Cookbook File upload

This example was orginal posted by Michael Snoyman, see http://permalink.gmane.org/gmane.comp.lang.haskell.yesod/323

{-# LANGUAGE OverloadedStrings, QuasiQuotes, TemplateHaskell,
	     TypeFamilies, MultiParamTypeClasses, FlexibleContexts #-}
import Yesod
import Control.Applicative
 
data File = File
 
instance Yesod File where
    approot _ = ""
instance RenderMessage File FormMessage where
    renderMessage _ _ = defaultFormMessage
 
mkYesod "File" [parseRoutes|
/ RootR GET POST
|]
 
form :: Html -> Form File File (FormResult (FileInfo, Maybe FileInfo), Widget)
form = renderDivs $ (,) <$> fileAFormReq "File" <*> fileAFormOpt "Optional file"
 
getRootR :: Handler RepHtml
getRootR = do
    ((_, widget), enctype) <- runFormPost form
    defaultLayout [whamlet|
<form method=post enctype=#{enctype}>
    ^{widget}
    <p>
	<input type=submit>
|]
 
postRootR :: Handler RepHtml
postRootR = do
    ((res, widget), enctype) <- runFormPost form
    defaultLayout $ case res of
	FormSuccess x -> [whamlet|<p>#{show x}|]
	_ -> [whamlet|
<form method=post enctype=#{enctype}>
    ^{widget}
    <p>
	<input type=submit>
|]
 
main :: IO ()
main = warpDebug 3000 File