Haskell, how can I traverse a [[String type]] to check whether a given string is in the list?

Question

I am having a problem, I have a list like this [["A1","A2","A3"],["B1","B2","B3"],["A1","B3""C3"]] I want to return a list if every string contains 2, the desired return should be as follows:

[["A1","A2","A3"],["B1","B2","B3"]]

my recent approach is

isIn :: Int -> [String] -> Bool
isIn a [] = False
isIn a (x:xs)
  | show a == x = True
  | otherwise = isIn a xs

And I tested it using the list above it gave me False, what did I wrong? How should I fix this?

Answer 1

By the type definition isIn :: Int -> [String] -> Bool I assume that the function is iterating the list of strings (for example ["A1","A2","A3"] ).

Therefore, the problem is in show a == x . x is equal to "A1" , "A2" , "A3" ; show 2 is "2" ; but "2" is not equal to any of these strings.

You'll need to use the function that verifies that a string contains "2", not equal to "2" (there's elem function)

Or implement it:

isInString :: Int -> [Char] -> Bool
isInString a [] = False
isInString a (x:xs)
  | intToDigit a == x = True
  | otherwise = isInString a xs

And then use it in isIn :

isIn :: Int -> [String] -> Bool
isIn a [] = False
isIn a (x:xs)
  | isInString a x = True
  | otherwise = isIn a xs