{--
JVM-Bridge -- bridge from FP languages and others to the Java VM
Copyright (C) 2001 Ashley Yakeley <ashley@semantic.org>

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.

This library 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
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
--}

module Data.Nothing where
	{
	import Foreign hiding (newForeignPtr);
	import Foreign.Concurrent (newForeignPtr);
	
	class HasNothing a where
		{
		nothing :: a;
		isNothing :: a -> Bool;
		};
	
	class (Monad m) => MonadHasNothing m a where
		{
		getNothing :: m a;
		getIsNothing :: a -> m Bool;
		};

{--	Can't do this unfortunately
	instance (Monad m,HasNothing a) => MonadHasNothing m a where
		{
		getNothing = return nothing;
		getIsNothing a = return (isNothing a);
		};
--}
	
	instance (Monad m) => MonadHasNothing m (Maybe a) where
		{
		getNothing = return nothing;
		getIsNothing a = return (isNothing a);
		};
	
	instance HasNothing (Maybe a) where
		{
		nothing = Nothing;
		isNothing Nothing = True;
		isNothing _ = False;
		};
	
	instance (Monad m) => MonadHasNothing m [a] where
		{
		getNothing = return nothing;
		getIsNothing a = return (isNothing a);
		};
	
	instance HasNothing [a] where
		{
		nothing = [];
		isNothing [] = True;
		isNothing _ = False;
		};
	
	instance (Monad m) => MonadHasNothing m (Ptr a) where
		{
		getNothing = return nothing;
		getIsNothing a = return (isNothing a);
		};
	
	instance HasNothing (Ptr a) where
		{
		nothing = nullPtr;
		isNothing a = a == nothing;
		};
	
	instance MonadHasNothing IO (ForeignPtr a) where
		{
		getNothing = do
			{
			n <- getNothing;
			newForeignPtr n (return ());
			};
		getIsNothing a = withForeignPtr a getIsNothing;
		};
	
	checkNothing :: (HasNothing a) => a -> Maybe a;
	checkNothing a = if (isNothing a) then Nothing else (Just a);
	
	failIfNothing :: (MonadHasNothing m a) => String -> a -> m ();
	failIfNothing failMessage a = do
		{
		noth <- getIsNothing a;
		if noth then (fail failMessage) else (return ());
		};
	}