ path $ \subject -> ok $ (sayHello subject)
             , seeOther "/hello/world" "/hello/world"
         ]