〈Haskell Tutorial（14）減輕型態負擔的型態參數〉的答案

Map.hs

data Map k v = Empty | Cm (k, v) (Map k v) 

fromList :: [(k, v)] -> Map k v
fromList [] = Empty
fromList (x:xs) = Cm x (fromList xs)

findValue :: (Eq k) => k -> Map k v -> Maybe v
findValue key Empty = Nothing
findValue key (Cm (k, v) xm) =
    if key == k then Just v else findValue key xm