Haskell Caesar Cipher

Question

i'm having trouble with this. my decrypt is not working as intended. i've been trying this for a while and still don't understand why it's showing error.

module CaesarCipher where

shift :: Char -> Int -> Char
shift ch shift =
  let newChar = ch + shift
      alphabet = ['a' .. 'z'] ++ ['A' .. 'Z'] ++ ['0' .. '9'] ++ [' ']
  in  if elem ch alphabet then
        let wrapAround = length alphabet
        in  alphabet !! ((newChar - head alphabet) `mod` wrapAround)
    else
        ch

encrypt :: String -> Int -> String
encrypt text shift = map (shift) text

decrypt :: String -> Int -> String
decrypt text shift = map (\ch -> shift ch (-shift)) text -- Lambda function for decryption

It looks like you have two things called shift, one being a function that shifts and the other being the amount of shift. Since you need both in the same scope, it would create a problem. Rename one. — n. m. could be an AI
– n. m. could be an AI, Commented May 9, 2024 at 15:51
@willeM_VanOnsem it shows this error: * Couldn't match expected type Char -> Int -> Char' with actual type Int' * The function shift' is applied to two value arguments, but its type Int' has none In the expression: shift ch (- shift) In the first argument of map', namely (\ ch -> shift ch (- shift))' | 17 | decrypt text shift = map (\ch -> shift ch (-shift)) text -- Lambda function for decryption — just coding
– just coding, Commented May 9, 2024 at 16:06

willeM_ Van Onsem · Accepted Answer · 2024-05-09 15:57:50Z

2

Don't give your parameter the same name as the function, use:

encrypt :: String -> Int -> String
encrypt text s = map (`shift` s) text

decrypt :: String -> Int -> String
decrypt text s = map (`shift` (-s)) text

as for the shift function itself, this will generate type errors, since you use newChar = ch + shift for example, but ch is a Char, and shift an Int, so you can not add these together. You can use elemIndex :: Eq a => a -> [a] -> Maybe Int ^[Hackage] to find out at what index an item is located in a list, to then shift, modulo, and obtain the element at the shifted index.

answered May 9, 2024 at 15:57

willeM_ Van Onsem

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2 Comments

just coding Over a year ago

hello, i've tried this and now it's showing error as you've said. im trying to do this without the help of external modules or libraries and only standard prelude. this is the current error: * Couldn't match type Char' with Int' Expected: [Int] Actual: [Char] * In the first argument of head', namely alphabet' In the second argument of (-)', namely head alphabet' In the first argument of mod', namely (newChar - head alphabet)' | 9 | in alphabet !! ((newChar - head alphabet) mod wrapAround)

willeM_ Van Onsem Over a year ago

@justcoding: head makes no sense, you have a Char, not a list of Chars. As said, use the elemIndex to get the position in the list.

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