Haskell - “我如何使用”如果“在“do”中的语句正确阻止？

本文介绍了Haskell - “我如何使用”如果“在“do”中的语句正确阻止？的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述 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 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 除非isdir handleWrong doOtherActions 请注意，当您尝试 action = do isdir if（not isdir） then do handleWrong else else doOtherActions 它被解析为 action = do isdir< - doesDirectoryExist path if（not isdir） then do handleWrong else do doOtherActions 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 () doOtherActionsEquivalently, you can use when from Control.Monad:action = do isdir <- doesDirectoryExist path when (not isdir) handleWrong doOtherActionsControl.Monad also has unless:action = do isdir <- doesDirectoryExist path unless isdir handleWrong doOtherActionsNote that when you triedaction = do isdir <- doesDirectoryExist path if(not isdir) then do handleWrong else do doOtherActionsit was parsed asaction = do isdir <- doesDirectoryExist path if(not isdir) then do handleWrong else do doOtherActions 这篇关于Haskell - “我如何使用”如果“在“do”中的语句正确阻止？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！