Использование библиотеки Parsec, для форматирования списка телефонных номеров

data Result a = Ok a | Error String deriving (Show, Eq)
 
errorInvalidFormat s = Error $ "Invalid phone number format: " ++ s
 
normalizePhoneNumber :: String -> Result String
normalizePhoneNumber n = case n of
    (    '8':num) -> check10 num n
    ('+':'7':num) -> check10 num n
    _             -> check7 n
    _              -> errorInvalidFormat n
    where
        check10 num orig
            | length num == 10  =  Ok ("+7" ++ num)
            | otherwise         =  errorInvalidFormat n
        check7 num
            | length num == 7   =  Ok ("+7343" ++ num)
            | otherwise         =  errorInvalidFormat n
 
normalizePhoneNumbers :: [String] -> ([String], [String])
normalizePhoneNumbers = foldl merge ([], []) . map normalizePhoneNumber
    where
        merge (oks, errors) (Ok    num) = (num:oks, errors)
        merge (oks, errors) (Error err) = (oks, err:errors)
 
testPhoneNumbers =
    [ "89006508005"
    , "9119990868"
    , "+79997109088"
    , "89015572218"
    , "89250475333"
    , "77717793929"
    , "+74973273032"
    , "8921134454454454"
    , "8900650800"
    ,"+791111123"
    , "2492996"
    ]
 
main = let (numbers, errors) = normalizePhoneNumbers testPhoneNumbers in do
    mapM_ putStrLn numbers
    putStrLn ""
    putStrLn "ERRORS:"
    mapM_ putStrLn errors

{-# Language OverloadedStrings #-}
module Lib (someFunc) where
 
import Data.DoList (DoList, fromList, toList)
import Control.Monad
import Text.Parsec
import Data.Monoid
 
someFunc :: IO ()
someFunc = do
    forM_ testPhoneNumbers $ \number -> do
        print $ parse phoneNumber "Number: " number
 
phoneNumber :: Parsec String u String
phoneNumber = plusNum <|> num
 
-- номера начинающиеся на +7
plusNum :: Parsec String u String
plusNum = do
    string "+7" <|> string "8"
    code    <- readCode
    number  <- readNumber
    pure $ "+7" <> code <> number
 
-- остальные номера
num :: Parsec String u String
num = do
    number <- readNumber
    pure $ "+7(343)" <> number
 
 
readCode :: Parsec String u String
readCode = do
    optional $ char '('
    code    <- count 3 digit
    optional $ char ')'
    pure $ "("<> code <> ")"
 
 
readNumber :: Parsec String u String
readNumber = do
    digs1 <- count 3 digit
    optional $ char '-'
    digs2 <- count 2 digit
    optional $ char '-'
    digs3 <- count 2 digit
    pure $ digs1 <> "-" <> digs2 <> "-" <> digs3
 
 
testPhoneNumbers :: [String]
testPhoneNumbers = toList $ do
    "89006508005"
    "9119990868"
    "+7(999)710-90-88"
    "89015572218"
    "89250475333"
    "77717793929"
    "+74973273032"
    "8921134454454454"
    "8900650800"
    "+791111123"
    "2492996"