Solution to SDS challenge 4

euler8.hs

import Data.Char
import Data.List
import Text.Read

main = do
  putStrLn "Number"
  number <- getLine
  putStrLn "Number of digits to multiply"
  digits <- getLine
  putStrLn $ pureMain digits number

pureMain digitsString numberString = let
  maybeNumberList = stringToIntList numberString
  maybeDigits = readMaybe digitsString
  in case largestProduct maybeDigits maybeNumberList of
      Right value -> "Largest product\n" ++ show value
      Left message -> message

largestProduct :: Maybe Int -> Maybe [Int] -> Either String Int
largestProduct _ Nothing = Left "Invalid number."
largestProduct Nothing _ = Left "Invalid digits."
largestProduct (Just digits) _
  | digits < 1 = Left "Digits must be a positive integer."
largestProduct (Just digits) (Just numberList) = let
  numberListRepetitions = take digits (tails numberList)
  in case products numberListRepetitions of
      [] -> Left "Number does not have as many digits."
      productList -> Right (maximum productList)

products :: [[Int]] -> [Int]
products [] = []
products [singleList] = singleList
products (firstList:restOfLists) = zipWith (*) firstList (products restOfLists)

stringToIntList :: String -> Maybe [Int]
stringToIntList "" = Nothing
stringToIntList [character]
  | isDigit character = Just [digitToInt character]
  | otherwise = Nothing
stringToIntList (first:restOfString) = let
  maybeFirstInt = stringToIntList [first]
  maybeRestofInts = stringToIntList restOfString
  in case (maybeFirstInt, maybeRestofInts) of
    (Just [firstInt], Just restOfInts) -> Just (firstInt:restOfInts)
    _ -> Nothing

Build steps:
ghc euler8.hs && ./euler8