Safe Haskell	Safe-Inferred
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 :: k -> Type Source # Methods from1 :: forall (a :: k). Maybe a -> Rep1 Maybe a Source # to1 :: forall (a :: k). 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 `ee = e` and `es = s = se` 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) :: Type -> Type Source # Methods from :: Maybe a -> Rep (Maybe a) x Source # to :: Rep (Maybe a) x -> Maybe a Source #
Read a => Read (Maybe a) Source #	Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS (Maybe a) Source # readList :: ReadS [Maybe a] Source # readPrec :: ReadPrec (Maybe a) Source # readListPrec :: ReadPrec [Maybe a] Source #
Show a => Show (Maybe a) Source #	Since: base-2.1
Instance details Defined in GHC.Show Methods showsPrec :: Int -> Maybe a -> ShowS Source # show :: Maybe a -> String Source # showList :: [Maybe a] -> ShowS Source #
Eq a => Eq (Maybe a) Source #	Since: base-2.1
Instance details Defined in GHC.Maybe Methods (==) :: Maybe a -> Maybe a -> Bool Source # (/=) :: Maybe a -> Maybe a -> Bool Source #
Ord a => Ord (Maybe a) Source #	Since: base-2.1
Instance details Defined in GHC.Maybe Methods compare :: Maybe a -> Maybe a -> Ordering Source # (<) :: Maybe a -> Maybe a -> Bool Source # (<=) :: Maybe a -> Maybe a -> Bool Source # (>) :: Maybe a -> Maybe a -> Bool Source # (>=) :: Maybe a -> Maybe a -> Bool Source # max :: Maybe a -> Maybe a -> Maybe a Source # min :: Maybe a -> Maybe a -> Maybe a Source #
type Rep1 Maybe Source #	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))
type Rep (Maybe a) Source #	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)))