Phone Number Kata

phonenumbers.hs

-- Given a list of phone numbers, determine if it is consistent.
-- In a consistent phone list no number is a prefix of another. For example:
--   Bob 91 12 54 26
--   Alice 97 625 992
--   Emergency 911
-- In this case, it is not possible to call Bob because the phone exchange
-- would direct your call to the emergency line as soon as you dialled the
-- first three digits of Bob's phone number. So this list would not be consistent.

module PhoneList(isConsistent) where

-- Rule:
-- A phonelist is consistent if the phonelist contains   none of that phonelist's  prefixes
isConsistent :: [String] -> Bool
isConsistent phoneList = not $ any phoneListContainsPrefix phoneList where
  phoneListContainsPrefix prefix = any (prefix `isPrefixOf `) phoneList where
    isPrefixOf [] []        = False
    isPrefixOf [] _         = True
    isPrefixOf _  []        = False
    isPrefixOf (x:xs)(y:ys) = x == y && xs `isPrefixOf` ys


-- *PhoneList> isConsistent ["911","1123456464","912111"]
-- True
-- *PhoneList> isConsistent ["911","1123456464","91111"]
-- False