This is the complete source code for a client-server web application that estimates the strength of a password the user enters, updating the estimate after every keystroke.

Note the total lack of html, javascript, routes, callbacks, or any of the usual web cruft. It's using the threepenny-gui haskell library and a little bit of FRP. Pretty awesome library, although the GUIs it renders only work when javascript is enabled.

  import qualified Graphics.UI.Threepenny as UI
  import Graphics.UI.Threepenny.Core
  import Text.Password.Strength (estimate)

  main = startGUI defaultConfig $ \window -> do
        prompt <- UI.input
        estimatedisplay <- UI.string ""
        getBody window #+ [ element prompt, element estimatedisplay ]
        promptIn <- stepper "" $ UI.valueChange prompt
        let showEstimate = show . (`estimate` []) <$> promptIn
        element estimatedisplay # sink value showEstimate
        return ()