Rendering Javascript from a Handler

To serve javascript with interpolated values we need to create a couple of instances of two Yesod classes, HasReps and ToContent.

Here is how to do so:

import Yesod.Routes.Class (Route)

newtype RepJavascript = RepJavascript Content
instance ToTypedContent RepJavascript where
  toTypedContent (RepJavascript c) = TypedContent typeJavascript c
instance HasContentType RepJavascript where
  getContentType _ = typeJavascript
deriving instance ToContent RepJavascript
instance ToContent Javascript where
  toContent = toContent . renderJavascript

juliusToRepJavascript :: (ToContent a, MonadHandler m) => 
                     => ((Route (HandlerSite m) -> [(Text,Text)] -> Text) -> a) -> m TypedContent
juliusToRepJavascript j =  do
  render <- getUrlRenderParams
  return $ toTypedContent $ RepJavascript (toContent $ j render)

Then you can use it like this:

getJavascript :: Handler TypedContent                      
getJavascript = do                                            
  juliusToRepJavascript $(juliusFile "templates/myfile.julius")

for version prior to Yesod 1.2:

newtype RepJavascript = RepJavascript Content

instance HasReps RepJavascript where
    chooseRep (RepJavascript c) _ = return (typeJavascript, c)
instance ToContent Javascript where
    toContent = toContent . renderJavascript

juliusToContent :: Monad mo
            => JavascriptUrl (Route master) -> GGHandler sub master mo Content
juliusToContent j = do
    render <- getUrlRenderParams
    return $ toContent $ j render

juliusToRepJavascript :: Monad mo
            => JavascriptUrl (Route master) -> GGHandler sub master mo RepJavascript
juliusToRepJavascript = liftM RepJavascript . juliusToContent

You can use this in your handler like so:

getHandlerR = do
    juliusToRepJavascript $(juliusFile "myfile")