Safe Haskell	None
Language	Haskell2010

Maybes

Contents

MaybeT

Synopsis

Documentation

module Data.Maybe

data MaybeErr err val Source #

Constructors

Succeeded val
Failed err

Instances

Monad (MaybeErr err) Source #
Methods (>>=) :: MaybeErr err a -> (a -> MaybeErr err b) -> MaybeErr err b # (>>) :: MaybeErr err a -> MaybeErr err b -> MaybeErr err b # return :: a -> MaybeErr err a # fail :: String -> MaybeErr err a #
Functor (MaybeErr err) Source #
Methods fmap :: (a -> b) -> MaybeErr err a -> MaybeErr err b # (<$) :: a -> MaybeErr err b -> MaybeErr err a #
Applicative (MaybeErr err) Source #
Methods pure :: a -> MaybeErr err a # (<>) :: MaybeErr err (a -> b) -> MaybeErr err a -> MaybeErr err b # liftA2 :: (a -> b -> c) -> MaybeErr err a -> MaybeErr err b -> MaybeErr err c # (>) :: MaybeErr err a -> MaybeErr err b -> MaybeErr err b # (<*) :: MaybeErr err a -> MaybeErr err b -> MaybeErr err a #

failME :: err -> MaybeErr err val Source #

isSuccess :: MaybeErr err val -> Bool Source #

orElse :: Maybe a -> a -> a infixr 4 Source #

Flipped version of fromMaybe, useful for chaining.

firstJust :: Maybe a -> Maybe a -> Maybe a Source #

firstJusts :: [Maybe a] -> Maybe a Source #

Takes a list of Maybes and returns the first Just if there is one, or Nothing otherwise.

whenIsJust :: Monad m => Maybe a -> (a -> m ()) -> m () Source #

expectJust :: HasCallStack => String -> Maybe a -> a Source #

MaybeT

newtype MaybeT (m :: * -> *) a :: (* -> *) -> * -> * #

The parameterizable maybe monad, obtained by composing an arbitrary monad with the Maybe monad.

Computations are actions that may produce a value or exit.

The return function yields a computation that produces that value, while >>= sequences two subcomputations, exiting if either computation does.

Constructors

MaybeT
Fields runMaybeT :: m (Maybe a)

Instances

MonadTrans MaybeT
Methods lift :: Monad m => m a -> MaybeT m a #
Monad m => Monad (MaybeT m)
Methods (>>=) :: MaybeT m a -> (a -> MaybeT m b) -> MaybeT m b # (>>) :: MaybeT m a -> MaybeT m b -> MaybeT m b # return :: a -> MaybeT m a # fail :: String -> MaybeT m a #
Functor m => Functor (MaybeT m)
Methods fmap :: (a -> b) -> MaybeT m a -> MaybeT m b # (<$) :: a -> MaybeT m b -> MaybeT m a #
MonadFix m => MonadFix (MaybeT m)
Methods mfix :: (a -> MaybeT m a) -> MaybeT m a #
Monad m => MonadFail (MaybeT m)
Methods fail :: String -> MaybeT m a #
(Functor m, Monad m) => Applicative (MaybeT m)
Methods pure :: a -> MaybeT m a # (<>) :: MaybeT m (a -> b) -> MaybeT m a -> MaybeT m b # liftA2 :: (a -> b -> c) -> MaybeT m a -> MaybeT m b -> MaybeT m c # (>) :: MaybeT m a -> MaybeT m b -> MaybeT m b # (<*) :: MaybeT m a -> MaybeT m b -> MaybeT m a #
Foldable f => Foldable (MaybeT f)
Methods fold :: Monoid m => MaybeT f m -> m # foldMap :: Monoid m => (a -> m) -> MaybeT f a -> m # foldr :: (a -> b -> b) -> b -> MaybeT f a -> b # foldr' :: (a -> b -> b) -> b -> MaybeT f a -> b # foldl :: (b -> a -> b) -> b -> MaybeT f a -> b # foldl' :: (b -> a -> b) -> b -> MaybeT f a -> b # foldr1 :: (a -> a -> a) -> MaybeT f a -> a # foldl1 :: (a -> a -> a) -> MaybeT f a -> a # toList :: MaybeT f a -> [a] # null :: MaybeT f a -> Bool # length :: MaybeT f a -> Int # elem :: Eq a => a -> MaybeT f a -> Bool # maximum :: Ord a => MaybeT f a -> a # minimum :: Ord a => MaybeT f a -> a # sum :: Num a => MaybeT f a -> a # product :: Num a => MaybeT f a -> a #
Traversable f => Traversable (MaybeT f)
Methods traverse :: Applicative f => (a -> f b) -> MaybeT f a -> f (MaybeT f b) # sequenceA :: Applicative f => MaybeT f (f a) -> f (MaybeT f a) # mapM :: Monad m => (a -> m b) -> MaybeT f a -> m (MaybeT f b) # sequence :: Monad m => MaybeT f (m a) -> m (MaybeT f a) #
Eq1 m => Eq1 (MaybeT m)
Methods liftEq :: (a -> b -> Bool) -> MaybeT m a -> MaybeT m b -> Bool #
Ord1 m => Ord1 (MaybeT m)
Methods liftCompare :: (a -> b -> Ordering) -> MaybeT m a -> MaybeT m b -> Ordering #
Read1 m => Read1 (MaybeT m)
Methods liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (MaybeT m a) # liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [MaybeT m a] # liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (MaybeT m a) # liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [MaybeT m a] #
Show1 m => Show1 (MaybeT m)
Methods liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> MaybeT m a -> ShowS # liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [MaybeT m a] -> ShowS #
MonadZip m => MonadZip (MaybeT m)
Methods mzip :: MaybeT m a -> MaybeT m b -> MaybeT m (a, b) # mzipWith :: (a -> b -> c) -> MaybeT m a -> MaybeT m b -> MaybeT m c # munzip :: MaybeT m (a, b) -> (MaybeT m a, MaybeT m b) #
MonadIO m => MonadIO (MaybeT m)
Methods liftIO :: IO a -> MaybeT m a #
(Functor m, Monad m) => Alternative (MaybeT m)
Methods empty :: MaybeT m a # (<\|>) :: MaybeT m a -> MaybeT m a -> MaybeT m a # some :: MaybeT m a -> MaybeT m [a] # many :: MaybeT m a -> MaybeT m [a] #
Monad m => MonadPlus (MaybeT m)
Methods mzero :: MaybeT m a # mplus :: MaybeT m a -> MaybeT m a -> MaybeT m a #
(Monad m, HasDynFlags m) => HasDynFlags (MaybeT m) Source #
Methods getDynFlags :: MaybeT m DynFlags Source #
(Eq1 m, Eq a) => Eq (MaybeT m a)
Methods (==) :: MaybeT m a -> MaybeT m a -> Bool # (/=) :: MaybeT m a -> MaybeT m a -> Bool #
(Ord1 m, Ord a) => Ord (MaybeT m a)
Methods compare :: MaybeT m a -> MaybeT m a -> Ordering # (<) :: MaybeT m a -> MaybeT m a -> Bool # (<=) :: MaybeT m a -> MaybeT m a -> Bool # (>) :: MaybeT m a -> MaybeT m a -> Bool # (>=) :: MaybeT m a -> MaybeT m a -> Bool # max :: MaybeT m a -> MaybeT m a -> MaybeT m a # min :: MaybeT m a -> MaybeT m a -> MaybeT m a #
(Read1 m, Read a) => Read (MaybeT m a)
Methods readsPrec :: Int -> ReadS (MaybeT m a) # readList :: ReadS [MaybeT m a] # readPrec :: ReadPrec (MaybeT m a) # readListPrec :: ReadPrec [MaybeT m a] #
(Show1 m, Show a) => Show (MaybeT m a)
Methods showsPrec :: Int -> MaybeT m a -> ShowS # show :: MaybeT m a -> String # showList :: [MaybeT m a] -> ShowS #

liftMaybeT :: Monad m => m a -> MaybeT m a Source #

tryMaybeT :: IO a -> MaybeT IO a Source #

Try performing an IO action, failing on error.