{- arch-tag: Euither utilities Copyright (C) 2004-2006 John Goerzen <jgoerzen@complete.org> 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 2 of the License, or (at your option) 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, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA -} {- | Module : Data.Either.Utils Copyright : Copyright (C) 2004-2006 John Goerzen License : GNU GPL, version 2 or above Maintainer : John Goerzen <jgoerzen@complete.org> Stability : provisional Portability: portable Utilities for working with the Either data type Copyright (c) 2004 John Goerzen, jgoerzen\@complete.org -} module Data.Either.Utils ( maybeToEither, forceEither, forceEitherMsg, eitherToMonadError, fromLeft, fromRight, fromEither ) where import Control.Monad.Error {- | Converts a Maybe value to an Either value, using the supplied parameter as the Left value if the Maybe is Nothing. This function can be interpreted as: @maybeToEither :: e -> Maybe a -> Either e a@ Its definition is given as it is so that it can be used in the Error and related monads. -} maybeToEither :: MonadError e m => e -- ^ (Left e) will be returned if the Maybe value is Nothing -> Maybe a -- ^ (Right a) will be returned if this is (Just a) -> m a maybeToEither errorval Nothing = throwError errorval maybeToEither _ (Just normalval) = return normalval {- | Pulls a "Right" value out of an Either value. If the Either value is Left, raises an exception with "error". -} forceEither :: Show e => Either e a -> a forceEither (Left x) = error (show x) forceEither (Right x) = x {- | Like 'forceEither', but can raise a specific message with the error. -} forceEitherMsg :: Show e => String -> Either e a -> a forceEitherMsg msg (Left x) = error $ msg ++ ": " ++ show x forceEitherMsg _ (Right x) = x {- | Takes an either and transforms it into something of the more generic MonadError class. -} eitherToMonadError :: MonadError e m => Either e a -> m a eitherToMonadError (Left x) = throwError x eitherToMonadError (Right x) = return x -- | Take a Left to a value, crashes on a Right fromLeft :: Either a b -> a fromLeft (Left a) = a fromLeft _ = error "Data.Either.Utils.fromLeft: Right" -- | Take a Right to a value, crashes on a Left fromRight :: Either a b -> b fromRight (Right a) = a fromRight _ = error "Data.Either.Utils.fromRight: Left" -- | Take an Either, and return the value inside it fromEither :: Either a a -> a fromEither (Left a) = a fromEither (Right a) = a