Safe Haskell	Safe-Inferred
Language	Haskell2010

Data.Maybe.Linear

Description

This module provides linear functions on the standard Maybe type.

Synopsis

data Maybe a
- = Nothing
- | Just a
maybe :: b -> (a %1 -> b) -> Maybe a %1 -> b
fromMaybe :: a -> Maybe a %1 -> a
maybeToList :: Maybe a %1 -> [a]
catMaybes :: [Maybe a] %1 -> [a]
mapMaybe :: (a %1 -> Maybe b) -> [a] %1 -> [b]

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

Instances details

Monad Maybe	Since: base-2.1
Instance details Defined in GHC.Base Methods (>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b # (>>) :: Maybe a -> Maybe b -> Maybe b # return :: a -> Maybe a #
Functor Maybe	Since: base-2.1
Instance details Defined in GHC.Base Methods fmap :: (a -> b) -> Maybe a -> Maybe b # (<$) :: a -> Maybe b -> Maybe a #
MonadFail Maybe	Since: base-4.9.0.0
Instance details Defined in Control.Monad.Fail Methods fail :: String -> Maybe a #
Applicative Maybe	Since: base-2.1
Instance details 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 #
Foldable Maybe	Since: base-2.1
Instance details 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
Instance details 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) #
Show1 Maybe	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> Maybe a -> ShowS # liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [Maybe a] -> ShowS #
Read1 Maybe	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (Maybe a) # liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [Maybe a] # liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (Maybe a) # liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [Maybe a] #
Ord1 Maybe	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftCompare :: (a -> b -> Ordering) -> Maybe a -> Maybe b -> Ordering #
Eq1 Maybe	Since: base-4.9.0.0
Instance details Defined in Data.Functor.Classes Methods liftEq :: (a -> b -> Bool) -> Maybe a -> Maybe b -> Bool #
MonadPlus Maybe	Since: base-2.1
Instance details Defined in GHC.Base Methods mzero :: Maybe a # mplus :: Maybe a -> Maybe a -> Maybe a #
Alternative Maybe	Since: base-2.1
Instance details Defined in GHC.Base Methods empty :: Maybe a # (<\|>) :: Maybe a -> Maybe a -> Maybe a # some :: Maybe a -> Maybe [a] # many :: Maybe a -> Maybe [a] #
Hashable1 Maybe
Instance details Defined in Data.Hashable.Class Methods liftHashWithSalt :: (Int -> a -> Int) -> Int -> Maybe a -> Int #
Functor Maybe Source #
Instance details Defined in Data.Functor.Linear.Internal.Functor Methods fmap :: (a %1 -> b) -> Maybe a %1 -> Maybe b Source #
Traversable Maybe Source #
Instance details Defined in Data.Functor.Linear.Internal.Traversable Methods traverse :: Applicative f => (a %1 -> f b) -> Maybe a %1 -> f (Maybe b) Source # sequence :: Applicative f => Maybe (f a) %1 -> f (Maybe a) Source #
Generic1 Maybe
Instance details Defined in GHC.Generics Associated Types type Rep1 Maybe :: k -> Type # Methods from1 :: forall (a :: k). Maybe a -> Rep1 Maybe a # to1 :: forall (a :: k). Rep1 Maybe a -> Maybe a #
Lift a => Lift (Maybe a :: Type)
Instance details Defined in Language.Haskell.TH.Syntax Methods lift :: Quote m => Maybe a -> m Exp # liftTyped :: forall (m :: Type -> Type). Quote m => Maybe a -> Code m (Maybe a) #
Eq a => Eq (Maybe a)	Since: base-2.1
Instance details Defined in GHC.Maybe Methods (==) :: Maybe a -> Maybe a -> Bool # (/=) :: Maybe a -> Maybe a -> Bool #
Ord a => Ord (Maybe a)	Since: base-2.1
Instance details 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 #
Read a => Read (Maybe a)	Since: base-2.1
Instance details 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
Instance details Defined in GHC.Show Methods showsPrec :: Int -> Maybe a -> ShowS # show :: Maybe a -> String # showList :: [Maybe a] -> ShowS #
Generic (Maybe a)
Instance details Defined in GHC.Generics Associated Types type Rep (Maybe a) :: Type -> Type # Methods from :: Maybe a -> Rep (Maybe a) x # to :: Rep (Maybe a) x -> Maybe a #
Semigroup a => Semigroup (Maybe a)	Since: base-4.9.0.0
Instance details 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 #
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 `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 # mappend :: Maybe a -> Maybe a -> Maybe a # mconcat :: [Maybe a] -> Maybe a #
SingKind a => SingKind (Maybe a)	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Associated Types type DemoteRep (Maybe a) Methods fromSing :: forall (a0 :: Maybe a). Sing a0 -> DemoteRep (Maybe a)
Storable a => Storable (Maybe a) Source #
Instance details Defined in Foreign.Marshal.Pure Methods sizeOf :: Maybe a -> Int # alignment :: Maybe a -> Int # peekElemOff :: Ptr (Maybe a) -> Int -> IO (Maybe a) # pokeElemOff :: Ptr (Maybe a) -> Int -> Maybe a -> IO () # peekByteOff :: Ptr b -> Int -> IO (Maybe a) # pokeByteOff :: Ptr b -> Int -> Maybe a -> IO () # peek :: Ptr (Maybe a) -> IO (Maybe a) # poke :: Ptr (Maybe a) -> Maybe a -> IO () #
Hashable a => Hashable (Maybe a)
Instance details Defined in Data.Hashable.Class Methods hashWithSalt :: Int -> Maybe a -> Int # hash :: Maybe a -> Int #
Consumable a => Consumable (Maybe a) Source #
Instance details Defined in Data.Unrestricted.Internal.Instances Methods consume :: Maybe a %1 -> () Source #
Dupable a => Dupable (Maybe a) Source #
Instance details Defined in Data.Unrestricted.Internal.Instances Methods dupV :: forall (n :: Nat). KnownNat n => Maybe a %1 -> V n (Maybe a) Source # dup2 :: Maybe a %1 -> (Maybe a, Maybe a) Source #
Movable a => Movable (Maybe a) Source #
Instance details Defined in Data.Unrestricted.Internal.Instances Methods move :: Maybe a %1 -> Ur (Maybe a) Source #
(Consumable a, Eq a) => Eq (Maybe a) Source #
Instance details Defined in Data.Ord.Linear.Internal.Eq Methods (==) :: Maybe a %1 -> Maybe a %1 -> Bool Source # (/=) :: Maybe a %1 -> Maybe a %1 -> Bool Source #
(Consumable a, Ord a) => Ord (Maybe a) Source #
Instance details Defined in Data.Ord.Linear.Internal.Ord Methods compare :: Maybe a %1 -> Maybe a %1 -> Ordering Source # (<=) :: Maybe a %1 -> Maybe a %1 -> Bool Source # (<) :: Maybe a %1 -> Maybe a %1 -> Bool Source # (>) :: Maybe a %1 -> Maybe a %1 -> Bool Source # (>=) :: Maybe a %1 -> Maybe a %1 -> Bool Source #
Representable a => Representable (Maybe a) Source #
Instance details Defined in Foreign.Marshal.Pure Associated Types type AsKnown (Maybe a) Source # Methods toKnown :: Maybe a %1 -> AsKnown (Maybe a) Source # ofKnown :: AsKnown (Maybe a) %1 -> Maybe a Source #
KnownRepresentable a => KnownRepresentable (Maybe a) Source #
Instance details Defined in Foreign.Marshal.Pure Methods storable :: Dict (Storable (Maybe a))
SingI ('Nothing :: Maybe a)	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods sing :: Sing 'Nothing
SingI a2 => SingI ('Just a2 :: Maybe a1)	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods sing :: Sing ('Just a2)
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))
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)))
data Sing (b :: Maybe a)
Instance details Defined in GHC.Generics data Sing (b :: Maybe a) where SNothing :: forall {a}. Sing ('Nothing :: Maybe a) SJust :: forall {a} (a1 :: a). Sing a1 -> Sing ('Just a1)
type DemoteRep (Maybe a)
Instance details Defined in GHC.Generics type DemoteRep (Maybe a) = Maybe (DemoteRep a)
type AsKnown (Maybe a) Source #
Instance details Defined in Foreign.Marshal.Pure type AsKnown (Maybe a) = Maybe (AsKnown a)

maybe :: b -> (a %1 -> b) -> Maybe a %1 -> b Source #

maybe b f m returns (f a) where a is in m if it exists and b otherwise

fromMaybe :: a -> Maybe a %1 -> a Source #

fromMaybe default m is the a in m if it exists and the default otherwise

maybeToList :: Maybe a %1 -> [a] Source #

maybeToList m creates a singleton or an empty list based on the Maybe a.

catMaybes :: [Maybe a] %1 -> [a] Source #

catMaybes xs discards the Nothings in xs and extracts the as

mapMaybe :: (a %1 -> Maybe b) -> [a] %1 -> [b] Source #

mapMaybe f xs = catMaybes (map f xs)