Antelude.Maybe

Documentation

data Maybe a

The Maybe type encapsulates an optional value. A value of type Maybe a either contains a value of type a (represented as Just a), or it is empty (represented as Nothing). Using Maybe is a good way to deal with errors or exceptional cases without resorting to drastic measures such as error.

The Maybe type is also a monad. It is a simple kind of error monad, where all errors are represented by Nothing. A richer error monad can be built using the Either type.

Constructors

Nothing

Just a

Instances

MonadFail Maybe

Since: base-4.9.0.0

Defined in Control.Monad.Fail

Methods

fail :: String -> Maybe a

Foldable Maybe

Since: base-2.1

Defined in Data.Foldable

Methods

fold :: Monoid m => Maybe m -> m

foldMap :: Monoid m => (a -> m) -> Maybe a -> m

foldMap' :: Monoid m => (a -> m) -> Maybe a -> m

foldr :: (a -> b -> b) -> b -> Maybe a -> b

foldr' :: (a -> b -> b) -> b -> Maybe a -> b

foldl :: (b -> a -> b) -> b -> Maybe a -> b

foldl' :: (b -> a -> b) -> b -> Maybe a -> b

foldr1 :: (a -> a -> a) -> Maybe a -> a

foldl1 :: (a -> a -> a) -> Maybe a -> a

toList :: Maybe a -> [a]

null :: Maybe a -> Bool

length :: Maybe a -> Int

elem :: Eq a => a -> Maybe a -> Bool

maximum :: Ord a => Maybe a -> a

minimum :: Ord a => Maybe a -> a

sum :: Num a => Maybe a -> a

product :: Num a => Maybe a -> a

Traversable Maybe

Since: base-2.1

Defined in Data.Traversable

Methods

traverse :: Applicative f => (a -> f b) -> Maybe a -> f (Maybe b)

sequenceA :: Applicative f => Maybe (f a) -> f (Maybe a)

mapM :: Monad m => (a -> m b) -> Maybe a -> m (Maybe b)

sequence :: Monad m => Maybe (m a) -> m (Maybe a)

Alternative Maybe

Picks the leftmost Just value, or, alternatively, Nothing.

Since: base-2.1

Defined in GHC.Base

Methods

empty :: Maybe a

(<|>) :: Maybe a -> Maybe a -> Maybe a

some :: Maybe a -> Maybe [a]

many :: Maybe a -> Maybe [a]

Applicative Maybe

Since: base-2.1

Defined in GHC.Base

Methods

pure :: a -> Maybe a

(<*>) :: Maybe (a -> b) -> Maybe a -> Maybe b

liftA2 :: (a -> b -> c) -> Maybe a -> Maybe b -> Maybe c

(*>) :: Maybe a -> Maybe b -> Maybe b

(<*) :: Maybe a -> Maybe b -> Maybe a

Functor Maybe

Since: base-2.1

Defined in GHC.Base

Methods

fmap :: (a -> b) -> Maybe a -> Maybe b

(<$) :: a -> Maybe b -> Maybe a

Monad Maybe

Since: base-2.1

Defined in GHC.Base

Methods

(>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b

(>>) :: Maybe a -> Maybe b -> Maybe b

return :: a -> Maybe a

MonadPlus Maybe

Picks the leftmost Just value, or, alternatively, Nothing.

Since: base-2.1

Defined in GHC.Base

Methods

mzero :: Maybe a

mplus :: Maybe a -> Maybe a -> Maybe a

Semigroup a => Monoid (Maybe a)

Lift a semigroup into Maybe forming a Monoid according to http://en.wikipedia.org/wiki/Monoid: "Any semigroup S may be turned into a monoid simply by adjoining an element e not in S and defining e*e = e and e*s = s = s*e for all s ∈ S."

Since 4.11.0: constraint on inner a value generalised from Monoid to Semigroup.

Since: base-2.1

Defined in GHC.Base

Methods

mempty :: Maybe a

mappend :: Maybe a -> Maybe a -> Maybe a

mconcat :: [Maybe a] -> Maybe a

Semigroup a => Semigroup (Maybe a)

Since: base-4.9.0.0

Defined in GHC.Base

Methods

(<>) :: Maybe a -> Maybe a -> Maybe a

sconcat :: NonEmpty (Maybe a) -> Maybe a

stimes :: Integral b => b -> Maybe a -> Maybe a

Read a => Read (Maybe a)

Since: base-2.1

Defined in GHC.Read

Methods

readsPrec :: Int -> ReadS (Maybe a)

readList :: ReadS [Maybe a]

readPrec :: ReadPrec (Maybe a)

readListPrec :: ReadPrec [Maybe a]

Show a => Show (Maybe a)

Since: base-2.1

Defined in GHC.Show

Methods

showsPrec :: Int -> Maybe a -> ShowS

show :: Maybe a -> String

showList :: [Maybe a] -> ShowS

Eq a => Eq (Maybe a)

Since: base-2.1

Defined in GHC.Maybe

Methods

(==) :: Maybe a -> Maybe a -> Bool

(/=) :: Maybe a -> Maybe a -> Bool

Ord a => Ord (Maybe a)

Since: base-2.1

Defined in GHC.Maybe

Methods

compare :: Maybe a -> Maybe a -> Ordering

(<) :: Maybe a -> Maybe a -> Bool

(<=) :: Maybe a -> Maybe a -> Bool

(>) :: Maybe a -> Maybe a -> Bool

(>=) :: Maybe a -> Maybe a -> Bool

max :: Maybe a -> Maybe a -> Maybe a

min :: Maybe a -> Maybe a -> Maybe a

Reexport from Maybe

isJust :: Maybe a -> Bool

The isJust function returns True iff its argument is of the form Just _.

Examples

Basic usage:

>>> isJust (Just 3)
True

>>> isJust (Just ())
True

>>> isJust Nothing
False

Only the outer constructor is taken into consideration:

>>> isJust (Just Nothing)
True

Reexport from Maybe

isNothing :: Maybe a -> Bool

The isNothing function returns True iff its argument is Nothing.

Examples

Basic usage:

>>> isNothing (Just 3)
False

>>> isNothing (Just ())
False

>>> isNothing Nothing
True

Only the outer constructor is taken into consideration:

>>> isNothing (Just Nothing)
False

fromEitherLeft :: Either left right -> Maybe left

Convert a 'Either a b' to an 'Maybe a', taking the Left case, or Nothing in the case of Right.

fromEitherRight :: Either left right -> Maybe right

Convert a 'Either a b' to an 'Maybe a', taking the Right case, or Nothing in the case of Left.

fromResult :: Result err ok -> Maybe ok

Convert a 'Result a b' to an 'Maybe a', taking the Ok case, or Nothing in the case of Err.

map :: (a -> b) -> Maybe a -> Maybe b

Defined as fmap. Apply a function to the contents of a Maybe, doing nothing if Nothing.

mapWithDefault :: b -> (a -> b) -> Maybe a -> b

A combination of map/fmap and withDefault. Given a default, a function, and a Maybe, get the mapped contents if Just, or the default if Nothing.

withDefault :: a -> Maybe a -> a

Get the contents of a Maybe, using a default if Nothing.