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In this gist we will first show that we can beat the arc challenge (, and then build the library that shows how we did it. This gist is Literate Haskell and is of course executable.

Let's start with some imports (for now, you can ignore these)

{-# LANGUAGE GADTs, TypeSynonymInstances #-}
module ArcChallenge where
import Control.Applicative
import Control.Applicative.Error (Failing (..))
import Control.Monad (ap)
import Control.Monad.Identity (Identity (..))
import Control.Monad.Reader
import Control.Concurrent.MVar
import Control.Monad.Trans
import Happstack.Server.SimpleHTTP hiding (Web)
import Text.XHtml.Strict (toHtml)
import qualified Data.ByteString.Lazy.Char8 as B
import qualified Data.Map as M
import qualified Text.XHtml.Strict as X
import qualified Text.XHtml.Strict.Formlets as F
import qualified Happstack.Server.SimpleHTTP as H

Here's the actual program for the arc challenge. It's only 13 nodes total:

arc = do  name <- input
          link "click here"
          display $ "you said: " ++ name

The nice thing about Haskell is that it has type inference. We made |input| polymorphic, so if we want input to return integers, we can do that.

The following program asks for your name, asks for two integers, shows a link "click here", shows the sum of the two integers and finally shows your name again. If the two integers x and y couldn't be parsed as integers, you get an error message and a continue link so that you can try again.

arc2 = do  name  <- input
           (x,y) <- input
           link "click here"
           display $ add x y
           display $ "you said:" ++ name

The method above consists of 25 nodes total.

Here's how you run all this:

main = runServer 8000 (M.fromList [("arc", arc), ("arc2", arc2)])

That's all there is to it. Our library is just a proof of concept, but it's included below. It is far from finished, but the interface of a complete library should remain the same. If you want to know how it is currently implemented, keep on reading.

For the sake of presentation, we consider the request body to be key/value pairs.

type RequestBody = [(String,String)]

In our module, a page is either a form of |a| or a basic webpage displaying something. The first parameters for |Form| is the rendering of the form, the second parameter is the parsing (which happens only after the user has submitted the data). Finally, we include a |Link| constructor that displays just a link.

data Page a where
  Form     :: X.Html -> (RequestBody -> Failing a) -> Page a
  Display  :: X.Html -> Page ()
  Link     :: String -> Page ()

A web continuation is just a function from |Request| to |Result|:

newtype Web a = Web {runWeb :: NextPage -> RequestBody -> Result a}

When running a web continuation, either the computation is completely finished, or it displays a page with a continuation.

data Result a  =  Done a
               |  Problem String (Web a)
               |  Step X.Html (Web a)
type NextPage = String

From a single page we can calculate a function that ignores the request and produces a |Result|:

runPage :: Page a ->  NextPage -> RequestBody -> Result a
runPage f@(Form msg parse) np _  = Step (makeForm msg) (Web (const (formResult (web f) . parse)))
runPage   (Display msg)    np _  = Step (continue msg      np "Continue") (return ())
runPage   (Link msg)       np _  = Step (continue X.noHtml np msg) (return ())

We can derive two smart constructors that lift a |Page| directly into the |Web| type:

display  :: X.HTML h => h -> Web ()
display  = web . Display . toHtml
input    :: DefaultForm a => Web a
input    = web . uncurry Form . runForm $ form
link     :: String -> Web ()
link     = web . Link

The |Web| newtype can easily be made an instance of |Monad|:

instance Monad Web where
  return   = Web . const . const . Done
  l >>= r  = Web $ \np req -> case (runWeb l np req) of
               Done x       -> runWeb (r x) np req
               Step msg l'  -> Step msg (l' >>= r) 
               Problem msg l' -> Problem msg (l' >>= r)

From a result we can calcute some HTML and possibly a continuation:

handleResult :: NextPage -> Result () -> (X.Html, Maybe (Web ()))
handleResult np (Done ())           = (toHtml "Done", Nothing)
handleResult np (Step msg cont)     = (msg, Just cont)
handleResult np (Problem msg retry) = (continue ("Problem: " ++ msg) np "Continue", Just retry)

Finally, some code to abstract working with forms. This makes use of the formlets library.

type Form a = F.XHtmlForm Identity a
runForm :: Form a -> (X.Html, [(String,String)] -> Failing a)
runForm f = let (_, Identity html, _) = F.runFormState [] f
                parse env = x where (Identity x, _, _) = F.runFormState (map (fmap Left) env) f
            in (html, parse)
class DefaultForm i           where form :: Form i
instance DefaultForm String  where form = F.input Nothing
instance DefaultForm Integer where form = F.inputInteger Nothing
instance (DefaultForm a, DefaultForm b) => DefaultForm (a,b) where 
  form = (,) <$> form <*> form
instance X.HTML Integer where toHtml = toHtml . show
instance Applicative Identity where pure = return; (<*>) = ap;

The |Env| maps a URL to a continuation. In a 'real' version of this library, it should keep continuations in the user's session.

type Env = M.Map String (Web ())
run :: Env -> String -> [(String, String)] -> (X.Html, Env) 
run env page reqBody = case M.lookup page env of
                       Nothing   -> (pageNotFound, env)
                       Just cont ->
                         let np = "/" ++ page
                             result        = runWeb cont np reqBody
                             (html, cont') = handleResult np result
                             env' = maybe (M.delete page env) (\x -> M.insert page x env) cont'
                         in (html, env')
createServerPart :: Env -> IO (ServerPart Response)
createServerPart e = do env <- newMVar e
                        return $ ServerPartT $ handle env
handle :: MVar Env -> ReaderT Request (WebT IO) Response
handle env = do req <- ask
                let contId     = foldr const "/" (rqPaths req)
                    formInputs = map (\(k,v) -> (k, B.unpack $ inputValue v)) $ rqInputs req
                e <- liftIO $ takeMVar env
                let (html, e') = run e contId formInputs
                liftIO $ putMVar env e'
                return $ toResponse html
runServer :: Int -> Env -> IO ()
runServer p env = do
    serverPart <- createServerPart env
    putStrLn $ "Running server at" ++ show  p
    simpleHTTP (nullConf { port = p }) serverPart

Some helper functions:

pageNotFound = X.toHtml "Page not found."
continue :: X.HTML x => x -> NextPage -> String -> X.Html
continue x np linkText = x X.+++ X.+++ (ahref np (toHtml linkText))
add :: Integer -> Integer -> Integer
add = (+)
ahref url text = X.anchor X.! [X.href url] X.<< text
makeForm f = X.form X.! [X.method "POST"] X.<< (f X.+++ X.submit "submit" "submit")
formResult frm (Success a)  = Done a
formResult frm (Failure xs) = Problem (unlines xs) frm
web = Web . runPage
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