Using endo monoid, writer monad and blaze html combinators for creating a web page

endotemplates.hs

{-# Language GeneralizedNewtypeDeriving #-}
{-# Language OverloadedStrings #-}
import Data.Monoid
import Text.Blaze.Html.Renderer.Text
import qualified Text.Blaze.Html5 as H
import qualified Text.Blaze.Html5.Attributes as A
import Text.Blaze.Html ((!), Html)
import Control.Monad.Writer
import Control.Monad.Identity

data Template = Template {
    scripts :: [Html]
  , title :: Html
  , body :: Html
  }

type EndoTemplate = Endo Template
newtype Page m a = Page (WriterT EndoTemplate m a) deriving (Monad, MonadWriter EndoTemplate)
runPage (Page w) = runWriterT w
emptyTemplate = (Template [] "" "")

setTitle :: (H.ToMarkup a, Monad m) => a -> Page m ()
setTitle x = tell . Endo $ \y -> y{title=H.toHtml x}

pushScript :: (H.ToValue a, Monad m) => a -> Page m ()
pushScript url = let
  script = H.script ! A.type_ "application/javascript" ! A.src (H.toValue url) $ mempty
  in tell . Endo $ \y -> y{scripts=script : scripts y}

addSnippet :: Monad m => Html -> Page m ()
addSnippet h = tell . Endo $ \y -> y{body=body y `mappend` h}

defaultLayout :: Page Identity () -> Html
defaultLayout p = let
  (_, endo) = runIdentity $ runPage p
  template = appEndo endo emptyTemplate
  in H.docTypeHtml $ do
    H.head $ do
      H.title (title template)
    H.body $ do
      (body template)
      foldr mappend mempty (scripts template)