```%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%

\begin{code}
module Maybes (
module Data.Maybe,

failME, isSuccess,

fmapM_maybe,
orElse,
mapCatMaybes,
allMaybes,
firstJust,
expectJust,
maybeToBool,

MaybeT(..)
) where

import Data.Maybe

infixr 4 `orElse`
\end{code}

%************************************************************************
%*                                                                      *
\subsection[Maybe type]{The @Maybe@ type}
%*                                                                      *
%************************************************************************

\begin{code}
maybeToBool :: Maybe a -> Bool
maybeToBool Nothing  = False
maybeToBool (Just _) = True

-- | Collects a list of @Justs@ into a single @Just@, returning @Nothing@ if
-- there are any @Nothings@.
allMaybes :: [Maybe a] -> Maybe [a]
allMaybes [] = Just []
allMaybes (Nothing : _)  = Nothing
allMaybes (Just x  : ms) = case allMaybes ms of
Nothing -> Nothing
Just xs -> Just (x:xs)

-- | Takes a list of @Maybes@ and returns the first @Just@ if there is one, or
-- @Nothing@ otherwise.
firstJust :: [Maybe a] -> Maybe a
firstJust [] = Nothing
firstJust (Just x  : _)  = Just x
firstJust (Nothing : ms) = firstJust ms
\end{code}

\begin{code}
expectJust :: String -> Maybe a -> a
{-# INLINE expectJust #-}
expectJust _   (Just x) = x
expectJust err Nothing  = error ("expectJust " ++ err)
\end{code}

\begin{code}
mapCatMaybes :: (a -> Maybe b) -> [a] -> [b]
mapCatMaybes _ [] = []
mapCatMaybes f (x:xs) = case f x of
Just y  -> y : mapCatMaybes f xs
Nothing -> mapCatMaybes f xs
\end{code}

\begin{code}
orElse :: Maybe a -> a -> a
(Just x) `orElse` _ = x
Nothing  `orElse` y = y
\end{code}

\begin{code}
fmapM_maybe :: Monad m => (a -> m b) -> Maybe a -> m (Maybe b)
fmapM_maybe _ Nothing = return Nothing
fmapM_maybe f (Just x) = do
x' <- f x
return \$ Just x'
\end{code}

%************************************************************************
%*									*
%*									*
%************************************************************************

\begin{code}

newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)}

instance Functor m => Functor (MaybeT m) where
fmap f x = MaybeT \$ fmap (fmap f) \$ runMaybeT x

return = MaybeT . return . Just
x >>= f = MaybeT \$ runMaybeT x >>= maybe (return Nothing) (runMaybeT . f)
fail _ = MaybeT \$ return Nothing

\end{code}

%************************************************************************
%*                                                                      *
\subsection[MaybeErr type]{The @MaybeErr@ type}
%*                                                                      *
%************************************************************************

\begin{code}
data MaybeErr err val = Succeeded val | Failed err