Basic yesod app from the first few chapters, just for including in a post.

links.hs

{-# LANGUAGE TypeFamilies, QuasiQuotes, MultiParamTypeClasses,
  TemplateHaskell, OverloadedStrings #-}

import Yesod

-- The app is parameterised on a custom datatype; this can also
-- contain other info of course:
data Links = Links

-- Routes, using template-haskell.  (I find "Links" as a string to be
-- a bit icky, but otherwise it works quite nicely). 'R' as a suffix
-- is just a convention, but means "Resource".  Note that HomeR, etc
-- become members of another data type created automatically.
mkYesod "Links" [parseRoutes|
/ HomeR GET
/page1 Page1R GET
/page2 Page2R GET
|]

-- Instantiate the Yesod interface for the app data type: the defaults
-- are fine here, but there's heaps of methods you can override to
-- specify things like the URL root, customise the default layout,
-- etc.
instance Yesod Links

-- Route handling.  The mian thing here -- the main thing in yesod,
-- really -- is that because it's (template-)haskell again these are
-- checked at compile-time, so they're type-safe.  Also, get*R is
-- generated by mkYesod above.
getHomeR, getPage1R, getPage2R :: Handler RepHtml
getHomeR = defaultLayout [whamlet|<a href=@{Page1R}>Go to page 1!|]
getPage1R = defaultLayout [whamlet|<a href=@{Page2R}>Go to page 2!|]
getPage2R = defaultLayout [whamlet|<a href=@{HomeR}>Go home!|]

main :: IO ()
main = warpDebug 3000 Links