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%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\begin{code}
module Maybes (
module Data.Maybe,
MaybeErr(..), -- Instance of Monad
failME, isSuccess,
orElse,
mapCatMaybes,
allMaybes,
firstJust, firstJusts,
whenIsJust,
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
whenIsJust :: Monad m => Maybe a -> (a -> m ()) -> m ()
whenIsJust (Just x) f = f x
whenIsJust Nothing _ = return ()
\end{code}
\begin{code}
-- | flipped version of @fromMaybe@.
orElse :: Maybe a -> a -> a
(Just x) `orElse` _ = x
Nothing `orElse` y = y
\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
instance Monad (MaybeErr err) where
return v = Succeeded v
Succeeded v >>= k = k v
Failed e >>= _ = Failed e
isSuccess :: MaybeErr err val -> Bool
isSuccess (Succeeded {}) = True
isSuccess (Failed {}) = False
failME :: err -> MaybeErr err val
failME e = Failed e
\end{code}
|
