vcard.hs

-- A spinoff of the Vcard.hs example from http://book.realworldhaskell.org/read/programming-with-monads.html
import Control.Monad

data Context = Home | Mobile | Business
               deriving (Eq, Show)

type Phone = String
type Phones = [Phone]

albulena = [(Home, "+35-1234")]

nils = [
  (Home, "1234"),
  (Business, "456"),
  (Home, "877788"),
  (Business, "8748753854")]

tumba = [(Mobile, "1757")]

onePersonalPhone :: [(Context, Phone)] -> Maybe Phone
onePersonalPhone ps = case lookup Home ps of
    Nothing -> lookup Mobile ps
    Just n -> Just n

allBusinessPhones :: [(Context, Phone)] -> Maybe Phones
allBusinessPhones ps = case filter (contextIs Business) ps of
    [] -> Nothing
    ns -> return $ map snd ns
    where contextIs a (b, _) = a == b


allPersonalPhones :: [(Context, Phone)] -> Maybe Phones
allPersonalPhones ps = filterM (\x -> Just $ Home == fst x) ps >>= mapM (Just . snd)


getPhonesOfType ty ps = filterM (\x -> Just $ ty == fst x) ps >>= mapM (Just . snd)

main = print . show $ getPhonesOfType Business nils