Haskell - “我如何使用”如果“在“do”中的语句正确阻止？ [英] Haskell - "How can I use "if" statement in "do" block properly?

问题描述

可能存在重复：

Haskell什么也不做 IO，或者如果没有其他的话

这些简单行中出现错误...

  action = do 
 isdir<  -  doesDirectoryExist路径 - 检查目录是否存在。 
 if（不是isdir）
然后处理错误
 doOtherActions  - 在这里编译错误。 
  

GHCi会投诉标识符，或者在添加<$ c $后不执行最后一行操作c> else do 。

我认为异常处理可能有效，但在这种常见的检查和做某事语句中是否有必要？ / p>

谢谢。

解决方案

if 在Haskell中必须始终有一个，然后和一个 else 。因此，这将工作：



  action = do 
 isdir<  -  doesDirectoryExist路径
如果不是isdir 
 then handleWrong 
 else return（）
 doOtherActions 
  

等价地，你可以在Control.Monad 时使用：



  action = do 
 isdir<  -  doesDirectoryExist path 
 when（not isdir）handleWrong 
 doOtherActions 
  

Control.Monad也有除非：

  action = do 
 isdir < -  doesDirectoryExist path 
除非isdir handleWrong 
 doOtherActions 
  





---

请注意，当您尝试

  action = do 
 isdir < -  doesDirectoryExist path 
 if（not isdir）
 then do handleWrong 
 else else 
 doOtherActions 
  

它被解析为

<$ p $ b $ action = do
isdir< - doesDirectoryExist path
if（not isdir）
then do handleWrong
else do doOtherActions


Possible Duplicate:
Haskell “do nothing” IO, or if without else

Something got wrong in these "easy" lines ...

action = do
    isdir <- doesDirectoryExist path  -- check if directory exists.
    if(not isdir)                     
        then do handleWrong
    doOtherActions                    -- compiling ERROR here.

GHCi will complaint about identifiers , or do not exec the last line action after I add else do .

I think exception handling may work, but is it necessary in such common "check and do something" statements ?

Thanks.

解决方案

if in Haskell must always have a then and an else. So this will work:

action = do
    isdir <- doesDirectoryExist path
    if not isdir
        then handleWrong
        else return ()
    doOtherActions

Equivalently, you can use when from Control.Monad:

action = do
    isdir <- doesDirectoryExist path
    when (not isdir) handleWrong
    doOtherActions

Control.Monad also has unless:

action = do
    isdir <- doesDirectoryExist path
    unless isdir handleWrong
    doOtherActions

---

Note that when you tried

action = do
    isdir <- doesDirectoryExist path
    if(not isdir)
        then do handleWrong
        else do
    doOtherActions

it was parsed as

action = do
    isdir <- doesDirectoryExist path
    if(not isdir)
        then do handleWrong
        else do doOtherActions

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