Util.hs

-- Copyright (C) 2009-2010 John Millikin <jmillikin@gmail.com>
-- 
-- This program is free software: you can redistribute it and/or modify
-- it under the terms of the GNU General Public License as published by
-- the Free Software Foundation, either version 3 of the License, or
-- any later version.
-- 
-- This program is distributed in the hope that it will be useful,
-- but WITHOUT ANY WARRANTY; without even the implied warranty of
-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-- GNU General Public License for more details.
-- 
-- You should have received a copy of the GNU General Public License
-- along with this program.  If not, see <http://www.gnu.org/licenses/>.
module DBus.Util where
import Text.Parsec (Parsec, parse)
import Data.Char (digitToInt)
import Data.List (isPrefixOf)

checkLength :: Int -> String -> Maybe String
checkLength length' s | length s <= length' = Just s
checkLength _ _ = Nothing

parseMaybe :: Parsec String () a -> String -> Maybe a
parseMaybe p = either (const Nothing) Just . parse p ""

mkUnsafe :: Show a => String -> (a -> Maybe b) -> a -> b
mkUnsafe label f x = case f x of
	Just x' -> x'
	Nothing -> error $ "Invalid " ++ label ++ ": " ++ show x

hexToInt :: String -> Int
hexToInt = foldl ((+) . (16 *)) 0 . map digitToInt

eitherToMaybe :: Either a b -> Maybe b
eitherToMaybe (Left  _) = Nothing
eitherToMaybe (Right x) = Just x

fromRight :: Either a b -> b
fromRight (Right x) = x
fromRight _ = error "DBus.Util.fromRight: Left"

maybeIndex :: [a] -> Int -> Maybe a
maybeIndex (x:_ ) 0         = Just x
maybeIndex (_:xs) n | n > 0 = maybeIndex xs (n - 1)
maybeIndex _ _ = Nothing

-- | Read values from a monad until a guard value is read; return all
-- values, including the guard.
--
readUntil :: (Monad m, Eq a) => [a] -> m a -> m [a]
readUntil guard getx = readUntil' [] where
	guard' = reverse guard
	step xs | isPrefixOf guard' xs = return . reverse $ xs
	        | otherwise            = readUntil' xs
	readUntil' xs = do
		x <- getx
		step $ x:xs

-- | Drop /n/ items from the end of a list
dropEnd :: Int -> [a] -> [a]
dropEnd n xs = take (length xs - n) xs