Maybes

Documentation

module Data.Maybe

data MaybeErr err val

Constructors

Succeeded val

Failed err

Instances

Monad (MaybeErr err)

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)

Methods

fmap :: (a -> b) -> MaybeErr err a -> MaybeErr err b

(<$) :: a -> MaybeErr err b -> MaybeErr err a

Applicative (MaybeErr err)

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

isSuccess :: MaybeErr err val -> Bool

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

Flipped version of fromMaybe, useful for chaining.

firstJust :: Maybe a -> Maybe a -> Maybe a

firstJusts :: [Maybe a] -> Maybe a

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 ()

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

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 f0 => (a -> f0 b) -> MaybeT f a -> f0 (MaybeT f b)

sequenceA :: Applicative f0 => MaybeT f (f0 a) -> f0 (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)

Methods

getDynFlags :: MaybeT m DynFlags

(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

tryMaybeT :: IO a -> MaybeT IO a

Try performing an IO action, failing on error.