Safe Haskell	None
Language	Haskell2010

GHC.Maybe

Description

Maybe type

Synopsis

data Maybe a
- = Nothing
- | Just a

Documentation

data Maybe a Source #

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

Instances details

MonadFail Maybe Source #

Since: base-4.9.0.0

Instance details

Defined in Control.Monad.Fail

Methods

fail :: String -> Maybe a Source #

MonadFix Maybe Source #

Since: base-2.1

Instance details

Defined in Control.Monad.Fix

Methods

mfix :: (a -> Maybe a) -> Maybe a Source #

MonadZip Maybe Source #

Since: base-4.8.0.0

Instance details

Defined in Control.Monad.Zip

Methods

mzip :: Maybe a -> Maybe b -> Maybe (a, b) Source #

mzipWith :: (a -> b -> c) -> Maybe a -> Maybe b -> Maybe c Source #

munzip :: Maybe (a, b) -> (Maybe a, Maybe b) Source #

Foldable Maybe Source #

Since: base-2.1

Instance details

Defined in Data.Foldable

Methods

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

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

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

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

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

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

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

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

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

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

null :: Maybe a -> Bool Source #

length :: Maybe a -> Int Source #

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

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

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

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

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

Eq1 Maybe Source #

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Classes

Methods

liftEq :: (a -> b -> Bool) -> Maybe a -> Maybe b -> Bool Source #

Ord1 Maybe Source #

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Classes

Methods

liftCompare :: (a -> b -> Ordering) -> Maybe a -> Maybe b -> Ordering Source #

Read1 Maybe Source #

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Classes

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (Maybe a) Source #

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [Maybe a] Source #

liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (Maybe a) Source #

liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [Maybe a] Source #

Show1 Maybe Source #

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Classes

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> Maybe a -> ShowS Source #

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [Maybe a] -> ShowS Source #

Traversable Maybe Source #

Since: base-2.1

Instance details

Defined in Data.Traversable

Methods

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

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

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

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

Alternative Maybe Source #

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

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

empty :: Maybe a Source #

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

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

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

Applicative Maybe Source #

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

pure :: a -> Maybe a Source #

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

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

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

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

Functor Maybe Source #

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

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

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

Monad Maybe Source #

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

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

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

return :: a -> Maybe a Source #

MonadPlus Maybe Source #

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

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

mzero :: Maybe a Source #

mplus :: Maybe a -> Maybe a -> Maybe a Source #

Generic1 Maybe Source #

Instance details

Defined in GHC.Generics

Associated Types

type Rep1 Maybe	Since: base-4.6.0.0
Instance details Defined in GHC.Generics type Rep1 Maybe = D1 ('MetaData "Maybe" "GHC.Maybe" "base" 'False) (C1 ('MetaCons "Nothing" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons "Just" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) Par1))

Methods

from1 :: Maybe a -> Rep1 Maybe a Source #

to1 :: Rep1 Maybe a -> Maybe a Source #

Data a => Data (Maybe a) Source #

Since: base-4.0.0.0

Instance details

Defined in Data.Data

Methods

gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> Maybe a -> c (Maybe a) Source #

gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (Maybe a) Source #

toConstr :: Maybe a -> Constr Source #

dataTypeOf :: Maybe a -> DataType Source #

dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (Maybe a)) Source #

dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (Maybe a)) Source #

gmapT :: (forall b. Data b => b -> b) -> Maybe a -> Maybe a Source #

gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Maybe a -> r Source #

gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Maybe a -> r Source #

gmapQ :: (forall d. Data d => d -> u) -> Maybe a -> [u] Source #

gmapQi :: Int -> (forall d. Data d => d -> u) -> Maybe a -> u Source #

gmapM :: Monad m => (forall d. Data d => d -> m d) -> Maybe a -> m (Maybe a) Source #

gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Maybe a -> m (Maybe a) Source #

gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Maybe a -> m (Maybe a) Source #

Semigroup a => Monoid (Maybe a) Source #

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

Instance details

Defined in GHC.Base

Methods

mempty :: Maybe a Source #

mappend :: Maybe a -> Maybe a -> Maybe a Source #

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

Semigroup a => Semigroup (Maybe a) Source #

Since: base-4.9.0.0

Instance details

Defined in GHC.Base

Methods

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

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

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

Generic (Maybe a) Source #

Instance details

Defined in GHC.Generics

Associated Types

type Rep (Maybe a)	Since: base-4.6.0.0
Instance details Defined in GHC.Generics type Rep (Maybe a) = D1 ('MetaData "Maybe" "GHC.Maybe" "base" 'False) (C1 ('MetaCons "Nothing" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons "Just" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 a)))