带有catch的Haskell类型错误

我正在玩一些在线示例,其中一个有这样的短语:

   do ... contents <- getDirectoryContents path `catch` const (return []) 

但它不会为我编译,并给出错误:

No instance for (Exception e0) arising from a use of `catch'
The type variable `e0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there are several potential instances:
  instance Exception NestedAtomically
    -- Defined in `Control.Exception.Base'
  instance Exception NoMethodError
    -- Defined in `Control.Exception.Base'
  instance Exception NonTermination
    -- Defined in `Control.Exception.Base'
  ...plus 7 others
In a stmt of a 'do' block:
  contents <- getDirectoryContents path `catch` const (return [])
In the expression:
  do { contents <- getDirectoryContents path
                   `catch` const (return []);

我不得不改变它以给出处理程序的类型,现在可以,但有点麻烦:

contents <- getDirectoryContents path `catch` (\(SomeException e) -> const (return []) e)

所以我想知道为什么我需要做出这个改变,如果这是最简洁的方法.(是的,我知道我可以使用替代形式的尝试,处理,...)