monad-example.hs

walktree :: Tree -> Counter Tree
walktree (Node left right) = do
	left' <- walktree left
	right' <- walktree right
	return (Node left' right')
--walktree (Node left right) =
--	walktree left >>= \left' ->
--	walktree right >>= \right' ->
--	return (Node left' right')
walktree (NormalLeaf x) = return x
walktree (VariableLeaf x) = do
	cnt <- nextCounter
	return (VariableLeaf cnt)
nextCounter :: Counter Integer
let nextCounter = do
	cnt <- get
	put (cnt+1)
	return cnt

type Counter t = cnt -> (t, Integer)
--type Counter t = State Integer t
instance Monad Counter where
	return x = (x, 0)
	-- (>>=) = bind
	(>>=) :: Counter t -> (t -> Counter u) -> Counter u
	(>>=) m f = \cnt ->
		let (result1, cnt1) = m cnt
		(f result1) cnt1

get :: Counter Integer
get = \cnt -> (cnt, cnt)

put :: Integer -> Counter ()
put cnt = \oldcnt -> ((), cnt)

runCounter :: Counter t -> t
runCounter m = let (result, cnt) = m 0 in result

runCounter (walktree mytree) :: Tree

--main :: IO ()
--main = do
--	something
--	main