This example shows how generalized hamlet templates allow the creation of
different types of values. The key component here is the HamletValue typeclass.
Yesod has instances for:

* Html

* Hamlet url (= (url -> [(String, String)] -> String) -> Html)

* GWidget s m ()

This example uses all three. You are of course free in your own code to make
your own instances.

> {-# LANGUAGE QuasiQuotes, TypeFamilies, MultiParamTypeClasses, OverloadedStrings, TemplateHaskell #-}
> import Yesod
> data NewHamlet = NewHamlet
> mkYesod "NewHamlet" [$parseRoutes|/ RootR GET|]
> instance Yesod NewHamlet where approot _ = ""
> type Widget = GWidget NewHamlet NewHamlet
> 
> myHtml :: Html
> myHtml = [$hamlet|<p>Just don't use any URLs in here!|]
>
> myInnerWidget :: Widget ()
> myInnerWidget = do
>     addHamlet [$hamlet|
>   <div #inner>Inner widget
>   #{myHtml}
> |]
>     addCassius [$cassius|
>#inner
>     color: red|]
> 
> myPlainTemplate :: Hamlet NewHamletRoute
> myPlainTemplate = [$hamlet|
> <p
>     <a href=@{RootR}>Link to home
> |]
> 
> myWidget :: Widget ()
> myWidget = [$hamlet|
>     <h1>Embed another widget
>     \^{myInnerWidget}
>     <h1>Embed a Hamlet
>     \^{addHamlet myPlainTemplate}
> |]
> 
> getRootR :: GHandler NewHamlet NewHamlet RepHtml
> getRootR = defaultLayout myWidget
> 
> main :: IO ()
> main = warpDebug 3000 NewHamlet