Copyright  (c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001 

License  BSDstyle (see the file libraries/base/LICENSE) 
Maintainer  libraries@haskell.org 
Stability  experimental 
Portability  portable 
Safe Haskell  Trustworthy 
Language  Haskell2010 
A type a
is a Monoid
if it provides an associative function (<>
)
that lets you combine any two values of type a
into one, and a neutral
element (mempty
) such that
a <> mempty == mempty <> a == a
A Monoid
is a Semigroup
with the added requirement of a neutral element.
Thus any Monoid
is a Semigroup
, but not the other way around.
Examples
The Sum
monoid is defined by the numerical addition operator and `0` as neutral element:
>>>
mempty :: Sum Int
Sum 0>>>
Sum 1 <> Sum 2 <> Sum 3 <> Sum 4 :: Sum Int
Sum {getSum = 10}
We can combine multiple values in a list into a single value using the mconcat
function.
Note that we have to specify the type here since Int
is a monoid under several different
operations:
>>>
mconcat [1,2,3,4] :: Sum Int
Sum {getSum = 10}>>>
mconcat [] :: Sum Int
Sum {getSum = 0}
Another valid monoid instance of Int
is Product
It is defined by multiplication
and `1` as neutral element:
>>>
Product 1 <> Product 2 <> Product 3 <> Product 4 :: Product Int
Product {getProduct = 24}>>>
mconcat [1,2,3,4] :: Product Int
Product {getProduct = 24}>>>
mconcat [] :: Product Int
Product {getProduct = 1}
Synopsis
 class Semigroup a => Monoid a where
 (<>) :: Semigroup a => a > a > a
 newtype Dual a = Dual {
 getDual :: a
 newtype Endo a = Endo {
 appEndo :: a > a
 newtype All = All {}
 newtype Any = Any {}
 newtype Sum a = Sum {
 getSum :: a
 newtype Product a = Product {
 getProduct :: a
 newtype First a = First {}
 newtype Last a = Last {}
 newtype Alt f a = Alt {
 getAlt :: f a
 newtype Ap f a = Ap {
 getAp :: f a
Monoid
typeclass
class Semigroup a => Monoid a where Source #
The class of monoids (types with an associative binary operation that has an identity). Instances should satisfy the following:
 Right identity
x
<>
mempty
= x Left identity
mempty
<>
x = x Associativity
x
(<>
(y<>
z) = (x<>
y)<>
zSemigroup
law) Concatenation
mconcat
=foldr
(<>
)mempty
The method names refer to the monoid of lists under concatenation, but there are many other instances.
Some types can be viewed as a monoid in more than one way,
e.g. both addition and multiplication on numbers.
In such cases we often define newtype
s and make those instances
of Monoid
, e.g. Sum
and Product
.
NOTE: Semigroup
is a superclass of Monoid
since base4.11.0.0.
Identity of mappend
mappend :: a > a > a Source #
An associative operation
NOTE: This method is redundant and has the default
implementation
since base4.11.0.0.mappend
= (<>
)
Fold a list using the monoid.
For most types, the default definition for mconcat
will be
used, but the function is included in the class definition so
that an optimized version can be provided for specific types.
Instances
Monoid Ordering #  Since: base2.1 
Monoid () #  Since: base2.1 
Monoid Any #  Since: base2.1 
Monoid All #  Since: base2.1 
Monoid Lifetime # 
Since: base4.8.0.0 
Monoid Event #  Since: base4.4.0.0 
Monoid [a] #  Since: base2.1 
Semigroup a => Monoid (Maybe a) #  Lift a semigroup into Since 4.11.0: constraint on inner Since: base2.1 
Monoid a => Monoid (IO a) #  Since: base4.9.0.0 
Monoid p => Monoid (Par1 p) #  Since: base4.12.0.0 
Monoid a => Monoid (Down a) #  Since: base4.11.0.0 
Num a => Monoid (Product a) #  Since: base2.1 
Num a => Monoid (Sum a) #  Since: base2.1 
Monoid (Endo a) #  Since: base2.1 
Monoid a => Monoid (Dual a) #  Since: base2.1 
Monoid (Last a) #  Since: base2.1 
Monoid (First a) #  Since: base2.1 
Monoid a => Monoid (Identity a) #  Since: base4.9.0.0 
Semigroup a => Monoid (Option a) #  Since: base4.9.0.0 
Monoid m => Monoid (WrappedMonoid m) #  Since: base4.9.0.0 
Defined in Data.Semigroup mempty :: WrappedMonoid m Source # mappend :: WrappedMonoid m > WrappedMonoid m > WrappedMonoid m Source # mconcat :: [WrappedMonoid m] > WrappedMonoid m Source #  
(Ord a, Bounded a) => Monoid (Max a) #  Since: base4.9.0.0 
(Ord a, Bounded a) => Monoid (Min a) #  Since: base4.9.0.0 
Monoid (Equivalence a) #  
Defined in Data.Functor.Contravariant mempty :: Equivalence a Source # mappend :: Equivalence a > Equivalence a > Equivalence a Source # mconcat :: [Equivalence a] > Equivalence a Source #  
Monoid (Comparison a) #  
Defined in Data.Functor.Contravariant mempty :: Comparison a Source # mappend :: Comparison a > Comparison a > Comparison a Source # mconcat :: [Comparison a] > Comparison a Source #  
Monoid (Predicate a) #  
Monoid b => Monoid (a > b) #  Since: base2.1 
Monoid (U1 p) #  Since: base4.12.0.0 
(Monoid a, Monoid b) => Monoid (a, b) #  Since: base2.1 
Monoid a => Monoid (ST s a) #  Since: base4.11.0.0 
Monoid (Proxy s) #  Since: base4.7.0.0 
Monoid a => Monoid (Op a b) #  
Monoid (f p) => Monoid (Rec1 f p) #  Since: base4.12.0.0 
(Monoid a, Monoid b, Monoid c) => Monoid (a, b, c) #  Since: base2.1 
Alternative f => Monoid (Alt f a) #  Since: base4.8.0.0 
(Applicative f, Monoid a) => Monoid (Ap f a) #  Since: base4.12.0.0 
Monoid a => Monoid (Const a b) #  Since: base4.9.0.0 
Monoid c => Monoid (K1 i c p) #  Since: base4.12.0.0 
(Monoid (f p), Monoid (g p)) => Monoid ((f :*: g) p) #  Since: base4.12.0.0 
(Monoid a, Monoid b, Monoid c, Monoid d) => Monoid (a, b, c, d) #  Since: base2.1 
Monoid (f p) => Monoid (M1 i c f p) #  Since: base4.12.0.0 
Monoid (f (g p)) => Monoid ((f :.: g) p) #  Since: base4.12.0.0 
(Monoid a, Monoid b, Monoid c, Monoid d, Monoid e) => Monoid (a, b, c, d, e) #  Since: base2.1 
The dual of a Monoid
, obtained by swapping the arguments of mappend
.
>>>
getDual (mappend (Dual "Hello") (Dual "World"))
"WorldHello"
Instances
Monad Dual #  Since: base4.8.0.0 
Functor Dual #  Since: base4.8.0.0 
MonadFix Dual #  Since: base4.8.0.0 
Applicative Dual #  Since: base4.8.0.0 
Foldable Dual #  Since: base4.8.0.0 
Defined in Data.Foldable fold :: Monoid m => Dual m > m Source # foldMap :: Monoid m => (a > m) > Dual a > m Source # foldMap' :: Monoid m => (a > m) > Dual a > m Source # foldr :: (a > b > b) > b > Dual a > b Source # foldr' :: (a > b > b) > b > Dual a > b Source # foldl :: (b > a > b) > b > Dual a > b Source # foldl' :: (b > a > b) > b > Dual a > b Source # foldr1 :: (a > a > a) > Dual a > a Source # foldl1 :: (a > a > a) > Dual a > a Source # toList :: Dual a > [a] Source # null :: Dual a > Bool Source # length :: Dual a > Int Source # elem :: Eq a => a > Dual a > Bool Source # maximum :: Ord a => Dual a > a Source # minimum :: Ord a => Dual a > a Source #  
Traversable Dual #  Since: base4.8.0.0 
MonadZip Dual #  Since: base4.8.0.0 
Bounded a => Bounded (Dual a) #  Since: base2.1 
Eq a => Eq (Dual a) #  Since: base2.1 
Data a => Data (Dual a) #  Since: base4.8.0.0 
Defined in Data.Data gfoldl :: (forall d b. Data d => c (d > b) > d > c b) > (forall g. g > c g) > Dual a > c (Dual a) Source # gunfold :: (forall b r. Data b => c (b > r) > c r) > (forall r. r > c r) > Constr > c (Dual a) Source # toConstr :: Dual a > Constr Source # dataTypeOf :: Dual a > DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) > Maybe (c (Dual a)) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) > Maybe (c (Dual a)) Source # gmapT :: (forall b. Data b => b > b) > Dual a > Dual a Source # gmapQl :: (r > r' > r) > r > (forall d. Data d => d > r') > Dual a > r Source # gmapQr :: forall r r'. (r' > r > r) > r > (forall d. Data d => d > r') > Dual a > r Source # gmapQ :: (forall d. Data d => d > u) > Dual a > [u] Source # gmapQi :: Int > (forall d. Data d => d > u) > Dual a > u Source # gmapM :: Monad m => (forall d. Data d => d > m d) > Dual a > m (Dual a) Source # gmapMp :: MonadPlus m => (forall d. Data d => d > m d) > Dual a > m (Dual a) Source # gmapMo :: MonadPlus m => (forall d. Data d => d > m d) > Dual a > m (Dual a) Source #  
Ord a => Ord (Dual a) #  Since: base2.1 
Defined in Data.Semigroup.Internal  
Read a => Read (Dual a) #  Since: base2.1 
Show a => Show (Dual a) #  Since: base2.1 
Generic (Dual a) #  Since: base4.7.0.0 
Semigroup a => Semigroup (Dual a) #  Since: base4.9.0.0 
Monoid a => Monoid (Dual a) #  Since: base2.1 
Generic1 Dual #  Since: base4.7.0.0 
type Rep (Dual a) #  
Defined in Data.Semigroup.Internal  
type Rep1 Dual #  
Defined in Data.Semigroup.Internal 
The monoid of endomorphisms under composition.
>>>
let computation = Endo ("Hello, " ++) <> Endo (++ "!")
>>>
appEndo computation "Haskell"
"Hello, Haskell!"
Bool
wrappers
Boolean monoid under conjunction (&&
).
>>>
getAll (All True <> mempty <> All False)
False
>>>
getAll (mconcat (map (\x > All (even x)) [2,4,6,7,8]))
False
Instances
Bounded All #  Since: base2.1 
Eq All #  Since: base2.1 
Data All #  Since: base4.8.0.0 
Defined in Data.Data gfoldl :: (forall d b. Data d => c (d > b) > d > c b) > (forall g. g > c g) > All > c All Source # gunfold :: (forall b r. Data b => c (b > r) > c r) > (forall r. r > c r) > Constr > c All Source # toConstr :: All > Constr Source # dataTypeOf :: All > DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) > Maybe (c All) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) > Maybe (c All) Source # gmapT :: (forall b. Data b => b > b) > All > All Source # gmapQl :: (r > r' > r) > r > (forall d. Data d => d > r') > All > r Source # gmapQr :: forall r r'. (r' > r > r) > r > (forall d. Data d => d > r') > All > r Source # gmapQ :: (forall d. Data d => d > u) > All > [u] Source # gmapQi :: Int > (forall d. Data d => d > u) > All > u Source # gmapM :: Monad m => (forall d. Data d => d > m d) > All > m All Source # gmapMp :: MonadPlus m => (forall d. Data d => d > m d) > All > m All Source # gmapMo :: MonadPlus m => (forall d. Data d => d > m d) > All > m All Source #  
Ord All #  Since: base2.1 
Read All #  Since: base2.1 
Show All #  Since: base2.1 
Generic All #  Since: base4.7.0.0 
Semigroup All #  Since: base4.9.0.0 
Monoid All #  Since: base2.1 
type Rep All #  
Defined in Data.Semigroup.Internal 
Boolean monoid under disjunction (
).
>>>
getAny (Any True <> mempty <> Any False)
True
>>>
getAny (mconcat (map (\x > Any (even x)) [2,4,6,7,8]))
True
Instances
Bounded Any #  Since: base2.1 
Eq Any #  Since: base2.1 
Data Any #  Since: base4.8.0.0 
Defined in Data.Data gfoldl :: (forall d b. Data d => c (d > b) > d > c b) > (forall g. g > c g) > Any > c Any Source # gunfold :: (forall b r. Data b => c (b > r) > c r) > (forall r. r > c r) > Constr > c Any Source # toConstr :: Any > Constr Source # dataTypeOf :: Any > DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) > Maybe (c Any) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) > Maybe (c Any) Source # gmapT :: (forall b. Data b => b > b) > Any > Any Source # gmapQl :: (r > r' > r) > r > (forall d. Data d => d > r') > Any > r Source # gmapQr :: forall r r'. (r' > r > r) > r > (forall d. Data d => d > r') > Any > r Source # gmapQ :: (forall d. Data d => d > u) > Any > [u] Source # gmapQi :: Int > (forall d. Data d => d > u) > Any > u Source # gmapM :: Monad m => (forall d. Data d => d > m d) > Any > m Any Source # gmapMp :: MonadPlus m => (forall d. Data d => d > m d) > Any > m Any Source # gmapMo :: MonadPlus m => (forall d. Data d => d > m d) > Any > m Any Source #  
Ord Any #  Since: base2.1 
Read Any #  Since: base2.1 
Show Any #  Since: base2.1 
Generic Any #  Since: base4.7.0.0 
Semigroup Any #  Since: base4.9.0.0 
Monoid Any #  Since: base2.1 
type Rep Any #  
Defined in Data.Semigroup.Internal 
Num
wrappers
Monoid under addition.
>>>
getSum (Sum 1 <> Sum 2 <> mempty)
3
Instances
Monad Sum #  Since: base4.8.0.0 
Functor Sum #  Since: base4.8.0.0 
MonadFix Sum #  Since: base4.8.0.0 
Applicative Sum #  Since: base4.8.0.0 
Foldable Sum #  Since: base4.8.0.0 
Defined in Data.Foldable fold :: Monoid m => Sum m > m Source # foldMap :: Monoid m => (a > m) > Sum a > m Source # foldMap' :: Monoid m => (a > m) > Sum a > m Source # foldr :: (a > b > b) > b > Sum a > b Source # foldr' :: (a > b > b) > b > Sum a > b Source # foldl :: (b > a > b) > b > Sum a > b Source # foldl' :: (b > a > b) > b > Sum a > b Source # foldr1 :: (a > a > a) > Sum a > a Source # foldl1 :: (a > a > a) > Sum a > a Source # toList :: Sum a > [a] Source # null :: Sum a > Bool Source # length :: Sum a > Int Source # elem :: Eq a => a > Sum a > Bool Source # maximum :: Ord a => Sum a > a Source # minimum :: Ord a => Sum a > a Source #  
Traversable Sum #  Since: base4.8.0.0 
MonadZip Sum #  Since: base4.8.0.0 
Bounded a => Bounded (Sum a) #  Since: base2.1 
Eq a => Eq (Sum a) #  Since: base2.1 
Data a => Data (Sum a) #  Since: base4.8.0.0 
Defined in Data.Data gfoldl :: (forall d b. Data d => c (d > b) > d > c b) > (forall g. g > c g) > Sum a > c (Sum a) Source # gunfold :: (forall b r. Data b => c (b > r) > c r) > (forall r. r > c r) > Constr > c (Sum a) Source # toConstr :: Sum a > Constr Source # dataTypeOf :: Sum a > DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) > Maybe (c (Sum a)) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) > Maybe (c (Sum a)) Source # gmapT :: (forall b. Data b => b > b) > Sum a > Sum a Source # gmapQl :: (r > r' > r) > r > (forall d. Data d => d > r') > Sum a > r Source # gmapQr :: forall r r'. (r' > r > r) > r > (forall d. Data d => d > r') > Sum a > r Source # gmapQ :: (forall d. Data d => d > u) > Sum a > [u] Source # gmapQi :: Int > (forall d. Data d => d > u) > Sum a > u Source # gmapM :: Monad m => (forall d. Data d => d > m d) > Sum a > m (Sum a) Source # gmapMp :: MonadPlus m => (forall d. Data d => d > m d) > Sum a > m (Sum a) Source # gmapMo :: MonadPlus m => (forall d. Data d => d > m d) > Sum a > m (Sum a) Source #  
Num a => Num (Sum a) #  Since: base4.7.0.0 
Ord a => Ord (Sum a) #  Since: base2.1 
Defined in Data.Semigroup.Internal  
Read a => Read (Sum a) #  Since: base2.1 
Show a => Show (Sum a) #  Since: base2.1 
Generic (Sum a) #  Since: base4.7.0.0 
Num a => Semigroup (Sum a) #  Since: base4.9.0.0 
Num a => Monoid (Sum a) #  Since: base2.1 
Generic1 Sum #  Since: base4.7.0.0 
type Rep (Sum a) #  
Defined in Data.Semigroup.Internal  
type Rep1 Sum #  
Defined in Data.Semigroup.Internal 
Monoid under multiplication.
>>>
getProduct (Product 3 <> Product 4 <> mempty)
12
Product  

Instances
Monad Product #  Since: base4.8.0.0 
Functor Product #  Since: base4.8.0.0 
MonadFix Product #  Since: base4.8.0.0 
Applicative Product #  Since: base4.8.0.0 
Defined in Data.Semigroup.Internal  
Foldable Product #  Since: base4.8.0.0 
Defined in Data.Foldable fold :: Monoid m => Product m > m Source # foldMap :: Monoid m => (a > m) > Product a > m Source # foldMap' :: Monoid m => (a > m) > Product a > m Source # foldr :: (a > b > b) > b > Product a > b Source # foldr' :: (a > b > b) > b > Product a > b Source # foldl :: (b > a > b) > b > Product a > b Source # foldl' :: (b > a > b) > b > Product a > b Source # foldr1 :: (a > a > a) > Product a > a Source # foldl1 :: (a > a > a) > Product a > a Source # toList :: Product a > [a] Source # null :: Product a > Bool Source # length :: Product a > Int Source # elem :: Eq a => a > Product a > Bool Source # maximum :: Ord a => Product a > a Source # minimum :: Ord a => Product a > a Source #  
Traversable Product #  Since: base4.8.0.0 
Defined in Data.Traversable  
MonadZip Product #  Since: base4.8.0.0 
Bounded a => Bounded (Product a) #  Since: base2.1 
Eq a => Eq (Product a) #  Since: base2.1 
Data a => Data (Product a) #  Since: base4.8.0.0 
Defined in Data.Data gfoldl :: (forall d b. Data d => c (d > b) > d > c b) > (forall g. g > c g) > Product a > c (Product a) Source # gunfold :: (forall b r. Data b => c (b > r) > c r) > (forall r. r > c r) > Constr > c (Product a) Source # toConstr :: Product a > Constr Source # dataTypeOf :: Product a > DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) > Maybe (c (Product a)) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) > Maybe (c (Product a)) Source # gmapT :: (forall b. Data b => b > b) > Product a > Product a Source # gmapQl :: (r > r' > r) > r > (forall d. Data d => d > r') > Product a > r Source # gmapQr :: forall r r'. (r' > r > r) > r > (forall d. Data d => d > r') > Product a > r Source # gmapQ :: (forall d. Data d => d > u) > Product a > [u] Source # gmapQi :: Int > (forall d. Data d => d > u) > Product a > u Source # gmapM :: Monad m => (forall d. Data d => d > m d) > Product a > m (Product a) Source # gmapMp :: MonadPlus m => (forall d. Data d => d > m d) > Product a > m (Product a) Source # gmapMo :: MonadPlus m => (forall d. Data d => d > m d) > Product a > m (Product a) Source #  
Num a => Num (Product a) #  Since: base4.7.0.0 
Defined in Data.Semigroup.Internal (+) :: Product a > Product a > Product a Source # () :: Product a > Product a > Product a Source # (*) :: Product a > Product a > Product a Source # negate :: Product a > Product a Source # abs :: Product a > Product a Source # signum :: Product a > Product a Source # fromInteger :: Integer > Product a Source #  
Ord a => Ord (Product a) #  Since: base2.1 
Defined in Data.Semigroup.Internal  
Read a => Read (Product a) #  Since: base2.1 
Show a => Show (Product a) #  Since: base2.1 
Generic (Product a) #  Since: base4.7.0.0 
Num a => Semigroup (Product a) #  Since: base4.9.0.0 
Num a => Monoid (Product a) #  Since: base2.1 
Generic1 Product #  Since: base4.7.0.0 
type Rep (Product a) #  
Defined in Data.Semigroup.Internal  
type Rep1 Product #  
Defined in Data.Semigroup.Internal 
Maybe
wrappers
To implement find
or findLast
on any Foldable
:
findLast :: Foldable t => (a > Bool) > t a > Maybe a findLast pred = getLast . foldMap (x > if pred x then Last (Just x) else Last Nothing)
Much of Map
s interface can be implemented with
alter
. Some of the rest can be implemented with a new
alterF
function and either First
or Last
:
alterF :: (Functor f, Ord k) => (Maybe a > f (Maybe a)) > k > Map k a > f (Map k a) instance Monoid a => Functor ((,) a)  from Data.Functor
insertLookupWithKey :: Ord k => (k > v > v > v) > k > v
> Map k v > (Maybe v, Map k v)
insertLookupWithKey combine key value =
Arrow.first getFirst . alterF
doChange key
where
doChange Nothing = (First Nothing, Just value)
doChange (Just oldValue) =
(First (Just oldValue),
Just (combine key value oldValue))
Maybe monoid returning the leftmost nonNothing value.
is isomorphic to First
a
, but precedes it
historically.Alt
Maybe
a
>>>
getFirst (First (Just "hello") <> First Nothing <> First (Just "world"))
Just "hello"
Use of this type is discouraged. Note the following equivalence:
Data.Monoid.First x === Maybe (Data.Semigroup.First x)
In addition to being equivalent in the structural sense, the two
also have Monoid
instances that behave the same. This type will
be marked deprecated in GHC 8.8, and removed in GHC 8.10.
Users are advised to use the variant from Data.Semigroup and wrap
it in Maybe
.
Instances
Monad First #  Since: base4.8.0.0 
Functor First #  Since: base4.8.0.0 
MonadFix First #  Since: base4.8.0.0 
Applicative First #  Since: base4.8.0.0 
Foldable First #  Since: base4.8.0.0 
Defined in Data.Foldable fold :: Monoid m => First m > m Source # foldMap :: Monoid m => (a > m) > First a > m Source # foldMap' :: Monoid m => (a > m) > First a > m Source # foldr :: (a > b > b) > b > First a > b Source # foldr' :: (a > b > b) > b > First a > b Source # foldl :: (b > a > b) > b > First a > b Source # foldl' :: (b > a > b) > b > First a > b Source # foldr1 :: (a > a > a) > First a > a Source # foldl1 :: (a > a > a) > First a > a Source # toList :: First a > [a] Source # null :: First a > Bool Source # length :: First a > Int Source # elem :: Eq a => a > First a > Bool Source # maximum :: Ord a => First a > a Source # minimum :: Ord a => First a > a Source #  
Traversable First #  Since: base4.8.0.0 
MonadZip First #  Since: base4.8.0.0 
Eq a => Eq (First a) #  Since: base2.1 
Data a => Data (First a) #  Since: base4.8.0.0 
Defined in Data.Data gfoldl :: (forall d b. Data d => c (d > b) > d > c b) > (forall g. g > c g) > First a > c (First a) Source # gunfold :: (forall b r. Data b => c (b > r) > c r) > (forall r. r > c r) > Constr > c (First a) Source # toConstr :: First a > Constr Source # dataTypeOf :: First a > DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) > Maybe (c (First a)) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) > Maybe (c (First a)) Source # gmapT :: (forall b. Data b => b > b) > First a > First a Source # gmapQl :: (r > r' > r) > r > (forall d. Data d => d > r') > First a > r Source # gmapQr :: forall r r'. (r' > r > r) > r > (forall d. Data d => d > r') > First a > r Source # gmapQ :: (forall d. Data d => d > u) > First a > [u] Source # gmapQi :: Int > (forall d. Data d => d > u) > First a > u Source # gmapM :: Monad m => (forall d. Data d => d > m d) > First a > m (First a) Source # gmapMp :: MonadPlus m => (forall d. Data d => d > m d) > First a > m (First a) Source # gmapMo :: MonadPlus m => (forall d. Data d => d > m d) > First a > m (First a) Source #  
Ord a => Ord (First a) #  Since: base2.1 
Read a => Read (First a) #  Since: base2.1 
Show a => Show (First a) #  Since: base2.1 
Generic (First a) #  Since: base4.7.0.0 
Semigroup (First a) #  Since: base4.9.0.0 
Monoid (First a) #  Since: base2.1 
Generic1 First #  Since: base4.7.0.0 
type Rep (First a) #  
Defined in Data.Monoid  
type Rep1 First #  
Defined in Data.Monoid 
Maybe monoid returning the rightmost nonNothing value.
is isomorphic to Last
a
, and thus to
Dual
(First
a)Dual
(Alt
Maybe
a)
>>>
getLast (Last (Just "hello") <> Last Nothing <> Last (Just "world"))
Just "world"
Use of this type is discouraged. Note the following equivalence:
Data.Monoid.Last x === Maybe (Data.Semigroup.Last x)
In addition to being equivalent in the structural sense, the two
also have Monoid
instances that behave the same. This type will
be marked deprecated in GHC 8.8, and removed in GHC 8.10.
Users are advised to use the variant from Data.Semigroup and wrap
it in Maybe
.
Instances
Monad Last #  Since: base4.8.0.0 
Functor Last #  Since: base4.8.0.0 
MonadFix Last #  Since: base4.8.0.0 
Applicative Last #  Since: base4.8.0.0 
Foldable Last #  Since: base4.8.0.0 
Defined in Data.Foldable fold :: Monoid m => Last m > m Source # foldMap :: Monoid m => (a > m) > Last a > m Source # foldMap' :: Monoid m => (a > m) > Last a > m Source # foldr :: (a > b > b) > b > Last a > b Source # foldr' :: (a > b > b) > b > Last a > b Source # foldl :: (b > a > b) > b > Last a > b Source # foldl' :: (b > a > b) > b > Last a > b Source # foldr1 :: (a > a > a) > Last a > a Source # foldl1 :: (a > a > a) > Last a > a Source # toList :: Last a > [a] Source # null :: Last a > Bool Source # length :: Last a > Int Source # elem :: Eq a => a > Last a > Bool Source # maximum :: Ord a => Last a > a Source # minimum :: Ord a => Last a > a Source #  
Traversable Last #  Since: base4.8.0.0 
MonadZip Last #  Since: base4.8.0.0 
Eq a => Eq (Last a) #  Since: base2.1 
Data a => Data (Last a) #  Since: base4.8.0.0 
Defined in Data.Data gfoldl :: (forall d b. Data d => c (d > b) > d > c b) > (forall g. g > c g) > Last a > c (Last a) Source # gunfold :: (forall b r. Data b => c (b > r) > c r) > (forall r. r > c r) > Constr > c (Last a) Source # toConstr :: Last a > Constr Source # dataTypeOf :: Last a > DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) > Maybe (c (Last a)) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) > Maybe (c (Last a)) Source # gmapT :: (forall b. Data b => b > b) > Last a > Last a Source # gmapQl :: (r > r' > r) > r > (forall d. Data d => d > r') > Last a > r Source # gmapQr :: forall r r'. (r' > r > r) > r > (forall d. Data d => d > r') > Last a > r Source # gmapQ :: (forall d. Data d => d > u) > Last a > [u] Source # gmapQi :: Int > (forall d. Data d => d > u) > Last a > u Source # gmapM :: Monad m => (forall d. Data d => d > m d) > Last a > m (Last a) Source # gmapMp :: MonadPlus m => (forall d. Data d => d > m d) > Last a > m (Last a) Source # gmapMo :: MonadPlus m => (forall d. Data d => d > m d) > Last a > m (Last a) Source #  
Ord a => Ord (Last a) #  Since: base2.1 
Defined in Data.Monoid  
Read a => Read (Last a) #  Since: base2.1 
Show a => Show (Last a) #  Since: base2.1 
Generic (Last a) #  Since: base4.7.0.0 
Semigroup (Last a) #  Since: base4.9.0.0 
Monoid (Last a) #  Since: base2.1 
Generic1 Last #  Since: base4.7.0.0 
type Rep (Last a) #  
Defined in Data.Monoid  
type Rep1 Last #  
Defined in Data.Monoid 
Alternative
wrapper
Monoid under <>
.
Since: base4.8.0.0
Instances
Generic1 (Alt f :: k > Type) #  Since: base4.8.0.0 
Monad f => Monad (Alt f) #  Since: base4.8.0.0 
Functor f => Functor (Alt f) #  Since: base4.8.0.0 
MonadFix f => MonadFix (Alt f) #  Since: base4.8.0.0 
Applicative f => Applicative (Alt f) #  Since: base4.8.0.0 
Foldable f => Foldable (Alt f) #  Since: base4.12.0.0 
Defined in Data.Foldable fold :: Monoid m => Alt f m > m Source # foldMap :: Monoid m => (a > m) > Alt f a > m Source # foldMap' :: Monoid m => (a > m) > Alt f a > m Source # foldr :: (a > b > b) > b > Alt f a > b Source # foldr' :: (a > b > b) > b > Alt f a > b Source # foldl :: (b > a > b) > b > Alt f a > b Source # foldl' :: (b > a > b) > b > Alt f a > b Source # foldr1 :: (a > a > a) > Alt f a > a Source # foldl1 :: (a > a > a) > Alt f a > a Source # toList :: Alt f a > [a] Source # null :: Alt f a > Bool Source # length :: Alt f a > Int Source # elem :: Eq a => a > Alt f a > Bool Source # maximum :: Ord a => Alt f a > a Source # minimum :: Ord a => Alt f a > a Source #  
Traversable f => Traversable (Alt f) #  Since: base4.12.0.0 
MonadPlus f => MonadPlus (Alt f) #  Since: base4.8.0.0 
Alternative f => Alternative (Alt f) #  Since: base4.8.0.0 
MonadZip f => MonadZip (Alt f) #  Since: base4.8.0.0 
Contravariant f => Contravariant (Alt f) #  
Enum (f a) => Enum (Alt f a) #  Since: base4.8.0.0 
Defined in Data.Semigroup.Internal succ :: Alt f a > Alt f a Source # pred :: Alt f a > Alt f a Source # toEnum :: Int > Alt f a Source # fromEnum :: Alt f a > Int Source # enumFrom :: Alt f a > [Alt f a] Source # enumFromThen :: Alt f a > Alt f a > [Alt f a] Source # enumFromTo :: Alt f a > Alt f a > [Alt f a] Source # enumFromThenTo :: Alt f a > Alt f a > Alt f a > [Alt f a] Source #  
Eq (f a) => Eq (Alt f a) #  Since: base4.8.0.0 
(Data (f a), Data a, Typeable f) => Data (Alt f a) #  Since: base4.8.0.0 
Defined in Data.Data gfoldl :: (forall d b. Data d => c (d > b) > d > c b) > (forall g. g > c g) > Alt f a > c (Alt f a) Source # gunfold :: (forall b r. Data b => c (b > r) > c r) > (forall r. r > c r) > Constr > c (Alt f a) Source # toConstr :: Alt f a > Constr Source # dataTypeOf :: Alt f a > DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) > Maybe (c (Alt f a)) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) > Maybe (c (Alt f a)) Source # gmapT :: (forall b. Data b => b > b) > Alt f a > Alt f a Source # gmapQl :: (r > r' > r) > r > (forall d. Data d => d > r') > Alt f a > r Source # gmapQr :: forall r r'. (r' > r > r) > r > (forall d. Data d => d > r') > Alt f a > r Source # gmapQ :: (forall d. Data d => d > u) > Alt f a > [u] Source # gmapQi :: Int > (forall d. Data d => d > u) > Alt f a > u Source # gmapM :: Monad m => (forall d. Data d => d > m d) > Alt f a > m (Alt f a) Source # gmapMp :: MonadPlus m => (forall d. Data d => d > m d) > Alt f a > m (Alt f a) Source # gmapMo :: MonadPlus m => (forall d. Data d => d > m d) > Alt f a > m (Alt f a) Source #  
Num (f a) => Num (Alt f a) #  Since: base4.8.0.0 
Defined in Data.Semigroup.Internal  
Ord (f a) => Ord (Alt f a) #  Since: base4.8.0.0 
Defined in Data.Semigroup.Internal  
Read (f a) => Read (Alt f a) #  Since: base4.8.0.0 
Show (f a) => Show (Alt f a) #  Since: base4.8.0.0 
Generic (Alt f a) #  Since: base4.8.0.0 
Alternative f => Semigroup (Alt f a) #  Since: base4.9.0.0 
Alternative f => Monoid (Alt f a) #  Since: base4.8.0.0 
type Rep1 (Alt f :: k > Type) #  
Defined in Data.Semigroup.Internal  
type Rep (Alt f a) #  
Defined in Data.Semigroup.Internal 
Applicative
wrapper
This data type witnesses the lifting of a Monoid
into an
Applicative
pointwise.
Since: base4.12.0.0
Instances
Generic1 (Ap f :: k > Type) #  Since: base4.12.0.0 
Monad f => Monad (Ap f) #  Since: base4.12.0.0 
Functor f => Functor (Ap f) #  Since: base4.12.0.0 
MonadFix f => MonadFix (Ap f) #  Since: base4.12.0.0 
MonadFail f => MonadFail (Ap f) #  Since: base4.12.0.0 
Applicative f => Applicative (Ap f) #  Since: base4.12.0.0 
Foldable f => Foldable (Ap f) #  Since: base4.12.0.0 
Defined in Data.Foldable fold :: Monoid m => Ap f m > m Source # foldMap :: Monoid m => (a > m) > Ap f a > m Source # foldMap' :: Monoid m => (a > m) > Ap f a > m Source # foldr :: (a > b > b) > b > Ap f a > b Source # foldr' :: (a > b > b) > b > Ap f a > b Source # foldl :: (b > a > b) > b > Ap f a > b Source # foldl' :: (b > a > b) > b > Ap f a > b Source # foldr1 :: (a > a > a) > Ap f a > a Source # foldl1 :: (a > a > a) > Ap f a > a Source # toList :: Ap f a > [a] Source # null :: Ap f a > Bool Source # length :: Ap f a > Int Source # elem :: Eq a => a > Ap f a > Bool Source # maximum :: Ord a => Ap f a > a Source # minimum :: Ord a => Ap f a > a Source #  
Traversable f => Traversable (Ap f) #  Since: base4.12.0.0 
MonadPlus f => MonadPlus (Ap f) #  Since: base4.12.0.0 
Alternative f => Alternative (Ap f) #  Since: base4.12.0.0 
(Applicative f, Bounded a) => Bounded (Ap f a) #  Since: base4.12.0.0 
Enum (f a) => Enum (Ap f a) #  Since: base4.12.0.0 
Defined in Data.Monoid succ :: Ap f a > Ap f a Source # pred :: Ap f a > Ap f a Source # toEnum :: Int > Ap f a Source # fromEnum :: Ap f a > Int Source # enumFrom :: Ap f a > [Ap f a] Source # enumFromThen :: Ap f a > Ap f a > [Ap f a] Source # enumFromTo :: Ap f a > Ap f a > [Ap f a] Source # enumFromThenTo :: Ap f a > Ap f a > Ap f a > [Ap f a] Source #  
Eq (f a) => Eq (Ap f a) #  Since: base4.12.0.0 
(Data (f a), Data a, Typeable f) => Data (Ap f a) #  Since: base4.12.0.0 
Defined in Data.Data gfoldl :: (forall d b. Data d => c (d > b) > d > c b) > (forall g. g > c g) > Ap f a > c (Ap f a) Source # gunfold :: (forall b r. Data b => c (b > r) > c r) > (forall r. r > c r) > Constr > c (Ap f a) Source # toConstr :: Ap f a > Constr Source # dataTypeOf :: Ap f a > DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) > Maybe (c (Ap f a)) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) > Maybe (c (Ap f a)) Source # gmapT :: (forall b. Data b => b > b) > Ap f a > Ap f a Source # gmapQl :: (r > r' > r) > r > (forall d. Data d => d > r') > Ap f a > r Source # gmapQr :: forall r r'. (r' > r > r) > r > (forall d. Data d => d > r') > Ap f a > r Source # gmapQ :: (forall d. Data d => d > u) > Ap f a > [u] Source # gmapQi :: Int > (forall d. Data d => d > u) > Ap f a > u Source # gmapM :: Monad m => (forall d. Data d => d > m d) > Ap f a > m (Ap f a) Source # gmapMp :: MonadPlus m => (forall d. Data d => d > m d) > Ap f a > m (Ap f a) Source # gmapMo :: MonadPlus m => (forall d. Data d => d > m d) > Ap f a > m (Ap f a) Source #  
(Applicative f, Num a) => Num (Ap f a) #  Since: base4.12.0.0 
Ord (f a) => Ord (Ap f a) #  Since: base4.12.0.0 
Defined in Data.Monoid  
Read (f a) => Read (Ap f a) #  Since: base4.12.0.0 
Show (f a) => Show (Ap f a) #  Since: base4.12.0.0 
Generic (Ap f a) #  Since: base4.12.0.0 
(Applicative f, Semigroup a) => Semigroup (Ap f a) #  Since: base4.12.0.0 
(Applicative f, Monoid a) => Monoid (Ap f a) #  Since: base4.12.0.0 
type Rep1 (Ap f :: k > Type) #  
Defined in Data.Monoid  
type Rep (Ap f a) #  
Defined in Data.Monoid 