module PhoneNumber where

import Prelude
import Data.Char.Unicode (isDigit)
import Data.Array (filter, head, tail, length)
import Data.String (toCharArray, fromCharArray)
import Data.Maybe (Maybe(..))

clean :: String -> Maybe (Array Char)
clean input = go where
  go
    | len == 10 = Just digits
    | len == 11 && head digits == Just '1' = tail digits
    | otherwise = Nothing
  digits = filter isDigit $ toCharArray input
  len = length digits

check:: Array Char -> Maybe String
check ['0', _] = Nothing
check ['1', _] = Nothing
check [ _ , _ , _ , '0', _] = Nothing
check [ _ , _ , _ , '1', _] = Nothing
check s = Just $ fromCharArray s


phoneNumber :: String -> Maybe String
phoneNumber input = clean input >>= check