Haskell使用辅助函数递归删除字符

Question

I have a function defined as follows:

rmCharsRec :: String -> String -> String

I have a helper function rmChar defined as follows that removes all occurrences of a character in a string:

rmChar :: Char -> String -> String

rmCharsRec should remove all the occurrences of each char in the first string, in the second string and return the response.

I have tried the following:

rmCharsRec :: String -> String -> String
rmCharsRec _ "" = ""
rmCharsRec c:chars str = rmChar ( c str ) ++ rmCharsRec chars

But it seems is not working. I always get the following error:

file.hs:140:1: Parse error in pattern: rmCharsRec

**This is not a homework problem*. I just want to learn Haskell from a set of exercises, and I got stuck to this as I don't get why I get an error.

Can somebody please notify me about what's the problem with my code?

Answer 1

If you want to write a pattern like c:chars you need to put in it brackets like so:

rmCharsRec (c:chars) str = ...

That will resolve the error message you mention, although I think that that's not the only issue you'll need to deal with.

Answer 2

您忘记了在模式匹配中在c:chars周围设置括号。