Safe Haskell	None
Language	Haskell2010

GHC.Maybe

Description

Maybe type

Synopsis

dataMaybe a
- =Nothing
- |Just a

Documentation

dataMaybe aSource #

TheMaybe type encapsulates an optional value. A value of typeMaybe a either contains a value of typea (represented asJust a), or it is empty (represented asNothing). UsingMaybe is a good way to deal with errors or exceptional cases without resorting to drastic measures such aserror.

TheMaybe type is also a monad. It is a simple kind of error monad, where all errors are represented byNothing. A richer error monad can be built using theEither type.

Constructors

Nothing
Just a

Instances

Monad MaybeSource #	Since: 2.1
Instance details Defined inGHC.Base Methods (>>=) ::Maybe a -> (a ->Maybe b) ->Maybe bSource # (>>) ::Maybe a ->Maybe b ->Maybe bSource # return :: a ->Maybe aSource # fail ::String ->Maybe aSource #
Functor MaybeSource #	Since: 2.1
Instance details Defined inGHC.Base Methods fmap :: (a -> b) ->Maybe a ->Maybe bSource # (<$) :: a ->Maybe b ->Maybe aSource #
MonadFix MaybeSource #	Since: 2.1
Instance details Defined inControl.Monad.Fix Methods mfix :: (a ->Maybe a) ->Maybe aSource #
MonadFail MaybeSource #	Since: 4.9.0.0
Instance details Defined inControl.Monad.Fail Methods fail ::String ->Maybe aSource #
Applicative MaybeSource #	Since: 2.1
Instance details Defined inGHC.Base Methods pure :: a ->Maybe aSource # (<>) ::Maybe (a -> b) ->Maybe a ->Maybe bSource # liftA2 :: (a -> b -> c) ->Maybe a ->Maybe b ->Maybe cSource # (>) ::Maybe a ->Maybe b ->Maybe bSource # (<*) ::Maybe a ->Maybe b ->Maybe aSource #
Foldable MaybeSource #	Since: 2.1
Instance details Defined inData.Foldable Methods fold ::Monoid m =>Maybe m -> mSource # foldMap ::Monoid m => (a -> m) ->Maybe a -> mSource # foldr :: (a -> b -> b) -> b ->Maybe a -> bSource # foldr' :: (a -> b -> b) -> b ->Maybe a -> bSource # foldl :: (b -> a -> b) -> b ->Maybe a -> bSource # foldl' :: (b -> a -> b) -> b ->Maybe a -> bSource # foldr1 :: (a -> a -> a) ->Maybe a -> aSource # foldl1 :: (a -> a -> a) ->Maybe a -> aSource # 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 -> aSource # minimum ::Ord a =>Maybe a -> aSource # sum ::Num a =>Maybe a -> aSource # product ::Num a =>Maybe a -> aSource #
Traversable MaybeSource #	Since: 2.1
Instance details Defined inData.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 #
MonadPlus MaybeSource #	Since: 2.1
Instance details Defined inGHC.Base Methods mzero ::Maybe aSource # mplus ::Maybe a ->Maybe a ->Maybe aSource #
Alternative MaybeSource #	Since: 2.1
Instance details Defined inGHC.Base Methods empty ::Maybe aSource # (<\|>) ::Maybe a ->Maybe a ->Maybe aSource # some ::Maybe a ->Maybe [a]Source # many ::Maybe a ->Maybe [a]Source #
MonadZip MaybeSource #	Since: 4.8.0.0
Instance details Defined inControl.Monad.Zip Methods mzip ::Maybe a ->Maybe b ->Maybe (a, b)Source # mzipWith :: (a -> b -> c) ->Maybe a ->Maybe b ->Maybe cSource # munzip ::Maybe (a, b) -> (Maybe a,Maybe b)Source #
Show1 MaybeSource #	Since: 4.9.0.0
Instance details Defined inData.Functor.Classes Methods liftShowsPrec :: (Int -> a ->ShowS) -> ([a] ->ShowS) ->Int ->Maybe a ->ShowS Source # liftShowList :: (Int -> a ->ShowS) -> ([a] ->ShowS) -> [Maybe a] ->ShowS Source #
Read1 MaybeSource #	Since: 4.9.0.0
Instance details Defined inData.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 #
Ord1 MaybeSource #	Since: 4.9.0.0
Instance details Defined inData.Functor.Classes Methods liftCompare :: (a -> b ->Ordering) ->Maybe a ->Maybe b ->Ordering Source #
Eq1 MaybeSource #	Since: 4.9.0.0
Instance details Defined inData.Functor.Classes Methods liftEq :: (a -> b ->Bool) ->Maybe a ->Maybe b ->Bool Source #
Eq a =>Eq (Maybe a)Source #	Since: 2.1
Instance details Defined inGHC.Maybe Methods (==) ::Maybe a ->Maybe a ->Bool # (/=) ::Maybe a ->Maybe a ->Bool #
Data a =>Data (Maybe a)Source #	Since: 4.0.0.0
Instance details Defined inData.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 aSource # gmapQl :: (r -> r' -> r) -> r -> (forall d.Data d => d -> r') ->Maybe a -> rSource # gmapQr :: (r' -> r -> r) -> r -> (forall d.Data d => d -> r') ->Maybe a -> rSource # gmapQ :: (forall d.Data d => d -> u) ->Maybe a -> [u]Source # gmapQi ::Int -> (forall d.Data d => d -> u) ->Maybe a -> uSource # 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 #
Ord a =>Ord (Maybe a)Source #	Since: 2.1
Instance details Defined inGHC.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#
Read a =>Read (Maybe a)Source #	Since: 2.1
Instance details Defined inGHC.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: 2.1
Instance details Defined inGHC.Show Methods showsPrec ::Int ->Maybe a ->ShowS Source # show ::Maybe a ->String Source # showList :: [Maybe a] ->ShowS Source #
Generic (Maybe a)Source #
Instance details Defined inGHC.Generics Associated Types typeRep (Maybe a) ::Type ->Type Source # Methods from ::Maybe a ->Rep (Maybe a) xSource # to ::Rep (Maybe a) x ->Maybe aSource #
Semigroup a =>Semigroup (Maybe a)Source #	Since: 4.9.0.0
Instance details Defined inGHC.Base Methods (<>) ::Maybe a ->Maybe a ->Maybe aSource # sconcat ::NonEmpty (Maybe a) ->Maybe aSource # stimes ::Integral b => b ->Maybe a ->Maybe aSource #
Semigroup a =>Monoid (Maybe a)Source #	Lift a semigroup into`Maybe` forming a`Monoid` according tohttp://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: 2.1*
Instance details Defined inGHC.Base Methods mempty ::Maybe aSource # mappend ::Maybe a ->Maybe a ->Maybe aSource # mconcat :: [Maybe a] ->Maybe aSource #
Generic1 MaybeSource #
Instance details Defined inGHC.Generics Associated Types typeRep1 Maybe :: k ->Type Source # Methods from1 ::Maybe a ->Rep1 Maybe aSource # to1 ::Rep1 Maybe a ->Maybe aSource #
typeRep (Maybe a)Source #	Since: 4.6.0.0
Instance details Defined inGHC.Generics typeRep (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)))
typeRep1 MaybeSource #	Since: 4.6.0.0
Instance details Defined inGHC.Generics typeRep1 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))

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