不在范围内:`fromMaybe' - haskell

Question

我有一个字符串和一个字符串列表,如果lst不包含给定的字符串,或者只是列出'where list'与lst相同,除了删除给定字符串的第一个出现之外,我应该返回任何内容.

allExcept :: [Char] -> [[Char]] -> Maybe [[Char]]

allExcept ch1 chars 
   | ch1  `notElem` chars = Nothing

allExcept ch [] = []      
allExcept ch (x:xs)
  | ch /= x = Just(x:(fromJust(allExcept ch xs)))
  | otherwise = Just(xs)  

我的代码的结果是:不在范围内:`fromMaybe'

Answer 1

您需要导入Data.Maybe模块:

import Data.Maybe