Created
September 8, 2012 11:29
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use State monad and WriterT to implement ListZipper
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import Control.Monad | |
import Control.Monad.State | |
import Control.Monad.Trans.Writer | |
type ListZipper a = ([a], [a]) | |
-- move focus forward, put previous root into breadcrumbs | |
goForward :: ListZipper a -> ListZipper a | |
goForward (x:xs, bs) = (xs, x:bs) | |
-- move focus back, restore previous root from breadcrumbs | |
goBack :: ListZipper a -> ListZipper a | |
goBack (xs, b:bs) = (b:xs, bs) | |
-- wrap goForward so it becomes a State | |
goForwardW :: WriterT [String] (State (ListZipper a)) [a] | |
goForwardW = lift $ state stateTrans where | |
stateTrans z = (fst newZ, newZ) where | |
newZ = goForward z | |
-- wrap goBack so it becomes a State | |
goBackW :: WriterT [String] (State (ListZipper a)) [a] | |
goBackW = lift $ state stateTrans where | |
stateTrans z = (fst newZ, newZ) where | |
newZ = goBack z | |
goForwardLog :: Show a => WriterT [String] (State (ListZipper a)) [a] | |
goForwardLog = do | |
l <- goForwardW | |
tell ["move forward, current focus:\t" ++ (show l)] | |
return l | |
goBackLog :: Show a => WriterT [String] (State (ListZipper a)) [a] | |
goBackLog = do | |
l <- goBackW | |
tell ["move back, current focus:\t" ++ (show l)] | |
return l | |
-- nothing but write out current focus | |
printLog :: Show a => WriterT [String] (State (ListZipper a)) [a] | |
printLog = do | |
l <- lift $ state $ \z -> (fst z, z) | |
tell ["print current focus:\t" ++ (show l)] | |
return l | |
-- return | |
listZipper :: [a] -> ListZipper a | |
listZipper xs = (xs, []) | |
_performTestCase1 :: Show a => WriterT [String] (State (ListZipper a)) [a] | |
_performTestCase1 = do | |
printLog | |
goForwardLog | |
goForwardLog | |
goBackLog | |
printLog | |
performTestCase1 = do | |
let ((a, w), s) = runState (runWriterT _performTestCase1) (listZipper [1..4]) | |
putStrLn $ "Focus:\t" ++ (show a) | |
putStrLn $ "Zipper:\t" ++ (show s) | |
putStrLn "Log:" | |
mapM_ putStrLn w | |
main = performTestCase1 |
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