Haskell - "我如何使用"如果"声明在"中"阻止"？

Question

可能重复:
Haskell"无所事事"IO,或者如果没有别的话

这些"简单"的线条出了点问题......

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

GHCi会投诉标识符,或者在我添加后不执行最后一行操作else do.

我认为异常处理可能有效,但是在这种常见的"检查并做某事"声明中是否有必要？

谢谢.

Answer 1

if在Haskell必须始终有一个then和一个else.所以这将有效:

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

同样,您可以使用whenControl.Monad:

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

Control.Monad还有unless:

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

请注意,当你尝试

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

它被解析为

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