我如何利用haskell的州和作家?
当我阅读了LYAH的最后一章并与之会面时,我给自己分配了一个任务,使其成为状态单子,以便源代码看起来更清晰,如:我如何利用haskell的州和作家?,haskell,monads,Haskell,Monads,当我阅读了LYAH的最后一章并与之会面时,我给自己分配了一个任务,使其成为状态单子,以便源代码看起来更清晰,如: manipList = do goForward goForward goBack 同时,我想利用Writer monad为这个过程保留一个日志,但我不知道如何将这两个monad组合在一起 我的解决方案是在状态中保留一个[String],我的源代码是 import Control.Monad import Control.Monad.State type
manipList = do
goForward
goForward
goBack
同时,我想利用Writer monad为这个过程保留一个日志,但我不知道如何将这两个monad组合在一起
我的解决方案是在状态中保留一个[String],我的源代码是
import Control.Monad
import Control.Monad.State
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
goForwardM :: State (ListZipper a) [a]
goForwardM = state stateTrans where
stateTrans z = (fst newZ, newZ) where
newZ = goForward z
-- wrap goBack so it becomes a State
goBackM :: State (ListZipper a) [a]
goBackM = state stateTrans where
stateTrans z = (fst newZ, newZ) where
newZ = goBack z
-- here I have tried to combine State with something like a Writer
-- so that I kept an extra [String] and add logs to it manually
-- nothing but write out current focus
printLog :: Show a => State (ListZipper a, [String]) [a]
printLog = state $ \(z, logs) -> (fst z, (z, ("print current focus: " ++ (show $ fst z)):logs))
-- wrap goForward and record this move
goForwardLog :: Show a => State (ListZipper a, [String]) [a]
goForwardLog = state stateTrans where
stateTrans (z, logs) = (fst newZ, (newZ, newLog:logs)) where
newZ = goForward z
newLog = "go forward, current focus: " ++ (show $ fst newZ)
-- wrap goBack and record this move
goBackLog :: Show a => State (ListZipper a, [String]) [a]
goBackLog = state stateTrans where
stateTrans (z, logs) = (fst newZ, (newZ, newLog:logs)) where
newZ = goBack z
newLog = "go back, current focus: " ++ (show $ fst newZ)
-- return
listZipper :: [a] -> ListZipper a
listZipper xs = (xs, [])
-- return
stateZipper :: [a] -> (ListZipper a, [String])
stateZipper xs = (listZipper xs, [])
_performTestCase1 = do
goForwardM
goForwardM
goBackM
performTestCase1 =
putStrLn $ show $ runState _performTestCase1 (listZipper [1..4])
_performTestCase2 = do
printLog
goForwardLog
goForwardLog
goBackLog
printLog
performTestCase2 = do
let (result2, (zipper2, log2)) = runState _performTestCase2 $ stateZipper [1..4]
putStrLn $ "Result: " ++ (show result2)
putStrLn $ "Zipper: " ++ (show zipper2)
putStrLn "Logs are: "
mapM_ putStrLn (reverse log2)
但问题是,我认为这不是一个好的解决方案,因为我必须手动维护日志。有没有其他方法可以混合State monad和Writer monad以便它们可以一起工作?您正在寻找。基本思想是定义一个类型,如WriterT
,它接受另一个monad并将其与Writer
组合,创建一个新类型(如WriterT log(states s)
)
注意:有一种惯例,变压器类型以大写字母T
结尾。因此,Maybe
和Writer
是正常的单子,MaybeT
和WriterT
是它们的转换器等价物
其核心思想非常简单:对于一组单子,您可以很容易地想象将它们的行为结合在一起。最简单的例子是可能。回想一下,可能
所做的一切就是在绑定时传播无内容
:
Nothing >>= f = Nothing
Just x >>= f = f x
因此,很容易想象用这种行为扩展任何monad。我们所要做的就是先检查Nothing
,然后使用旧的monad绑定。MaybeT
类型正是这样做的:它包装了一个现有的monad,并在每个绑定的前面加上这样的检查。您还必须实现return
,方法是将值包装在中,然后使用内部monad的return
。还有更多的管道使一切正常工作,但这是一个重要的想法
您可以想象Writer
的一个非常类似的行为:首先我们组合任何新的输出,然后使用旧的monad绑定。这本质上就是WriterT
的行为。其中还涉及一些其他细节,但基本思想相当简单和有用
单子变形金刚是一种非常常见的方式来“组合”你想要的单子。有一些版本的最常用的monad作为转换器,除了IO
之外,它必须始终位于monad堆栈的底部。在您的情况下,WriterT
和StateT
都存在,并且可以用于您的程序。Tikhon Jelvis用monad transformers给出了一个很好的答案。然而,也有一个快速的解决方案
mtl
中的模块导出RWS
monad,它是读卡器
、写卡器
和状态
monad的组合。这正是我想要的!我试过了,他们两个(WriterT和StateT)都很好,泰!