エラー処理

Eitherを利用すると失敗の場合にも成功の場合にもデータ(Right)が付随する。

Real World Haskell―実戦で学ぶ関数型言語プログラミング
Bryan O'Sullivan,John Goerzen,Don Stewart
オライリージャパン / ￥ 3,990 ()
在庫あり。

tryとhandleも使える。

try :: IO a -> IO (Either Exception a)
handle :: (Exception e) => (e -> IO a) -> IO a -> IO a

19.3.1

import Control.Monad.Error
import Control.Monad.State
import qualified Data.ByteString.Char8 as B

data ParseError = NumericOverflow
                | EndOfInput
                | Chatty String
                  deriving (Eq, Ord, Show)

instance Error ParseError where
    noMsg  = Chatty "oh noes!"
    strMsg = Chatty

newtype Parser a = P {
      runP :: ErrorT ParseError (State B.ByteString) a
    } deriving (Monad, MonadError ParseError)

liftP :: State B.ByteString a -> Parser a
liftP m = P (lift m)

satisfy :: (Char -> Bool) -> Parser Char
satisfy p = do
  s <- liftP get
  case B.uncons s of
    Nothing ->  throwError EndOfInput
    Just (c, s')
         | p c ->  liftP (put s') >> return c
         | otherwise -> throwError (Chatty "satisfy failed")

optional :: Parser a -> Parser (Maybe a)
optional p = (Just `liftM` p) `catchError` \_ -> return Nothing

runParser :: Parser a -> B.ByteString -> Either ParseError (a, B.ByteString)
runParser p bs = case runState (runErrorT (runP p)) bs of
                   (Left err, _) -> Left err
                   (Right r, bs) -> Right (r, bs)