% % (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, firstJusts,
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)

firstJust :: Maybe a -> Maybe a -> Maybe a
firstJust (Just a) _ = Just a
firstJust Nothing  b = b

-- | Takes a list of @Maybes@ and returns the first @Just@ if there is one, or
-- @Nothing@ otherwise.
firstJusts :: [Maybe a] -> Maybe a
firstJusts = foldr firstJust Nothing

\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 $Just x'  \end{code} %************************************************************************ %* * \subsection[MaybeT type]{The @MaybeT@ monad transformer} %* * %************************************************************************ \begin{code}  newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)} instance Functor m => Functor (MaybeT m) where fmap f x = MaybeT$ fmap (fmap f) $runMaybeT x instance Monad m => Monad (MaybeT m) where return = MaybeT . return . Just x >>= f = MaybeT$ 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