在Haskell List Comprehensions中做一些替换

Question

我的问题是,如果我输入一个包含诸如Hello, today is a Nice Day!!如何摆脱空格和标点符号以及用小写字母替换大写字母的字符串？

我知道如何删除它们但不知道如何替换它们.

还要摆脱标点符号.

对不起,我不知道如何乱用字符串,只有数字.

testList xs = [if x = [,|.|?|!] then " "  | x<-xs] 

Answer 1

import Data.Char

如果要将标点符号转换为空格,将字符从大写转换为小写:

testList xs = [if x `elem` ",.?!" then ' ' else toLower x | x<-xs]

例: testList "TeST,LiST!" == "test list "

如果要删除标点符号并将字符从大写转换为小写:

testList2 xs = [toLower x | x<-xs, not (x `elem` ",.?!")]

例: testList2 "Te..S,!t LiS?T" == "test list"

如果您不想或不能导入Data.Char,这是toLower的实现:

toLower' :: Char -> Char
toLower' char 
    | isNotUppercase = char -- no change required
    | otherwise = toEnum (codeChar + diffLowerUpperChar) -- char lowered
    where
      codeChar = fromEnum char -- each character has a numeric code
      code_A = 65
      code_Z = 90
      code_a = 97
      isNotUppercase = codeChar < code_A || codeChar > code_Z
      diffLowerUpperChar = code_a - code_A