On Thu, May 24, 2012 at 03:09:24PM +0400, Dmitriy Matrosov wrote:
and the last one with monadic go function and monadic user-defined folding function:
foldTapeD2 :: (Monoid m) => (a -> State Int m) -> Tape a -> m foldTapeD2 f t = fst $ runState (foldTape (go f) t) 0 where go :: (Monoid m) => (a -> State Int m) -> a -> [State Int m] -> State Int m go f name xs = do cs <- get z <- f name put (cs + 1) foldr (go' (cs + 1)) (return z) xs go' :: (Monoid m) => Int -> State Int m -> State Int m -> State Int m go' cs mx mz = do x <- mx put cs z <- mz put cs return (x `mappend` z)
By the way, for this sort of pattern where you change the state for some subcomputation and then restore it after the subcomputation returns, it can be much nicer to use the Reader monad with the 'local' function instead of the State monad. That might actually go a long way towards making the monadic version nicer to read. =) -Brent