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我只是在学习Haskell而我正在尝试编写一个简单的程序来消除a中的前n个字符String.这就是我得到的:

cutString :: (Num n, String str) => n -> str -> str

cutString n str = case n of
        0 -> tail str
        n -> cutString (n-1) (tail str)

虽然GHC给了我这个错误,我无法弄清楚原因:

`String' is applied to too many type arguments
 In the type signature for `cutString':
 cutString :: (Num n, String str) => n -> str -> str