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/Haskell 9

GHC.Maybe

Copyright	(c) The University of Glasgow 1997-2002
License	see libraries/base/LICENSE
Maintainer	[email protected]
Stability	internal
Portability	non-portable (GHC Extensions)
Safe Haskell	Safe-Inferred
Language	Haskell2010

Description

The Maybe type.

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

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

Alternative Maybe Source

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

Since: base-2.1

Instance details

Defined in GHC.Internal.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.Internal.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.Internal.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.Internal.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.Internal.Base

Methods

mzero :: Maybe a Source

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

MonadFail Maybe Source

Since: base-4.9.0.0

Instance details

Defined in GHC.Internal.Control.Monad.Fail

Methods

fail :: String -> Maybe a Source

MonadFix Maybe Source

Since: base-2.1

Instance details

Defined in GHC.Internal.Control.Monad.Fix

Methods

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

MonadZip Maybe Source

Since: ghc-internal-4.8.0.0

Instance details

Defined in GHC.Internal.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 GHC.Internal.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

Traversable Maybe Source

Since: base-2.1

Instance details

Defined in GHC.Internal.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

Generic1 Maybe Source

Instance details

Defined in GHC.Internal.Generics

Associated Types

type Rep1 Maybe	Since: base-4.6.0.0
Instance details Defined in GHC.Internal.Generics type Rep1 Maybe = D1 ('MetaData "Maybe" "GHC.Internal.Maybe" "ghc-internal" '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

Lift a => Lift (Maybe a :: Type) Source

Instance details

Defined in GHC.Internal.TH.Lift

Methods

lift :: Quote m => Maybe a -> m Exp Source

liftTyped :: forall (m :: Type -> Type). Quote m => Maybe a -> Code 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.Internal.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.Internal.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

Data a => Data (Maybe a) Source

Since: base-4.0.0.0

Instance details

Defined in GHC.Internal.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

Generic (Maybe a) Source

Instance details

Defined in GHC.Internal.Generics

Associated Types

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