Reads values of property from file

properties.hs

import System.Environment
import Data.List
import Data.Maybe


data Arguments = Arguments String String
    | UnexpectedArguments [String]
    | NotEnoughArguments
    deriving Show

parseArgs :: [String] -> Arguments
parseArgs ( path : k : [] ) = Arguments path k
parseArgs ( path : k : rest ) = UnexpectedArguments rest
parseArgs _ = NotEnoughArguments


data Property = Property String String
    | MalformedProperty
    deriving Show

match :: String -> Property -> Bool
match s ( Property k v ) = k == s
match s MalformedProperty = False

value :: Property -> String
value ( Property k v ) = v

parse :: String -> Property
parse s
    | ( not $ null k ) && ( not $ null v ) = Property k v
    | otherwise = MalformedProperty
    where
        v = drop 1 _v
        ( k , _v ) = break ( == '=' ) s

filter :: String -> [Property] -> [Property]
filter k pp = Data.List.filter ( match k ) pp


main = do
    args <- getArgs
    case parseArgs args of
        Arguments path key -> do
            contents <- readFile path
            putStrLn $ foldl (++) "" $ foundValues key contents
            where
                foundValues k = map value . foundProperties k
                foundProperties k = Main.filter k . properties
                properties = map parse . lines
        UnexpectedArguments aa -> do
            putStrLn $ "Unexpected arguments: " ++ intercalate " " aa
        NotEnoughArguments -> do
            putStrLn "Not enough arguments"