planet-mitchell-0.1.0: Planet Mitchell

Safe HaskellSafe
LanguageHaskell2010

Maybe

Contents

Synopsis

Maybe

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
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 #

fail :: String -> 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 #

MonadFix Maybe

Since: base-2.1

Instance details

Defined in Control.Monad.Fix

Methods

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

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) #

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] #

ToJSON1 Maybe 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

liftToJSON :: (a -> Value) -> ([a] -> Value) -> Maybe a -> Value #

liftToJSONList :: (a -> Value) -> ([a] -> Value) -> [Maybe a] -> Value #

liftToEncoding :: (a -> Encoding) -> ([a] -> Encoding) -> Maybe a -> Encoding #

liftToEncodingList :: (a -> Encoding) -> ([a] -> Encoding) -> [Maybe a] -> Encoding #

FromJSON1 Maybe 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

liftParseJSON :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser (Maybe a) #

liftParseJSONList :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser [Maybe a] #

MonadPlus Maybe

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

mzero :: Maybe a #

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

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 #

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 #

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] #

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 #

MonadZip Maybe

Since: base-4.8.0.0

Instance details

Defined in Control.Monad.Zip

Methods

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

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

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

Compactable Maybe 
Instance details

Defined in Control.Compactable

Methods

compact :: Maybe (Maybe a) -> Maybe a #

separate :: Maybe (Either l r) -> (Maybe l, Maybe r) #

filter :: (a -> Bool) -> Maybe a -> Maybe a #

partition :: (a -> Bool) -> Maybe a -> (Maybe a, Maybe a) #

fmapMaybe :: Functor Maybe => (a -> Maybe b) -> Maybe a -> Maybe b #

fmapEither :: Functor Maybe => (a -> Either l r) -> Maybe a -> (Maybe l, Maybe r) #

applyMaybe :: Applicative Maybe => Maybe (a -> Maybe b) -> Maybe a -> Maybe b #

applyEither :: Applicative Maybe => Maybe (a -> Either l r) -> Maybe a -> (Maybe l, Maybe r) #

bindMaybe :: Monad Maybe => Maybe a -> (a -> Maybe (Maybe b)) -> Maybe b #

bindEither :: Monad Maybe => Maybe a -> (a -> Maybe (Either l r)) -> (Maybe l, Maybe r) #

traverseMaybe :: (Applicative g, Traversable Maybe) => (a -> g (Maybe b)) -> Maybe a -> g (Maybe b) #

traverseEither :: (Applicative g, Traversable Maybe) => (a -> g (Either l r)) -> Maybe a -> g (Maybe l, Maybe r) #

CompactFold Maybe 
Instance details

Defined in Control.Compactable

Methods

compactFold :: Foldable g => Maybe (g a) -> Maybe a #

separateFold :: Bifoldable g => Maybe (g a b) -> (Maybe a, Maybe b) #

fmapFold :: (Functor Maybe, Foldable g) => (a -> g b) -> Maybe a -> Maybe b #

fmapBifold :: (Functor Maybe, Bifoldable g) => (a -> g l r) -> Maybe a -> (Maybe l, Maybe r) #

applyFold :: (Applicative Maybe, Foldable g) => Maybe (a -> g b) -> Maybe a -> Maybe b #

applyBifold :: (Applicative Maybe, Bifoldable g) => Maybe (a -> g l r) -> Maybe a -> (Maybe l, Maybe r) #

bindFold :: (Monad Maybe, Foldable g) => Maybe a -> (a -> Maybe (g b)) -> Maybe b #

bindBifold :: (Monad Maybe, Bifoldable g) => Maybe a -> (a -> Maybe (g l r)) -> (Maybe l, Maybe r) #

traverseFold :: (Applicative h, Foldable g, Traversable Maybe) => (a -> h (g b)) -> Maybe a -> h (Maybe b) #

traverseBifold :: (Applicative h, Bifoldable g, Traversable Maybe) => (a -> h (g l r)) -> Maybe a -> h (Maybe l, Maybe r) #

NFData1 Maybe

Since: deepseq-1.4.3.0

Instance details

Defined in Control.DeepSeq

Methods

liftRnf :: (a -> ()) -> Maybe a -> () #

MonadThrow Maybe 
Instance details

Defined in Control.Monad.Catch

Methods

throwM :: Exception e => e -> Maybe a #

Hashable1 Maybe 
Instance details

Defined in Data.Hashable.Class

Methods

liftHashWithSalt :: (Int -> a -> Int) -> Int -> Maybe a -> Int #

Apply Maybe 
Instance details

Defined in Data.Functor.Bind.Class

Methods

(<.>) :: Maybe (a -> b) -> Maybe a -> Maybe b #

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

(<.) :: Maybe a -> Maybe b -> Maybe a #

liftF2 :: (a -> b -> c) -> Maybe a -> Maybe b -> Maybe c #

Pointed Maybe 
Instance details

Defined in Data.Pointed

Methods

point :: a -> Maybe a #

Plus Maybe 
Instance details

Defined in Data.Functor.Plus

Methods

zero :: Maybe a #

Alt Maybe 
Instance details

Defined in Data.Functor.Alt

Methods

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

some :: Applicative Maybe => Maybe a -> Maybe [a] #

many :: Applicative Maybe => Maybe a -> Maybe [a] #

Bind Maybe 
Instance details

Defined in Data.Functor.Bind.Class

Methods

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

join :: Maybe (Maybe a) -> Maybe a #

Extend Maybe 
Instance details

Defined in Data.Functor.Extend

Methods

duplicated :: Maybe a -> Maybe (Maybe a) #

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

MonadError () Maybe

Since: mtl-2.2.2

Instance details

Defined in Control.Monad.Error.Class

Methods

throwError :: () -> Maybe a #

catchError :: Maybe a -> (() -> Maybe a) -> Maybe a #

MonadBase Maybe Maybe 
Instance details

Defined in Control.Monad.Base

Methods

liftBase :: Maybe α -> Maybe α #

(Selector s, GToJSON enc arity (K1 i (Maybe a) :: * -> *), KeyValuePair enc pairs, Monoid pairs) => RecordToPairs enc pairs arity (S1 s (K1 i (Maybe a) :: * -> *)) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

recordToPairs :: Options -> ToArgs enc arity a0 -> S1 s (K1 i (Maybe a)) a0 -> pairs

() :=> (Functor Maybe) 
Instance details

Defined in Data.Constraint

Methods

ins :: () :- Functor Maybe #

() :=> (Applicative Maybe) 
Instance details

Defined in Data.Constraint

Methods

ins :: () :- Applicative Maybe #

() :=> (Alternative Maybe) 
Instance details

Defined in Data.Constraint

Methods

ins :: () :- Alternative Maybe #

() :=> (MonadPlus Maybe) 
Instance details

Defined in Data.Constraint

Methods

ins :: () :- MonadPlus Maybe #

(Selector s, FromJSON a) => FromRecord arity (S1 s (K1 i (Maybe a) :: * -> *)) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseRecord :: Options -> FromArgs arity a0 -> Object -> Parser (S1 s (K1 i (Maybe a)) a0)

Eq a => Eq (Maybe a) 
Instance details

Defined in GHC.Base

Methods

(==) :: Maybe a -> Maybe a -> Bool #

(/=) :: Maybe a -> Maybe a -> Bool #

Data a => Data (Maybe a)

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) #

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

toConstr :: Maybe a -> Constr #

dataTypeOf :: Maybe a -> DataType #

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

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

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

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

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

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

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

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

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

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

Ord a => Ord (Maybe a) 
Instance details

Defined in GHC.Base

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

Show a => Show (Maybe a) 
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) :: * -> * #

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 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 #

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

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

Lift a => Lift (Maybe a) 
Instance details

Defined in Language.Haskell.TH.Syntax

Methods

lift :: Maybe a -> Q Exp #

Hashable a => Hashable (Maybe a) 
Instance details

Defined in Data.Hashable.Class

Methods

hashWithSalt :: Int -> Maybe a -> Int #

hash :: Maybe a -> Int #

ToJSON a => ToJSON (Maybe a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

FromJSON a => FromJSON (Maybe a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

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 :: Sing a0 -> DemoteRep (Maybe a)

NFData a => NFData (Maybe a) 
Instance details

Defined in Control.DeepSeq

Methods

rnf :: Maybe a -> () #

Ixed (Maybe a) 
Instance details

Defined in Control.Lens.At

Methods

ix :: Index (Maybe a) -> Traversal' (Maybe a) (IxValue (Maybe a)) #

At (Maybe a) 
Instance details

Defined in Control.Lens.At

Methods

at :: Index (Maybe a) -> Lens' (Maybe a) (Maybe (IxValue (Maybe a))) #

AsEmpty (Maybe a) 
Instance details

Defined in Control.Lens.Empty

Methods

_Empty :: Prism' (Maybe a) () #

Pretty a => Pretty (Maybe a)

Ignore Nothings, print Just contents.

>>> pretty (Just True)
True
>>> braces (pretty (Nothing :: Maybe Bool))
{}
>>> pretty [Just 1, Nothing, Just 3, Nothing]
[1, 3]
Instance details

Defined in Data.Text.Prettyprint.Doc.Internal

Methods

pretty :: Maybe a -> Doc ann #

prettyList :: [Maybe a] -> Doc ann #

Lower (Maybe a) 
Instance details

Defined in Data.Semilattice.Lower

Methods

lowerBound :: Maybe a #

Serialise a => Serialise (Maybe a)

Since: serialise-0.2.0.0

Instance details

Defined in Codec.Serialise.Class

Generic1 Maybe 
Instance details

Defined in GHC.Generics

Associated Types

type Rep1 Maybe :: k -> * #

Methods

from1 :: Maybe a -> Rep1 Maybe a #

to1 :: Rep1 Maybe a -> Maybe a #

SingI (Nothing :: Maybe a)

Since: base-4.9.0.0

Instance details

Defined in GHC.Generics

Methods

sing :: Sing Nothing

(Eq a) :=> (Eq (Maybe a)) 
Instance details

Defined in Data.Constraint

Methods

ins :: Eq a :- Eq (Maybe a) #

(Ord a) :=> (Ord (Maybe a)) 
Instance details

Defined in Data.Constraint

Methods

ins :: Ord a :- Ord (Maybe a) #

(Read a) :=> (Read (Maybe a)) 
Instance details

Defined in Data.Constraint

Methods

ins :: Read a :- Read (Maybe a) #

(Show a) :=> (Show (Maybe a)) 
Instance details

Defined in Data.Constraint

Methods

ins :: Show a :- Show (Maybe a) #

(Semigroup a) :=> (Semigroup (Maybe a)) 
Instance details

Defined in Data.Constraint

Methods

ins :: Semigroup a :- Semigroup (Maybe a) #

(Monoid a) :=> (Monoid (Maybe a)) 
Instance details

Defined in Data.Constraint

Methods

ins :: Monoid a :- Monoid (Maybe a) #

Each (Maybe a) (Maybe b) a b
each :: Traversal (Maybe a) (Maybe b) a b
Instance details

Defined in Control.Lens.Each

Methods

each :: Traversal (Maybe a) (Maybe b) a b #

SingI a2 => SingI (Just a2 :: Maybe a1)

Since: base-4.9.0.0

Instance details

Defined in GHC.Generics

Methods

sing :: Sing (Just a2)

(Selector c, ToJSON a) => GtoJson (M1 S c (K1 i (Maybe a) :: * -> *)) 
Instance details

Defined in Generics.Generic.Aeson

Methods

gtoJSONf :: Settings -> Bool -> Bool -> M1 S c (K1 i (Maybe a)) a0 -> Either [Value] [(Text, Value)] #

(Selector c, FromJSON a) => GfromJson (M1 S c (K1 i (Maybe a) :: * -> *)) 
Instance details

Defined in Generics.Generic.Aeson

Methods

gparseJSONf :: Settings -> Bool -> Bool -> Bool -> StateT [Value] Parser (M1 S c (K1 i (Maybe a)) a0) #

type Rep (Maybe a) 
Instance details

Defined in GHC.Generics

type Rep (Maybe a) = D1 (MetaData "Maybe" "GHC.Base" "base" False) (C1 (MetaCons "Nothing" PrefixI False) (U1 :: * -> *) :+: 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
type DemoteRep (Maybe a) 
Instance details

Defined in GHC.Generics

type DemoteRep (Maybe a) = Maybe (DemoteRep a)
type Index (Maybe a) 
Instance details

Defined in Control.Lens.At

type Index (Maybe a) = ()
type IxValue (Maybe a) 
Instance details

Defined in Control.Lens.At

type IxValue (Maybe a) = a
type Rep1 Maybe 
Instance details

Defined in GHC.Generics

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

The maybe function takes a default value, a function, and a Maybe value. If the Maybe value is Nothing, the function returns the default value. Otherwise, it applies the function to the value inside the Just and returns the result.

Examples

Expand

Basic usage:

>>> maybe False odd (Just 3)
True
>>> maybe False odd Nothing
False

Read an integer from a string using readMaybe. If we succeed, return twice the integer; that is, apply (*2) to it. If instead we fail to parse an integer, return 0 by default:

>>> import Text.Read ( readMaybe )
>>> maybe 0 (*2) (readMaybe "5")
10
>>> maybe 0 (*2) (readMaybe "")
0

Apply show to a Maybe Int. If we have Just n, we want to show the underlying Int n. But if we have Nothing, we return the empty string instead of (for example) "Nothing":

>>> maybe "" show (Just 5)
"5"
>>> maybe "" show Nothing
""

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

Monadic generalisation of maybe.

isJust :: Maybe a -> Bool #

The isJust function returns True iff its argument is of the form Just _.

Examples

Expand

Basic usage:

>>> isJust (Just 3)
True
>>> isJust (Just ())
True
>>> isJust Nothing
False

Only the outer constructor is taken into consideration:

>>> isJust (Just Nothing)
True

isNothing :: Maybe a -> Bool #

The isNothing function returns True iff its argument is Nothing.

Examples

Expand

Basic usage:

>>> isNothing (Just 3)
False
>>> isNothing (Just ())
False
>>> isNothing Nothing
True

Only the outer constructor is taken into consideration:

>>> isNothing (Just Nothing)
False

fromMaybe :: a -> Maybe a -> a #

The fromMaybe function takes a default value and and Maybe value. If the Maybe is Nothing, it returns the default values; otherwise, it returns the value contained in the Maybe.

Examples

Expand

Basic usage:

>>> fromMaybe "" (Just "Hello, World!")
"Hello, World!"
>>> fromMaybe "" Nothing
""

Read an integer from a string using readMaybe. If we fail to parse an integer, we want to return 0 by default:

>>> import Text.Read ( readMaybe )
>>> fromMaybe 0 (readMaybe "5")
5
>>> fromMaybe 0 (readMaybe "")
0

listToMaybe :: [a] -> Maybe a #

The listToMaybe function returns Nothing on an empty list or Just a where a is the first element of the list.

Examples

Expand

Basic usage:

>>> listToMaybe []
Nothing
>>> listToMaybe [9]
Just 9
>>> listToMaybe [1,2,3]
Just 1

Composing maybeToList with listToMaybe should be the identity on singleton/empty lists:

>>> maybeToList $ listToMaybe [5]
[5]
>>> maybeToList $ listToMaybe []
[]

But not on lists with more than one element:

>>> maybeToList $ listToMaybe [1,2,3]
[1]

maybeToList :: Maybe a -> [a] #

The maybeToList function returns an empty list when given Nothing or a singleton list when not given Nothing.

Examples

Expand

Basic usage:

>>> maybeToList (Just 7)
[7]
>>> maybeToList Nothing
[]

One can use maybeToList to avoid pattern matching when combined with a function that (safely) works on lists:

>>> import Text.Read ( readMaybe )
>>> sum $ maybeToList (readMaybe "3")
3
>>> sum $ maybeToList (readMaybe "")
0

catMaybes :: [Maybe a] -> [a] #

The catMaybes function takes a list of Maybes and returns a list of all the Just values.

Examples

Expand

Basic usage:

>>> catMaybes [Just 1, Nothing, Just 3]
[1,3]

When constructing a list of Maybe values, catMaybes can be used to return all of the "success" results (if the list is the result of a map, then mapMaybe would be more appropriate):

>>> import Text.Read ( readMaybe )
>>> [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]
[Just 1,Nothing,Just 3]
>>> catMaybes $ [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]
[1,3]

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

The mapMaybe function is a version of map which can throw out elements. In particular, the functional argument returns something of type Maybe b. If this is Nothing, no element is added on to the result list. If it is Just b, then b is included in the result list.

Examples

Expand

Using mapMaybe f x is a shortcut for catMaybes $ map f x in most cases:

>>> import Text.Read ( readMaybe )
>>> let readMaybeInt = readMaybe :: String -> Maybe Int
>>> mapMaybe readMaybeInt ["1", "Foo", "3"]
[1,3]
>>> catMaybes $ map readMaybeInt ["1", "Foo", "3"]
[1,3]

If we map the Just constructor, the entire list should be returned:

>>> mapMaybe Just [1,2,3]
[1,2,3]

Prisms

_Just :: (Choice p, Applicative f) => p a (f b) -> p (Maybe a) (f (Maybe b)) #

This Prism provides a Traversal for tweaking the target of the value of Just in a Maybe.

>>> over _Just (+1) (Just 2)
Just 3

Unlike traverse this is a Prism, and so you can use it to inject as well:

>>> _Just # 5
Just 5
>>> 5^.re _Just
Just 5

Interestingly,

m ^? _Just ≡ m
>>> Just x ^? _Just
Just x
>>> Nothing ^? _Just
Nothing

_Nothing :: (Choice p, Applicative f) => p () (f ()) -> p (Maybe a) (f (Maybe a)) #

This Prism provides the Traversal of a Nothing in a Maybe.

>>> Nothing ^? _Nothing
Just ()
>>> Just () ^? _Nothing
Nothing

But you can turn it around and use it to construct Nothing as well:

>>> _Nothing # ()
Nothing

Newtypes

newtype Option a #

Option is effectively Maybe with a better instance of Monoid, built off of an underlying Semigroup instead of an underlying Monoid.

Ideally, this type would not exist at all and we would just fix the Monoid instance of Maybe

Constructors

Option 

Fields

Instances
Monad Option

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Methods

(>>=) :: Option a -> (a -> Option b) -> Option b #

(>>) :: Option a -> Option b -> Option b #

return :: a -> Option a #

fail :: String -> Option a #

Functor Option

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Methods

fmap :: (a -> b) -> Option a -> Option b #

(<$) :: a -> Option b -> Option a #

MonadFix Option

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Methods

mfix :: (a -> Option a) -> Option a #

Applicative Option

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Methods

pure :: a -> Option a #

(<*>) :: Option (a -> b) -> Option a -> Option b #

liftA2 :: (a -> b -> c) -> Option a -> Option b -> Option c #

(*>) :: Option a -> Option b -> Option b #

(<*) :: Option a -> Option b -> Option a #

Foldable Option

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Methods

fold :: Monoid m => Option m -> m #

foldMap :: Monoid m => (a -> m) -> Option a -> m #

foldr :: (a -> b -> b) -> b -> Option a -> b #

foldr' :: (a -> b -> b) -> b -> Option a -> b #

foldl :: (b -> a -> b) -> b -> Option a -> b #

foldl' :: (b -> a -> b) -> b -> Option a -> b #

foldr1 :: (a -> a -> a) -> Option a -> a #

foldl1 :: (a -> a -> a) -> Option a -> a #

toList :: Option a -> [a] #

null :: Option a -> Bool #

length :: Option a -> Int #

elem :: Eq a => a -> Option a -> Bool #

maximum :: Ord a => Option a -> a #

minimum :: Ord a => Option a -> a #

sum :: Num a => Option a -> a #

product :: Num a => Option a -> a #

Traversable Option

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Methods

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

sequenceA :: Applicative f => Option (f a) -> f (Option a) #

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

sequence :: Monad m => Option (m a) -> m (Option a) #

Alternative Option

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Methods

empty :: Option a #

(<|>) :: Option a -> Option a -> Option a #

some :: Option a -> Option [a] #

many :: Option a -> Option [a] #

ToJSON1 Option 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

liftToJSON :: (a -> Value) -> ([a] -> Value) -> Option a -> Value #

liftToJSONList :: (a -> Value) -> ([a] -> Value) -> [Option a] -> Value #

liftToEncoding :: (a -> Encoding) -> ([a] -> Encoding) -> Option a -> Encoding #

liftToEncodingList :: (a -> Encoding) -> ([a] -> Encoding) -> [Option a] -> Encoding #

FromJSON1 Option 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

liftParseJSON :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser (Option a) #

liftParseJSONList :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser [Option a] #

MonadPlus Option

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Methods

mzero :: Option a #

mplus :: Option a -> Option a -> Option a #

Compactable Option 
Instance details

Defined in Control.Compactable

Methods

compact :: Option (Maybe a) -> Option a #

separate :: Option (Either l r) -> (Option l, Option r) #

filter :: (a -> Bool) -> Option a -> Option a #

partition :: (a -> Bool) -> Option a -> (Option a, Option a) #

fmapMaybe :: Functor Option => (a -> Maybe b) -> Option a -> Option b #

fmapEither :: Functor Option => (a -> Either l r) -> Option a -> (Option l, Option r) #

applyMaybe :: Applicative Option => Option (a -> Maybe b) -> Option a -> Option b #

applyEither :: Applicative Option => Option (a -> Either l r) -> Option a -> (Option l, Option r) #

bindMaybe :: Monad Option => Option a -> (a -> Option (Maybe b)) -> Option b #

bindEither :: Monad Option => Option a -> (a -> Option (Either l r)) -> (Option l, Option r) #

traverseMaybe :: (Applicative g, Traversable Option) => (a -> g (Maybe b)) -> Option a -> g (Option b) #

traverseEither :: (Applicative g, Traversable Option) => (a -> g (Either l r)) -> Option a -> g (Option l, Option r) #

CompactFold Option 
Instance details

Defined in Control.Compactable

Methods

compactFold :: Foldable g => Option (g a) -> Option a #

separateFold :: Bifoldable g => Option (g a b) -> (Option a, Option b) #

fmapFold :: (Functor Option, Foldable g) => (a -> g b) -> Option a -> Option b #

fmapBifold :: (Functor Option, Bifoldable g) => (a -> g l r) -> Option a -> (Option l, Option r) #

applyFold :: (Applicative Option, Foldable g) => Option (a -> g b) -> Option a -> Option b #

applyBifold :: (Applicative Option, Bifoldable g) => Option (a -> g l r) -> Option a -> (Option l, Option r) #

bindFold :: (Monad Option, Foldable g) => Option a -> (a -> Option (g b)) -> Option b #

bindBifold :: (Monad Option, Bifoldable g) => Option a -> (a -> Option (g l r)) -> (Option l, Option r) #

traverseFold :: (Applicative h, Foldable g, Traversable Option) => (a -> h (g b)) -> Option a -> h (Option b) #

traverseBifold :: (Applicative h, Bifoldable g, Traversable Option) => (a -> h (g l r)) -> Option a -> h (Option l, Option r) #

NFData1 Option

Since: deepseq-1.4.3.0

Instance details

Defined in Control.DeepSeq

Methods

liftRnf :: (a -> ()) -> Option a -> () #

Apply Option 
Instance details

Defined in Data.Functor.Bind.Class

Methods

(<.>) :: Option (a -> b) -> Option a -> Option b #

(.>) :: Option a -> Option b -> Option b #

(<.) :: Option a -> Option b -> Option a #

liftF2 :: (a -> b -> c) -> Option a -> Option b -> Option c #

Pointed Option 
Instance details

Defined in Data.Pointed

Methods

point :: a -> Option a #

Plus Option 
Instance details

Defined in Data.Functor.Plus

Methods

zero :: Option a #

Alt Option 
Instance details

Defined in Data.Functor.Alt

Methods

(<!>) :: Option a -> Option a -> Option a #

some :: Applicative Option => Option a -> Option [a] #

many :: Applicative Option => Option a -> Option [a] #

Bind Option 
Instance details

Defined in Data.Functor.Bind.Class

Methods

(>>-) :: Option a -> (a -> Option b) -> Option b #

join :: Option (Option a) -> Option a #

(Selector s, GToJSON enc arity (K1 i (Maybe a) :: * -> *), KeyValuePair enc pairs, Monoid pairs) => RecordToPairs enc pairs arity (S1 s (K1 i (Option a) :: * -> *)) 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

recordToPairs :: Options -> ToArgs enc arity a0 -> S1 s (K1 i (Option a)) a0 -> pairs

(Selector s, FromJSON a) => FromRecord arity (S1 s (K1 i (Option a) :: * -> *)) 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

parseRecord :: Options -> FromArgs arity a0 -> Object -> Parser (S1 s (K1 i (Option a)) a0)

Eq a => Eq (Option a) 
Instance details

Defined in Data.Semigroup

Methods

(==) :: Option a -> Option a -> Bool #

(/=) :: Option a -> Option a -> Bool #

Data a => Data (Option a) 
Instance details

Defined in Data.Semigroup

Methods

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

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

toConstr :: Option a -> Constr #

dataTypeOf :: Option a -> DataType #

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

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

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

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

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

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

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

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

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

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

Ord a => Ord (Option a) 
Instance details

Defined in Data.Semigroup

Methods

compare :: Option a -> Option a -> Ordering #

(<) :: Option a -> Option a -> Bool #

(<=) :: Option a -> Option a -> Bool #

(>) :: Option a -> Option a -> Bool #

(>=) :: Option a -> Option a -> Bool #

max :: Option a -> Option a -> Option a #

min :: Option a -> Option a -> Option a #

Read a => Read (Option a) 
Instance details

Defined in Data.Semigroup

Show a => Show (Option a) 
Instance details

Defined in Data.Semigroup

Methods

showsPrec :: Int -> Option a -> ShowS #

show :: Option a -> String #

showList :: [Option a] -> ShowS #

Generic (Option a) 
Instance details

Defined in Data.Semigroup

Associated Types

type Rep (Option a) :: * -> * #

Methods

from :: Option a -> Rep (Option a) x #

to :: Rep (Option a) x -> Option a #

Semigroup a => Semigroup (Option a)

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Methods

(<>) :: Option a -> Option a -> Option a #

sconcat :: NonEmpty (Option a) -> Option a #

stimes :: Integral b => b -> Option a -> Option a #

Semigroup a => Monoid (Option a)

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Methods

mempty :: Option a #

mappend :: Option a -> Option a -> Option a #

mconcat :: [Option a] -> Option a #

Hashable a => Hashable (Option a) 
Instance details

Defined in Data.Hashable.Class

Methods

hashWithSalt :: Int -> Option a -> Int #

hash :: Option a -> Int #

ToJSON a => ToJSON (Option a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

FromJSON a => FromJSON (Option a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

NFData a => NFData (Option a)

Since: deepseq-1.4.2.0

Instance details

Defined in Control.DeepSeq

Methods

rnf :: Option a -> () #

Wrapped (Option a) 
Instance details

Defined in Control.Lens.Wrapped

Associated Types

type Unwrapped (Option a) :: * #

Methods

_Wrapped' :: Iso' (Option a) (Unwrapped (Option a)) #

Serialise a => Serialise (Option a)

Since: serialise-0.2.0.0

Instance details

Defined in Codec.Serialise.Class

Generic1 Option 
Instance details

Defined in Data.Semigroup

Associated Types

type Rep1 Option :: k -> * #

Methods

from1 :: Option a -> Rep1 Option a #

to1 :: Rep1 Option a -> Option a #

t ~ Option b => Rewrapped (Option a) t 
Instance details

Defined in Control.Lens.Wrapped

type Rep (Option a) 
Instance details

Defined in Data.Semigroup

type Rep (Option a) = D1 (MetaData "Option" "Data.Semigroup" "base" True) (C1 (MetaCons "Option" PrefixI True) (S1 (MetaSel (Just "getOption") NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec0 (Maybe a))))
type Unwrapped (Option a) 
Instance details

Defined in Control.Lens.Wrapped

type Unwrapped (Option a) = Maybe a
type Rep1 Option 
Instance details

Defined in Data.Semigroup

type Rep1 Option = D1 (MetaData "Option" "Data.Semigroup" "base" True) (C1 (MetaCons "Option" PrefixI True) (S1 (MetaSel (Just "getOption") NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec1 Maybe)))

option :: b -> (a -> b) -> Option a -> b #

Fold an Option case-wise, just like maybe.

newtype First a #

Maybe monoid returning the leftmost non-Nothing value.

First a is isomorphic to Alt Maybe a, but precedes it historically.

>>> getFirst (First (Just "hello") <> First Nothing <> First (Just "world"))
Just "hello"

Constructors

First 

Fields

Instances
Monad First 
Instance details

Defined in Data.Monoid

Methods

(>>=) :: First a -> (a -> First b) -> First b #

(>>) :: First a -> First b -> First b #

return :: a -> First a #

fail :: String -> First a #

Functor First 
Instance details

Defined in Data.Monoid

Methods

fmap :: (a -> b) -> First a -> First b #

(<$) :: a -> First b -> First a #

MonadFix First

Since: base-4.8.0.0

Instance details

Defined in Control.Monad.Fix

Methods

mfix :: (a -> First a) -> First a #

Applicative First 
Instance details

Defined in Data.Monoid

Methods

pure :: a -> First a #

(<*>) :: First (a -> b) -> First a -> First b #

liftA2 :: (a -> b -> c) -> First a -> First b -> First c #

(*>) :: First a -> First b -> First b #

(<*) :: First a -> First b -> First a #

Foldable First

Since: base-4.8.0.0

Instance details

Defined in Data.Foldable

Methods

fold :: Monoid m => First m -> m #

foldMap :: Monoid m => (a -> m) -> First a -> m #

foldr :: (a -> b -> b) -> b -> First a -> b #

foldr' :: (a -> b -> b) -> b -> First a -> b #

foldl :: (b -> a -> b) -> b -> First a -> b #

foldl' :: (b -> a -> b) -> b -> First a -> b #

foldr1 :: (a -> a -> a) -> First a -> a #

foldl1 :: (a -> a -> a) -> First a -> a #

toList :: First a -> [a] #

null :: First a -> Bool #

length :: First a -> Int #

elem :: Eq a => a -> First a -> Bool #

maximum :: Ord a => First a -> a #

minimum :: Ord a => First a -> a #

sum :: Num a => First a -> a #

product :: Num a => First a -> a #

Traversable First

Since: base-4.8.0.0

Instance details

Defined in Data.Traversable

Methods

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

sequenceA :: Applicative f => First (f a) -> f (First a) #

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

sequence :: Monad m => First (m a) -> m (First a) #

ToJSON1 First 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

liftToJSON :: (a -> Value) -> ([a] -> Value) -> First a -> Value #

liftToJSONList :: (a -> Value) -> ([a] -> Value) -> [First a] -> Value #

liftToEncoding :: (a -> Encoding) -> ([a] -> Encoding) -> First a -> Encoding #

liftToEncodingList :: (a -> Encoding) -> ([a] -> Encoding) -> [First a] -> Encoding #

FromJSON1 First 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

liftParseJSON :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser (First a) #

liftParseJSONList :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser [First a] #

MonadZip First

Since: base-4.8.0.0

Instance details

Defined in Control.Monad.Zip

Methods

mzip :: First a -> First b -> First (a, b) #

mzipWith :: (a -> b -> c) -> First a -> First b -> First c #

munzip :: First (a, b) -> (First a, First b) #

NFData1 First

Since: deepseq-1.4.3.0

Instance details

Defined in Control.DeepSeq

Methods

liftRnf :: (a -> ()) -> First a -> () #

Apply First 
Instance details

Defined in Data.Functor.Bind.Class

Methods

(<.>) :: First (a -> b) -> First a -> First b #

(.>) :: First a -> First b -> First b #

(<.) :: First a -> First b -> First a #

liftF2 :: (a -> b -> c) -> First a -> First b -> First c #

Pointed First 
Instance details

Defined in Data.Pointed

Methods

point :: a -> First a #

Plus First 
Instance details

Defined in Data.Functor.Plus

Methods

zero :: First a #

Alt First 
Instance details

Defined in Data.Functor.Alt

Methods

(<!>) :: First a -> First a -> First a #

some :: Applicative First => First a -> First [a] #

many :: Applicative First => First a -> First [a] #

Bind First 
Instance details

Defined in Data.Functor.Bind.Class

Methods

(>>-) :: First a -> (a -> First b) -> First b #

join :: First (First a) -> First a #

Eq a => Eq (First a) 
Instance details

Defined in Data.Monoid

Methods

(==) :: First a -> First a -> Bool #

(/=) :: First a -> First a -> Bool #

Data a => Data (First a)

Since: base-4.8.0.0

Instance details

Defined in Data.Data

Methods

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

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

toConstr :: First a -> Constr #

dataTypeOf :: First a -> DataType #

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

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

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

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

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

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

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

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

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

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

Ord a => Ord (First a) 
Instance details

Defined in Data.Monoid

Methods

compare :: First a -> First a -> Ordering #

(<) :: First a -> First a -> Bool #

(<=) :: First a -> First a -> Bool #

(>) :: First a -> First a -> Bool #

(>=) :: First a -> First a -> Bool #

max :: First a -> First a -> First a #

min :: First a -> First a -> First a #

Read a => Read (First a) 
Instance details

Defined in Data.Monoid

Show a => Show (First a) 
Instance details

Defined in Data.Monoid

Methods

showsPrec :: Int -> First a -> ShowS #

show :: First a -> String #

showList :: [First a] -> ShowS #

Generic (First a) 
Instance details

Defined in Data.Monoid

Associated Types

type Rep (First a) :: * -> * #

Methods

from :: First a -> Rep (First a) x #

to :: Rep (First a) x -> First a #

Semigroup (First a)

Since: base-4.9.0.0

Instance details

Defined in Data.Monoid

Methods

(<>) :: First a -> First a -> First a #

sconcat :: NonEmpty (First a) -> First a #

stimes :: Integral b => b -> First a -> First a #

Monoid (First a)

Since: base-2.1

Instance details

Defined in Data.Monoid

Methods

mempty :: First a #

mappend :: First a -> First a -> First a #

mconcat :: [First a] -> First a #

ToJSON a => ToJSON (First a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

FromJSON a => FromJSON (First a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

NFData a => NFData (First a)

Since: deepseq-1.4.0.0

Instance details

Defined in Control.DeepSeq

Methods

rnf :: First a -> () #

Wrapped (First a) 
Instance details

Defined in Control.Lens.Wrapped

Associated Types

type Unwrapped (First a) :: * #

Methods

_Wrapped' :: Iso' (First a) (Unwrapped (First a)) #

AsEmpty (First a) 
Instance details

Defined in Control.Lens.Empty

Methods

_Empty :: Prism' (First a) () #

Lower (First a) 
Instance details

Defined in Data.Semilattice.Lower

Methods

lowerBound :: First a #

Serialise a => Serialise (First a)

Since: serialise-0.2.0.0

Instance details

Defined in Codec.Serialise.Class

Generic1 First 
Instance details

Defined in Data.Monoid

Associated Types

type Rep1 First :: k -> * #

Methods

from1 :: First a -> Rep1 First a #

to1 :: Rep1 First a -> First a #

t ~ First b => Rewrapped (First a) t 
Instance details

Defined in Control.Lens.Wrapped

type Rep (First a) 
Instance details

Defined in Data.Monoid

type Rep (First a) = D1 (MetaData "First" "Data.Monoid" "base" True) (C1 (MetaCons "First" PrefixI True) (S1 (MetaSel (Just "getFirst") NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec0 (Maybe a))))
type Unwrapped (First a) 
Instance details

Defined in Control.Lens.Wrapped

type Unwrapped (First a) = Maybe a
type Rep1 First 
Instance details

Defined in Data.Monoid

type Rep1 First = D1 (MetaData "First" "Data.Monoid" "base" True) (C1 (MetaCons "First" PrefixI True) (S1 (MetaSel (Just "getFirst") NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec1 Maybe)))

newtype Last a #

Maybe monoid returning the rightmost non-Nothing value.

Last a is isomorphic to Dual (First a), and thus to Dual (Alt Maybe a)

>>> getLast (Last (Just "hello") <> Last Nothing <> Last (Just "world"))
Just "world"

Constructors

Last 

Fields

Instances
Monad Last 
Instance details

Defined in Data.Monoid

Methods

(>>=) :: Last a -> (a -> Last b) -> Last b #

(>>) :: Last a -> Last b -> Last b #

return :: a -> Last a #

fail :: String -> Last a #

Functor Last 
Instance details

Defined in Data.Monoid

Methods

fmap :: (a -> b) -> Last a -> Last b #

(<$) :: a -> Last b -> Last a #

MonadFix Last

Since: base-4.8.0.0

Instance details

Defined in Control.Monad.Fix

Methods

mfix :: (a -> Last a) -> Last a #

Applicative Last 
Instance details

Defined in Data.Monoid

Methods

pure :: a -> Last a #

(<*>) :: Last (a -> b) -> Last a -> Last b #

liftA2 :: (a -> b -> c) -> Last a -> Last b -> Last c #

(*>) :: Last a -> Last b -> Last b #

(<*) :: Last a -> Last b -> Last a #

Foldable Last

Since: base-4.8.0.0

Instance details

Defined in Data.Foldable

Methods

fold :: Monoid m => Last m -> m #

foldMap :: Monoid m => (a -> m) -> Last a -> m #

foldr :: (a -> b -> b) -> b -> Last a -> b #

foldr' :: (a -> b -> b) -> b -> Last a -> b #

foldl :: (b -> a -> b) -> b -> Last a -> b #

foldl' :: (b -> a -> b) -> b -> Last a -> b #

foldr1 :: (a -> a -> a) -> Last a -> a #

foldl1 :: (a -> a -> a) -> Last a -> a #

toList :: Last a -> [a] #

null :: Last a -> Bool #

length :: Last a -> Int #

elem :: Eq a => a -> Last a -> Bool #

maximum :: Ord a => Last a -> a #

minimum :: Ord a => Last a -> a #

sum :: Num a => Last a -> a #

product :: Num a => Last a -> a #

Traversable Last

Since: base-4.8.0.0

Instance details

Defined in Data.Traversable

Methods

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

sequenceA :: Applicative f => Last (f a) -> f (Last a) #

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

sequence :: Monad m => Last (m a) -> m (Last a) #

ToJSON1 Last 
Instance details

Defined in Data.Aeson.Types.ToJSON

Methods

liftToJSON :: (a -> Value) -> ([a] -> Value) -> Last a -> Value #

liftToJSONList :: (a -> Value) -> ([a] -> Value) -> [Last a] -> Value #

liftToEncoding :: (a -> Encoding) -> ([a] -> Encoding) -> Last a -> Encoding #

liftToEncodingList :: (a -> Encoding) -> ([a] -> Encoding) -> [Last a] -> Encoding #

FromJSON1 Last 
Instance details

Defined in Data.Aeson.Types.FromJSON

Methods

liftParseJSON :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser (Last a) #

liftParseJSONList :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser [Last a] #

MonadZip Last

Since: base-4.8.0.0

Instance details

Defined in Control.Monad.Zip

Methods

mzip :: Last a -> Last b -> Last (a, b) #

mzipWith :: (a -> b -> c) -> Last a -> Last b -> Last c #

munzip :: Last (a, b) -> (Last a, Last b) #

NFData1 Last

Since: deepseq-1.4.3.0

Instance details

Defined in Control.DeepSeq

Methods

liftRnf :: (a -> ()) -> Last a -> () #

Apply Last 
Instance details

Defined in Data.Functor.Bind.Class

Methods

(<.>) :: Last (a -> b) -> Last a -> Last b #

(.>) :: Last a -> Last b -> Last b #

(<.) :: Last a -> Last b -> Last a #

liftF2 :: (a -> b -> c) -> Last a -> Last b -> Last c #

Pointed Last 
Instance details

Defined in Data.Pointed

Methods

point :: a -> Last a #

Plus Last 
Instance details

Defined in Data.Functor.Plus

Methods

zero :: Last a #

Alt Last 
Instance details

Defined in Data.Functor.Alt

Methods

(<!>) :: Last a -> Last a -> Last a #

some :: Applicative Last => Last a -> Last [a] #

many :: Applicative Last => Last a -> Last [a] #

Bind Last 
Instance details

Defined in Data.Functor.Bind.Class

Methods

(>>-) :: Last a -> (a -> Last b) -> Last b #

join :: Last (Last a) -> Last a #

Eq a => Eq (Last a) 
Instance details

Defined in Data.Monoid

Methods

(==) :: Last a -> Last a -> Bool #

(/=) :: Last a -> Last a -> Bool #

Data a => Data (Last a)

Since: base-4.8.0.0

Instance details

Defined in Data.Data

Methods

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

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

toConstr :: Last a -> Constr #

dataTypeOf :: Last a -> DataType #

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

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

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

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

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

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

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

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

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

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

Ord a => Ord (Last a) 
Instance details

Defined in Data.Monoid

Methods

compare :: Last a -> Last a -> Ordering #

(<) :: Last a -> Last a -> Bool #

(<=) :: Last a -> Last a -> Bool #

(>) :: Last a -> Last a -> Bool #

(>=) :: Last a -> Last a -> Bool #

max :: Last a -> Last a -> Last a #

min :: Last a -> Last a -> Last a #

Read a => Read (Last a) 
Instance details

Defined in Data.Monoid

Show a => Show (Last a) 
Instance details

Defined in Data.Monoid

Methods

showsPrec :: Int -> Last a -> ShowS #

show :: Last a -> String #

showList :: [Last a] -> ShowS #

Generic (Last a) 
Instance details

Defined in Data.Monoid

Associated Types

type Rep (Last a) :: * -> * #

Methods

from :: Last a -> Rep (Last a) x #

to :: Rep (Last a) x -> Last a #

Semigroup (Last a)

Since: base-4.9.0.0

Instance details

Defined in Data.Monoid

Methods

(<>) :: Last a -> Last a -> Last a #

sconcat :: NonEmpty (Last a) -> Last a #

stimes :: Integral b => b -> Last a -> Last a #

Monoid (Last a)

Since: base-2.1

Instance details

Defined in Data.Monoid

Methods

mempty :: Last a #

mappend :: Last a -> Last a -> Last a #

mconcat :: [Last a] -> Last a #

ToJSON a => ToJSON (Last a) 
Instance details

Defined in Data.Aeson.Types.ToJSON

FromJSON a => FromJSON (Last a) 
Instance details

Defined in Data.Aeson.Types.FromJSON

NFData a => NFData (Last a)

Since: deepseq-1.4.0.0

Instance details

Defined in Control.DeepSeq

Methods

rnf :: Last a -> () #

Wrapped (Last a) 
Instance details

Defined in Control.Lens.Wrapped

Associated Types

type Unwrapped (Last a) :: * #

Methods

_Wrapped' :: Iso' (Last a) (Unwrapped (Last a)) #

AsEmpty (Last a) 
Instance details

Defined in Control.Lens.Empty

Methods

_Empty :: Prism' (Last a) () #

Lower (Last a) 
Instance details

Defined in Data.Semilattice.Lower

Methods

lowerBound :: Last a #

Serialise a => Serialise (Last a)

Since: serialise-0.2.0.0

Instance details

Defined in Codec.Serialise.Class

Methods

encode :: Last a -> Encoding #

decode :: Decoder s (Last a) #

encodeList :: [Last a] -> Encoding #

decodeList :: Decoder s [Last a] #

Generic1 Last 
Instance details

Defined in Data.Monoid

Associated Types

type Rep1 Last :: k -> * #

Methods

from1 :: Last a -> Rep1 Last a #

to1 :: Rep1 Last a -> Last a #

t ~ Last b => Rewrapped (Last a) t 
Instance details

Defined in Control.Lens.Wrapped

type Rep (Last a) 
Instance details

Defined in Data.Monoid

type Rep (Last a) = D1 (MetaData "Last" "Data.Monoid" "base" True) (C1 (MetaCons "Last" PrefixI True) (S1 (MetaSel (Just "getLast") NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec0 (Maybe a))))
type Unwrapped (Last a) 
Instance details

Defined in Control.Lens.Wrapped

type Unwrapped (Last a) = Maybe a
type Rep1 Last 
Instance details

Defined in Data.Monoid

type Rep1 Last = D1 (MetaData "Last" "Data.Monoid" "base" True) (C1 (MetaCons "Last" PrefixI True) (S1 (MetaSel (Just "getLast") NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec1 Maybe)))