-- case-Ausdrücke zusammen mit alg. Datentypen

data Eintrag s i = Eintrag s i
   deriving (Show, Eq, Ord, Read)


type Datenbank s i = [Eintrag s i]

type Name = String; type Nummer = String
persTelBuch, offTelBuch :: Datenbank Name Nummer

persTelBuch = [Eintrag "Anne" "23247654", Eintrag "Tom" "74574547"]
offTelBuch = [Eintrag "Anne" "0124675", Eintrag "Maria" "46534765"]

such :: Eq s => s -> Datenbank s i -> Maybe i

such x ((Eintrag sch info):db) 
   | x == sch = Just info
   | otherwise = such x db
such x [] = Nothing

globalSuch :: Eq k => k -> [Datenbank k i] -> Maybe i
globalSuch n [] = Nothing
globalSuch n (db:dbs) = case (such n db) of
   Nothing -> globalSuch n dbs
   Just info -> Just info

telNum :: Name -> Maybe Nummer
telNum n = globalSuch n [persTelBuch, offTelBuch]
 
test1, test2, test3 :: Maybe Nummer
test1 = globalSuch "Maria" [persTelBuch, offTelBuch]
test2 = globalSuch "Anne" [persTelBuch, offTelBuch]
test3 = globalSuch "Anton" [persTelBuch, offTelBuch]