如何在Haskell中检查字符串的每个字符

Question

I've got a homework for a function that checks if a username is valid or not. Allowed chars are underscore, letters, and digits. I'm not allowed to use indexing (!!) and Length

My code so far:

isValid' :: Char -> Bool
isValid' x
     | x == '_'    = True
     | x `elem` ['0'..'9'] = True
     | x `elem` ['a'..'z'] = True
     | x `elem` ['A'..'Z'] = True
     | otherwise           = False

isValidUsername :: [Char] -> Bool
isValidUsername x
     | map isValid' x = True
     | otherwise     = False

I want to run my isValid function on all chars of the string i put in isValidUsername. For example:

isValidUsername "MyUsername_123" should return True

isValidUsername "not@v@lidusern@me* *2" should return False

I just can't figure out how to run through all the chars of my string.

Answer 1

Well if you want all the elements to satisfy the isValid' function, you can use the all :: (a -> Bool) -> [a] -> Bool function, so you can rewrite your function to:

isValidUsername :: [Char] -> Bool
isValidUsername x = all isValid' x

or even shorter:

isValidUsername :: [Char] -> Bool
isValidUsername = all isValid'

Note that here it means that the empty string "" is a valid username as well, since for an empty string, all characters (there are no characters) are in the alphanumerical range.

Answer 2

Okay, i figured it out:

isValidUsername :: [Char] -> Bool
isValidUsername x
     | False `elem` (map isValid x) = False
     | otherwise                    = True