Haskell最佳实践：在Haskeline中提前终止 [英] Haskell Best Practise: Early termination in Haskeline

问题描述

我使用的是 Haskeline 包，我想从中获取三个字符串在我做任何事情之前的命令行，我已经想出了对我来说似乎是一个很好的解决方案。但我相信可能有更好的方法来做到这一点。我在使用Haskeline软件包时寻找最佳实践。请评估以下示例代码的优点：

  import System.Console.Haskeline 
 import Control.Monad.Trans 
 import Control.Monad.Maybe 
 import Data.Maybe 
 import Control.Monad 
 $ b $ main :: IO（）
 main = runInputT defaultSettings（runMaybeT getStrings）>> = print 
 
 getStrings :: MaybeT（InputT IO）（String，String，String）
 getStrings = do 
 mone<  -  lift $ getInputLine食品>
 notNothing mone 
 mtwo<  -  lift $ getInputLinedrink>
 notNothing mtwo 
 mthree<  -  lift $ getInputLinedessert>
 notNothing mthree 
 return（fromJust mone，fromJust mtwo，fromJth mthree）
 where 
 notNothing a = guard（a / = Nothing）
  

您可以看到它完成了提前终止的任务，但它看起来仍然有点不自然。我正在考虑尝试将notNothing和getInputLine转换为一行，如下所示：

  mone<  -  notNothing =< ;< lift $ getInputLinefood> - 不检查
  

我认为这看起来并不好。我认为这是非常清晰和简洁的（尽管它没有输入检查，所以我将不得不编写一个版本）。

然而，这是我拥有的最好的拿出来，我的问题最后是：你会如何改善这个代码更整洁，更容易阅读？我甚至在正确的轨道上吗？

编辑：如果你的警卫不是'a / = Nothing'我发现的是：

$ p $ myGuard s = guard（someConditionFunc s）>> s


因为您可以编写（如）：

  mone < -  myGuard =<< （lift $ getInputLine提示符）
  

这非常酷。但是，如果你仅仅匹配Nothing，那么TomMD的答案会更好。

解决方案为什么不利用<$ c $对于Maybe monad，c> fail _ = Nothing

  mthree < -  lift $ getInputLine甜点>
 notNothing mthree 
  

变成

 只需要mthree<  -  lift $ getInputLinedessert>
  

I am using the Haskeline package and I want to get three strings in a row from the command line before I do anything and I have come up with what seems to be a neat solution to me. But I am sure that there might be a better way to do it. I am looking for best practices while using the Haskeline package. Please evaluate the merits of the following example code:

import System.Console.Haskeline
import Control.Monad.Trans
import Control.Monad.Maybe
import Data.Maybe
import Control.Monad

main :: IO ()
main = runInputT defaultSettings (runMaybeT getStrings) >>= print

getStrings :: MaybeT (InputT IO) (String, String, String)
getStrings = do
   mone <- lift $ getInputLine "food> "
   notNothing mone
   mtwo <- lift $ getInputLine "drink> "
   notNothing mtwo
   mthree <- lift $ getInputLine "dessert> "
   notNothing mthree
   return (fromJust mone, fromJust mtwo, fromJust mthree)
      where
         notNothing a = guard (a /= Nothing)

As you can see it accomplishes the task of early termination but it looks a bit yucky still. I'm thinking of trying to convert the notNothing's and the getInputLine's into a single line like:

mone <- notNothing =<< lift $ getInputLine "food> " -- does not type check

Which I think does not look that bad. I think that is pretty clear and concise (though it does not type check so I will have to write a version that does).

However, this is the best I have come up with and my question finally is: How would you go about improving this code to be neater and more readily readable? Am I even on the right track?

Edit: If your guard is something other than 'a /= Nothing' then a nice helper function that I just discovered is:

myGuard s = guard (someConditionFunc s) >> s

Because then you can write (as luqui suggested):

mone <- myGuard =<< (lift $ getInputLine prompt)

Which is pretty cool. But if you are matching against only Nothing then TomMD's answer is better.

解决方案

Why not just leverage the fact that fail _ = Nothing for the Maybe monad?

mthree <- lift $ getInputLine "dessert> "
notNothing mthree

becomes

Just mthree <- lift $ getInputLine "dessert> "

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