[web-devel] Fail-back monad

Alberto G. Corona agocorona at gmail.com
Wed Mar 28 02:34:29 CEST 2012

Hi Haskellers.

In my package MFlow [1] I program an entire web  navigation in a
single procedure. That happened  in the good-old WASH web application
The problem is the back button in the Browser.
To go back in the code to the previous interactions when the data
input does not match the expected because the user pressed the back
button one or more times, i came across this Monad specimen,: that
solves the problem.

data FailBack a = BackPoint a   -- will go back to this point
                             | NoBack a       . -- Normal outcome
                             | GoBack            -- "exception":: must
go to the last backPoint

newtype BackT m a = BackT { runBackT :: m (FailBack a ) }

-- this monad nas a loop

instance Monad m => Monad (BackT  m) where
    fail   _ = BackT $ return GoBack
    return x = BackT . return $ NoBack x
    x >>= f  = BackT $ loop
     loop = do
        v <- runBackT x
        case v of
            NoBack y  -> runBackT (f y)  -- business as usual
            BackPoint y  -> do
                 z <- runBackT (f y)
                 case z of
                  GoBack  -> loop          -- if x was a backpoint,
then redirects the flow to this backpoint
                  other -> return other
            GoBack -> return  GoBack --propagate the signal back

 This monad does not perform exploration of alternatives as is the
case of MonadPlus instances. It does not perform the kind of
backtracking of "nondeterministic"  three navigations in the Prolog
style. It just go back to the last point where the computation can
restart again in a  sequence of actions.

In this example:

liftBackPoint f= BackT $ f >>= \x -> return $ BackPoint x

test= runBackT $ do
       lift $ print "will not return back here"
        liftBackPoint $ print "will return here"
        n2  &lt;- lift $ getLine
        lift $ print "second input"
        n3  &lt;- lift $  getLine
        if n3 == "back"
                   then  fail ""
                   else lift $ print  $ n2++n3

Whenever the second input is "back" The procedure will go back to
where liftBackPoint is. Otherwise, it will return the concatenation of
the two inputs. If the underlying monad is an instance of MonadState,
it can transport the state that caused the failure to the backpoint.

Are there something similar? May it be functionally equivalent to
something simpler or with more grounds?
 I looked at some exception monads out there, but they did not seems
to share the same idea

[1] http://haskell-web.blogspot.com.es/2012/02/web-application-server-with-stateful.html

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