Step by Step solution about How to resolve the algorithm SEDOLs step by step in the Haskell programming language

The code above implements a program that checks if a certain string has a valid SEDOL. A SEDOL is a code that identifies a security in the UK, comprising of seven characters:

• six characters which identify the security
• one check digit that is calculated from the other six characters

The program first attempts to convert the string into a list of integers, using the sedolValue function. The sedolValue function takes a single character and either returns an integer or an error message.

The sedolValue function checks if the character is a vowel, in which case it returns an error message. If the character is a digit, it returns the integer value of the digit. If the character is an upper case letter, it returns the numerical value of the letter (e.g. 'A' = 10, 'B' = 11, etc.)

If the sedolValue function returns a list of integers, the program then calculates the check digit using the checkSumFromSedolValues function. The checkSumFromSedolValues function takes a list of integers and returns the check digit. The check digit is calculated by multiplying each integer in the list by a weighting factor, summing the results, and then taking the remainder of the sum when divided by 10. The result is then subtracted from 10, and the remainder of that result when divided by 10 is the check digit.

If the checkSumFromSedolValues function returns a valid check digit, the program then prints the check digit to the console. If the checkSumFromSedolValues function returns an error message, the program prints the error message to the console.

The main function of the program takes a list of strings and applies the checkSum function to each string. The checkSum function takes a string and returns either the check digit or an error message. The main function then prints the check digit or error message to the console for each string.

import Data.Char (isAsciiUpper, isDigit, ord)

-------------------------- SEDOLS ------------------------

checkSum :: String -> String
checkSum x =
  case traverse sedolValue x of
    Right xs -> (show . checkSumFromSedolValues) xs
    Left annotated -> annotated

checkSumFromSedolValues :: [Int] -> Int
checkSumFromSedolValues xs =
  rem
    ( 10
        - rem
          ( sum $
              zipWith
                (*)
                [1, 3, 1, 7, 3, 9]
                xs
          )
          10
    )
    10

sedolValue :: Char -> Either String Int
sedolValue c
  | c `elem` "AEIOU" = Left " ← Unexpected vowel."
  | isDigit c = Right (ord c - ord '0')
  | isAsciiUpper c = Right (ord c - ord 'A' + 10)

--------------------------- TEST -------------------------
main :: IO ()
main =
  mapM_
    (putStrLn . ((<>) <*> checkSum))
    [ "710889",
      "B0YBKJ",
      "406566",
      "B0YBLH",
      "228276",
      "B0YBKL",
      "557910",
      "B0YBKR",
      "585284",
      "B0YBKT",
      "BOYBKT", -- Ill formed test case - illegal vowel.
      "B00030"
    ]