-- This is an example from the "Learn You A Haskell" tuturial.
-- The story is that there is a guy walking with a Pole where birds
-- can land on the left and right. If the difference between birds
-- on the left and right gets too big he falls off the rope. This
-- is indicated by Nothing.

import Debug.Hoed(runO,observe,observe',Identifier(..),(*>>=),(>>==),(>>=*))

type Birds = Int
type Pole  = (Birds,Birds)

landLeft :: Birds -> Identifier -> (Pole -> Maybe Pole, Int)
landLeft n d
  = let (f,i) = observe' "landLeft" d (\n' p' -> {-# SCC "landLeft" #-} landLeft' n' p')
    in (f n, i)
landLeft' n (left,right)
  | abs ((left + n) - right) < 4 = Just (left + n, right)
  | otherwise                    = Nothing

landRight :: Birds -> Identifier -> (Pole -> Maybe Pole, Int)
landRight n d 
  = let (f,i) = observe' "landRight" d (\n' p' -> {-# SCC "landRight" #-} landRight' n' p')
    in  (f n, i)
landRight' n (left,right)
  | abs (left - (right + n)) < 4 = Just (left, right + n)
  | otherwise                    = Nothing
        where x + y = x Prelude.+ (abs y)

walk :: Maybe Pole
walk = observe "walk" $ {-# SCC "walk" #-}
        return (0,0) *>>= landRight 1   >>== landLeft 1
          >>== landRight 2 >>== landRight (-1) >>=* landRight 1

main = runO $ print walk