Safe Haskell | Safe-Inferred |
---|---|
Language | Haskell2010 |
Synopsis
- class Semigroup a => Monoid a where
- newtype First a = First {}
- newtype Last a = Last {}
- newtype Ap (f :: k -> Type) (a :: k) = Ap {
- getAp :: f 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 Alt (f :: k -> Type) (a :: k) = Alt {
- getAlt :: f a
- (<>) :: Semigroup a => a -> a -> a
Documentation
class Semigroup a => Monoid a where #
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 base-4.11.0.0.
Identity of mappend
>>>
"Hello world" <> mempty
"Hello world"
An associative operation
NOTE: This method is redundant and has the default
implementation
since base-4.11.0.0.
Should it be implemented manually, since mappend
= (<>
)mappend
is a synonym for
(<>
), it is expected that the two functions are defined the same
way. In a future GHC release mappend
will be removed from Monoid
.
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.
>>>
mconcat ["Hello", " ", "Haskell", "!"]
"Hello Haskell!"
Instances
Monoid Ordering | Since: base-2.1 |
Monoid () | Since: base-2.1 |
Monoid All | Since: base-2.1 |
Monoid Any | Since: base-2.1 |
Monoid [a] | Since: base-2.1 |
Semigroup a => Monoid (Maybe a) | Lift a semigroup into Since 4.11.0: constraint on inner Since: base-2.1 |
Monoid a => Monoid (IO a) | Since: base-4.9.0.0 |
Monoid (Predicate a) | |
Monoid (Comparison a) | |
Defined in Data.Functor.Contravariant mempty :: Comparison a # mappend :: Comparison a -> Comparison a -> Comparison a # mconcat :: [Comparison a] -> Comparison a # | |
Monoid (Equivalence a) | |
Defined in Data.Functor.Contravariant mempty :: Equivalence a # mappend :: Equivalence a -> Equivalence a -> Equivalence a # mconcat :: [Equivalence a] -> Equivalence a # | |
(Ord a, Bounded a) => Monoid (Min a) | Since: base-4.9.0.0 |
(Ord a, Bounded a) => Monoid (Max a) | Since: base-4.9.0.0 |
Monoid m => Monoid (WrappedMonoid m) | Since: base-4.9.0.0 |
Defined in Data.Semigroup mempty :: WrappedMonoid m # mappend :: WrappedMonoid m -> WrappedMonoid m -> WrappedMonoid m # mconcat :: [WrappedMonoid m] -> WrappedMonoid m # | |
Semigroup a => Monoid (Option a) | Since: base-4.9.0.0 |
Monoid a => Monoid (Identity a) | Since: base-4.9.0.0 |
Monoid (First a) | Since: base-2.1 |
Monoid (Last a) | Since: base-2.1 |
Monoid a => Monoid (Dual a) | Since: base-2.1 |
Monoid (Endo a) | Since: base-2.1 |
Num a => Monoid (Sum a) | Since: base-2.1 |
Num a => Monoid (Product a) | Since: base-2.1 |
Monoid b => Monoid (a -> b) | Since: base-2.1 |
(Monoid a, Monoid b) => Monoid (a, b) | Since: base-2.1 |
Monoid a => Monoid (ST s a) | Since: base-4.11.0.0 |
Monoid a => Monoid (Op a b) | |
Monoid (Proxy s) | Since: base-4.7.0.0 |
(Monoid a, Monoid b, Monoid c) => Monoid (a, b, c) | Since: base-2.1 |
Monoid a => Monoid (Const a b) | Since: base-4.9.0.0 |
(Applicative f, Monoid a) => Monoid (Ap f a) | Since: base-4.12.0.0 |
Alternative f => Monoid (Alt f a) | Since: base-4.8.0.0 |
(Monoid a, Monoid b, Monoid c, Monoid d) => Monoid (a, b, c, d) | Since: base-2.1 |
(Monoid a, Monoid b, Monoid c, Monoid d, Monoid e) => Monoid (a, b, c, d, e) | Since: base-2.1 |
Maybe monoid returning the leftmost non-Nothing 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: base-4.8.0.0 |
Functor First | Since: base-4.8.0.0 |
Applicative First | Since: base-4.8.0.0 |
Foldable First | Since: base-4.8.0.0 |
Defined in Data.Foldable fold :: Monoid m => First m -> m # foldMap :: Monoid m => (a -> m) -> First a -> 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 # elem :: Eq a => a -> First a -> Bool # maximum :: Ord a => First a -> a # minimum :: Ord a => First a -> a # | |
Traversable First | Since: base-4.8.0.0 |
Eq a => Eq (First a) | Since: base-2.1 |
Ord a => Ord (First a) | Since: base-2.1 |
Read a => Read (First a) | Since: base-2.1 |
Show a => Show (First a) | Since: base-2.1 |
Generic (First a) | Since: base-4.7.0.0 |
Semigroup (First a) | Since: base-4.9.0.0 |
Monoid (First a) | Since: base-2.1 |
Generic1 First | Since: base-4.7.0.0 |
type Rep (First a) | |
Defined in Data.Monoid | |
type Rep1 First | |
Defined in Data.Monoid |
Maybe monoid returning the rightmost non-Nothing 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: base-4.8.0.0 |
Functor Last | Since: base-4.8.0.0 |
Applicative Last | Since: base-4.8.0.0 |
Foldable Last | Since: base-4.8.0.0 |
Defined in Data.Foldable fold :: Monoid m => Last m -> m # foldMap :: Monoid m => (a -> m) -> Last a -> 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 # elem :: Eq a => a -> Last a -> Bool # maximum :: Ord a => Last a -> a # | |
Traversable Last | Since: base-4.8.0.0 |
Eq a => Eq (Last a) | Since: base-2.1 |
Ord a => Ord (Last a) | Since: base-2.1 |
Read a => Read (Last a) | Since: base-2.1 |
Show a => Show (Last a) | Since: base-2.1 |
Generic (Last a) | Since: base-4.7.0.0 |
Semigroup (Last a) | Since: base-4.9.0.0 |
Monoid (Last a) | Since: base-2.1 |
Generic1 Last | Since: base-4.7.0.0 |
type Rep (Last a) | |
Defined in Data.Monoid | |
type Rep1 Last | |
Defined in Data.Monoid |
newtype Ap (f :: k -> Type) (a :: k) #
This data type witnesses the lifting of a Monoid
into an
Applicative
pointwise.
Since: base-4.12.0.0
Instances
Generic1 (Ap f :: k -> Type) | Since: base-4.12.0.0 |
Monad f => Monad (Ap f) | Since: base-4.12.0.0 |
Functor f => Functor (Ap f) | Since: base-4.12.0.0 |
MonadFail f => MonadFail (Ap f) | Since: base-4.12.0.0 |
Defined in Data.Monoid | |
Applicative f => Applicative (Ap f) | Since: base-4.12.0.0 |
Foldable f => Foldable (Ap f) | Since: base-4.12.0.0 |
Defined in Data.Foldable fold :: Monoid m => Ap f m -> m # foldMap :: Monoid m => (a -> m) -> Ap f a -> m # foldMap' :: Monoid m => (a -> m) -> Ap f a -> m # foldr :: (a -> b -> b) -> b -> Ap f a -> b # foldr' :: (a -> b -> b) -> b -> Ap f a -> b # foldl :: (b -> a -> b) -> b -> Ap f a -> b # foldl' :: (b -> a -> b) -> b -> Ap f a -> b # foldr1 :: (a -> a -> a) -> Ap f a -> a # foldl1 :: (a -> a -> a) -> Ap f a -> a # elem :: Eq a => a -> Ap f a -> Bool # maximum :: Ord a => Ap f a -> a # | |
Traversable f => Traversable (Ap f) | Since: base-4.12.0.0 |
Alternative f => Alternative (Ap f) | Since: base-4.12.0.0 |
MonadPlus f => MonadPlus (Ap f) | Since: base-4.12.0.0 |
(Applicative f, Bounded a) => Bounded (Ap f a) | Since: base-4.12.0.0 |
Enum (f a) => Enum (Ap f a) | Since: base-4.12.0.0 |
Defined in Data.Monoid | |
Eq (f a) => Eq (Ap f a) | Since: base-4.12.0.0 |
(Applicative f, Num a) => Num (Ap f a) | Since: base-4.12.0.0 |
Ord (f a) => Ord (Ap f a) | Since: base-4.12.0.0 |
Read (f a) => Read (Ap f a) | Since: base-4.12.0.0 |
Show (f a) => Show (Ap f a) | Since: base-4.12.0.0 |
Generic (Ap f a) | Since: base-4.12.0.0 |
(Applicative f, Semigroup a) => Semigroup (Ap f a) | Since: base-4.12.0.0 |
(Applicative f, Monoid a) => Monoid (Ap f a) | Since: base-4.12.0.0 |
type Rep1 (Ap f :: k -> Type) | |
Defined in Data.Monoid | |
type Rep (Ap f a) | |
Defined in Data.Monoid |
The dual of a Monoid
, obtained by swapping the arguments of mappend
.
>>>
getDual (mappend (Dual "Hello") (Dual "World"))
"WorldHello"
Instances
Monad Dual | Since: base-4.8.0.0 |
Functor Dual | Since: base-4.8.0.0 |
Applicative Dual | Since: base-4.8.0.0 |
Foldable Dual | Since: base-4.8.0.0 |
Defined in Data.Foldable fold :: Monoid m => Dual m -> m # foldMap :: Monoid m => (a -> m) -> Dual a -> m # foldMap' :: Monoid m => (a -> m) -> Dual a -> m # foldr :: (a -> b -> b) -> b -> Dual a -> b # foldr' :: (a -> b -> b) -> b -> Dual a -> b # foldl :: (b -> a -> b) -> b -> Dual a -> b # foldl' :: (b -> a -> b) -> b -> Dual a -> b # foldr1 :: (a -> a -> a) -> Dual a -> a # foldl1 :: (a -> a -> a) -> Dual a -> a # elem :: Eq a => a -> Dual a -> Bool # maximum :: Ord a => Dual a -> a # | |
Traversable Dual | Since: base-4.8.0.0 |
Bounded a => Bounded (Dual a) | Since: base-2.1 |
Eq a => Eq (Dual a) | Since: base-2.1 |
Ord a => Ord (Dual a) | Since: base-2.1 |
Read a => Read (Dual a) | Since: base-2.1 |
Show a => Show (Dual a) | Since: base-2.1 |
Generic (Dual a) | Since: base-4.7.0.0 |
Semigroup a => Semigroup (Dual a) | Since: base-4.9.0.0 |
Monoid a => Monoid (Dual a) | Since: base-2.1 |
Generic1 Dual | Since: base-4.7.0.0 |
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!"
Boolean monoid under conjunction (&&
).
>>>
getAll (All True <> mempty <> All False)
False
>>>
getAll (mconcat (map (\x -> All (even x)) [2,4,6,7,8]))
False
Boolean monoid under disjunction (||
).
>>>
getAny (Any True <> mempty <> Any False)
True
>>>
getAny (mconcat (map (\x -> Any (even x)) [2,4,6,7,8]))
True
Monoid under addition.
>>>
getSum (Sum 1 <> Sum 2 <> mempty)
3
Instances
Monad Sum | Since: base-4.8.0.0 |
Functor Sum | Since: base-4.8.0.0 |
Applicative Sum | Since: base-4.8.0.0 |
Foldable Sum | Since: base-4.8.0.0 |
Defined in Data.Foldable fold :: Monoid m => Sum m -> m # foldMap :: Monoid m => (a -> m) -> Sum a -> m # foldMap' :: Monoid m => (a -> m) -> Sum a -> m # foldr :: (a -> b -> b) -> b -> Sum a -> b # foldr' :: (a -> b -> b) -> b -> Sum a -> b # foldl :: (b -> a -> b) -> b -> Sum a -> b # foldl' :: (b -> a -> b) -> b -> Sum a -> b # foldr1 :: (a -> a -> a) -> Sum a -> a # foldl1 :: (a -> a -> a) -> Sum a -> a # elem :: Eq a => a -> Sum a -> Bool # maximum :: Ord a => Sum a -> a # | |
Traversable Sum | Since: base-4.8.0.0 |
Bounded a => Bounded (Sum a) | Since: base-2.1 |
Eq a => Eq (Sum a) | Since: base-2.1 |
Num a => Num (Sum a) | Since: base-4.7.0.0 |
Ord a => Ord (Sum a) | Since: base-2.1 |
Read a => Read (Sum a) | Since: base-2.1 |
Show a => Show (Sum a) | Since: base-2.1 |
Generic (Sum a) | Since: base-4.7.0.0 |
Num a => Semigroup (Sum a) | Since: base-4.9.0.0 |
Num a => Monoid (Sum a) | Since: base-2.1 |
Generic1 Sum | Since: base-4.7.0.0 |
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: base-4.8.0.0 |
Functor Product | Since: base-4.8.0.0 |
Applicative Product | Since: base-4.8.0.0 |
Foldable Product | Since: base-4.8.0.0 |
Defined in Data.Foldable fold :: Monoid m => Product m -> m # foldMap :: Monoid m => (a -> m) -> Product a -> m # foldMap' :: Monoid m => (a -> m) -> Product a -> m # foldr :: (a -> b -> b) -> b -> Product a -> b # foldr' :: (a -> b -> b) -> b -> Product a -> b # foldl :: (b -> a -> b) -> b -> Product a -> b # foldl' :: (b -> a -> b) -> b -> Product a -> b # foldr1 :: (a -> a -> a) -> Product a -> a # foldl1 :: (a -> a -> a) -> Product a -> a # elem :: Eq a => a -> Product a -> Bool # maximum :: Ord a => Product a -> a # minimum :: Ord a => Product a -> a # | |
Traversable Product | Since: base-4.8.0.0 |
Bounded a => Bounded (Product a) | Since: base-2.1 |
Eq a => Eq (Product a) | Since: base-2.1 |
Num a => Num (Product a) | Since: base-4.7.0.0 |
Defined in Data.Semigroup.Internal | |
Ord a => Ord (Product a) | Since: base-2.1 |
Defined in Data.Semigroup.Internal | |
Read a => Read (Product a) | Since: base-2.1 |
Show a => Show (Product a) | Since: base-2.1 |
Generic (Product a) | Since: base-4.7.0.0 |
Num a => Semigroup (Product a) | Since: base-4.9.0.0 |
Num a => Monoid (Product a) | Since: base-2.1 |
Generic1 Product | Since: base-4.7.0.0 |
type Rep (Product a) | |
Defined in Data.Semigroup.Internal | |
type Rep1 Product | |
Defined in Data.Semigroup.Internal |
newtype Alt (f :: k -> Type) (a :: k) #
Monoid under <|>
.
>>>
getAlt (Alt (Just 12) <> Alt (Just 24))
Just 12
>>>
getAlt $ Alt Nothing <> Alt (Just 24)
Just 24
Since: base-4.8.0.0
Instances
Generic1 (Alt f :: k -> Type) | Since: base-4.8.0.0 |
Monad f => Monad (Alt f) | Since: base-4.8.0.0 |
Functor f => Functor (Alt f) | Since: base-4.8.0.0 |
Applicative f => Applicative (Alt f) | Since: base-4.8.0.0 |
Foldable f => Foldable (Alt f) | Since: base-4.12.0.0 |
Defined in Data.Foldable fold :: Monoid m => Alt f m -> m # foldMap :: Monoid m => (a -> m) -> Alt f a -> m # foldMap' :: Monoid m => (a -> m) -> Alt f a -> m # foldr :: (a -> b -> b) -> b -> Alt f a -> b # foldr' :: (a -> b -> b) -> b -> Alt f a -> b # foldl :: (b -> a -> b) -> b -> Alt f a -> b # foldl' :: (b -> a -> b) -> b -> Alt f a -> b # foldr1 :: (a -> a -> a) -> Alt f a -> a # foldl1 :: (a -> a -> a) -> Alt f a -> a # elem :: Eq a => a -> Alt f a -> Bool # maximum :: Ord a => Alt f a -> a # minimum :: Ord a => Alt f a -> a # | |
Traversable f => Traversable (Alt f) | Since: base-4.12.0.0 |
Contravariant f => Contravariant (Alt f) | |
Alternative f => Alternative (Alt f) | Since: base-4.8.0.0 |
MonadPlus f => MonadPlus (Alt f) | Since: base-4.8.0.0 |
Enum (f a) => Enum (Alt f a) | Since: base-4.8.0.0 |
Eq (f a) => Eq (Alt f a) | Since: base-4.8.0.0 |
Num (f a) => Num (Alt f a) | Since: base-4.8.0.0 |
Ord (f a) => Ord (Alt f a) | Since: base-4.8.0.0 |
Read (f a) => Read (Alt f a) | Since: base-4.8.0.0 |
Show (f a) => Show (Alt f a) | Since: base-4.8.0.0 |
Generic (Alt f a) | Since: base-4.8.0.0 |
Alternative f => Semigroup (Alt f a) | Since: base-4.9.0.0 |
Alternative f => Monoid (Alt f a) | Since: base-4.8.0.0 |
type Rep1 (Alt f :: k -> Type) | |
Defined in Data.Semigroup.Internal | |
type Rep (Alt f a) | |
Defined in Data.Semigroup.Internal |