| Copyright | (c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001 |
|---|---|
| License | BSD-style (see the file libraries/base/LICENSE) |
| Maintainer | libraries@haskell.org |
| Stability | experimental |
| Portability | portable |
| Safe Haskell | Trustworthy |
| Language | Haskell2010 |
Data.Monoid
Description
A class for monoids (types with an associative binary operation that has an identity) with various general-purpose instances.
- class Monoid a where
- (<>) :: Monoid m => m -> m -> m
- 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
Monoid typeclass
The class of monoids (types with an associative binary operation that has an identity). Instances should satisfy the following laws:
mappend mempty x = x
mappend x mempty = x
mappend x (mappend y z) = mappend (mappend x y) z
mconcat =
foldrmappend 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 newtypes and make those instances
of Monoid, e.g. Sum and Product.
Methods
Identity of mappend
mappend :: a -> a -> a Source #
An associative operation
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 Source # | Since: 2.1 |
| Monoid () Source # | Since: 2.1 |
| Monoid Any Source # | Since: 2.1 |
| Monoid All Source # | Since: 2.1 |
| Monoid Lifetime Source # |
Since: 4.8.0.0 |
| Monoid Event Source # | Since: 4.3.1.0 |
| Monoid [a] Source # | Since: 2.1 |
| Monoid a => Monoid (Maybe a) Source # | Lift a semigroup into Since: 2.1 |
| Monoid a => Monoid (IO a) Source # | Since: 4.9.0.0 |
| Monoid (Last a) Source # | Since: 2.1 |
| Monoid (First a) Source # | Since: 2.1 |
| Num a => Monoid (Product a) Source # | Since: 2.1 |
| Num a => Monoid (Sum a) Source # | Since: 2.1 |
| Monoid (Endo a) Source # | Since: 2.1 |
| Monoid a => Monoid (Dual a) Source # | Since: 2.1 |
| Monoid a => Monoid (Identity a) Source # | |
| Semigroup a => Monoid (Option a) Source # | Since: 4.9.0.0 |
| Monoid m => Monoid (WrappedMonoid m) Source # | Since: 4.9.0.0 |
| (Ord a, Bounded a) => Monoid (Max a) Source # | Since: 4.9.0.0 |
| (Ord a, Bounded a) => Monoid (Min a) Source # | Since: 4.9.0.0 |
| Monoid b => Monoid (a -> b) Source # | Since: 2.1 |
| (Monoid a, Monoid b) => Monoid (a, b) Source # | Since: 2.1 |
| Monoid (Proxy k s) Source # | Since: 4.7.0.0 |
| (Monoid a, Monoid b, Monoid c) => Monoid (a, b, c) Source # | Since: 2.1 |
| Alternative f => Monoid (Alt * f a) Source # | Since: 4.8.0.0 |
| Monoid a => Monoid (Const k a b) Source # | |
| (Monoid a, Monoid b, Monoid c, Monoid d) => Monoid (a, b, c, d) Source # | Since: 2.1 |
| (Monoid a, Monoid b, Monoid c, Monoid d, Monoid e) => Monoid (a, b, c, d, e) Source # | Since: 2.1 |
Instances
| Monad Dual Source # | Since: 4.8.0.0 |
| Functor Dual Source # | Since: 4.8.0.0 |
| MonadFix Dual Source # | Since: 4.8.0.0 |
| Applicative Dual Source # | Since: 4.8.0.0 |
| Foldable Dual Source # | Since: 4.8.0.0 |
| Traversable Dual Source # | Since: 4.8.0.0 |
| MonadZip Dual Source # | Since: 4.8.0.0 |
| Bounded a => Bounded (Dual a) Source # | |
| Eq a => Eq (Dual a) Source # | |
| Data a => Data (Dual a) Source # | Since: 4.8.0.0 |
| Ord a => Ord (Dual a) Source # | |
| Read a => Read (Dual a) Source # | |
| Show a => Show (Dual a) Source # | |
| Generic (Dual a) Source # | |
| Semigroup a => Semigroup (Dual a) Source # | Since: 4.9.0.0 |
| Monoid a => Monoid (Dual a) Source # | Since: 2.1 |
| Generic1 * Dual Source # | |
| type Rep (Dual a) Source # | |
| type Rep1 * Dual Source # | |
The monoid of endomorphisms under composition.
Bool wrappers
Boolean monoid under conjunction (&&).
Boolean monoid under disjunction (||).
Num wrappers
Monoid under addition.
Instances
| Monad Sum Source # | Since: 4.8.0.0 |
| Functor Sum Source # | Since: 4.8.0.0 |
| MonadFix Sum Source # | Since: 4.8.0.0 |
| Applicative Sum Source # | Since: 4.8.0.0 |
| Foldable Sum Source # | Since: 4.8.0.0 |
| Traversable Sum Source # | Since: 4.8.0.0 |
| MonadZip Sum Source # | Since: 4.8.0.0 |
| Bounded a => Bounded (Sum a) Source # | |
| Eq a => Eq (Sum a) Source # | |
| Data a => Data (Sum a) Source # | Since: 4.8.0.0 |
| Num a => Num (Sum a) Source # | |
| Ord a => Ord (Sum a) Source # | |
| Read a => Read (Sum a) Source # | |
| Show a => Show (Sum a) Source # | |
| Generic (Sum a) Source # | |
| Num a => Semigroup (Sum a) Source # | Since: 4.9.0.0 |
| Num a => Monoid (Sum a) Source # | Since: 2.1 |
| Generic1 * Sum Source # | |
| type Rep (Sum a) Source # | |
| type Rep1 * Sum Source # | |
Monoid under multiplication.
Constructors
| Product | |
Fields
| |
Instances
| Monad Product Source # | Since: 4.8.0.0 |
| Functor Product Source # | Since: 4.8.0.0 |
| MonadFix Product Source # | Since: 4.8.0.0 |
| Applicative Product Source # | Since: 4.8.0.0 |
| Foldable Product Source # | Since: 4.8.0.0 |
| Traversable Product Source # | Since: 4.8.0.0 |
| MonadZip Product Source # | Since: 4.8.0.0 |
| Bounded a => Bounded (Product a) Source # | |
| Eq a => Eq (Product a) Source # | |
| Data a => Data (Product a) Source # | Since: 4.8.0.0 |
| Num a => Num (Product a) Source # | |
| Ord a => Ord (Product a) Source # | |
| Read a => Read (Product a) Source # | |
| Show a => Show (Product a) Source # | |
| Generic (Product a) Source # | |
| Num a => Semigroup (Product a) Source # | Since: 4.9.0.0 |
| Num a => Monoid (Product a) Source # | Since: 2.1 |
| Generic1 * Product Source # | |
| type Rep (Product a) Source # | |
| type Rep1 * Product Source # | |
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 Data.Map's interface can be implemented with
Data.Map.alter. Some of the rest can be implemented with a new
alterA function and either First or Last:
alterA :: (Applicative f, Ord k) =>
(Maybe a -> f (Maybe a)) -> k -> Map k a -> f (Map k a)
instance Monoid a => Applicative ((,) a) -- from Control.ApplicativeinsertLookupWithKey :: Ord k => (k -> v -> v -> v) -> k -> v
-> Map k v -> (Maybe v, Map k v)
insertLookupWithKey combine key value =
Arrow.first getFirst . alterA 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 non-Nothing value.
is isomorphic to First a, but precedes it
historically.Alt Maybe a
Instances
| Monad First Source # | |
| Functor First Source # | |
| MonadFix First Source # | Since: 4.8.0.0 |
| Applicative First Source # | |
| Foldable First Source # | Since: 4.8.0.0 |
| Traversable First Source # | Since: 4.8.0.0 |
| MonadZip First Source # | Since: 4.8.0.0 |
| Eq a => Eq (First a) Source # | |
| Data a => Data (First a) Source # | Since: 4.8.0.0 |
| Ord a => Ord (First a) Source # | |
| Read a => Read (First a) Source # | |
| Show a => Show (First a) Source # | |
| Generic (First a) Source # | |
| Semigroup (First a) Source # | Since: 4.9.0.0 |
| Monoid (First a) Source # | Since: 2.1 |
| Generic1 * First Source # | |
| type Rep (First a) Source # | |
| type Rep1 * First Source # | |
Maybe monoid returning the rightmost non-Nothing value.
is isomorphic to Last a, and thus to
Dual (First a)Dual (Alt Maybe a)
Instances
| Monad Last Source # | |
| Functor Last Source # | |
| MonadFix Last Source # | Since: 4.8.0.0 |
| Applicative Last Source # | |
| Foldable Last Source # | Since: 4.8.0.0 |
| Traversable Last Source # | Since: 4.8.0.0 |
| MonadZip Last Source # | Since: 4.8.0.0 |
| Eq a => Eq (Last a) Source # | |
| Data a => Data (Last a) Source # | Since: 4.8.0.0 |
| Ord a => Ord (Last a) Source # | |
| Read a => Read (Last a) Source # | |
| Show a => Show (Last a) Source # | |
| Generic (Last a) Source # | |
| Semigroup (Last a) Source # | Since: 4.9.0.0 |
| Monoid (Last a) Source # | Since: 2.1 |
| Generic1 * Last Source # | |
| type Rep (Last a) Source # | |
| type Rep1 * Last Source # | |
Alternative wrapper
Monoid under <|>.
Since: 4.8.0.0
Instances
| Generic1 k (Alt k f) Source # | |
| Monad f => Monad (Alt * f) Source # | |
| Functor f => Functor (Alt * f) Source # | |
| MonadFix f => MonadFix (Alt * f) Source # | Since: 4.8.0.0 |
| Applicative f => Applicative (Alt * f) Source # | |
| MonadPlus f => MonadPlus (Alt * f) Source # | |
| Alternative f => Alternative (Alt * f) Source # | |
| MonadZip f => MonadZip (Alt * f) Source # | Since: 4.8.0.0 |
| Enum (f a) => Enum (Alt k f a) Source # | |
| Eq (f a) => Eq (Alt k f a) Source # | |
| (Data (f a), Data a, Typeable (* -> *) f) => Data (Alt * f a) Source # | Since: 4.8.0.0 |
| Num (f a) => Num (Alt k f a) Source # | |
| Ord (f a) => Ord (Alt k f a) Source # | |
| Read (f a) => Read (Alt k f a) Source # | |
| Show (f a) => Show (Alt k f a) Source # | |
| Generic (Alt k f a) Source # | |
| Alternative f => Semigroup (Alt * f a) Source # | Since: 4.9.0.0 |
| Alternative f => Monoid (Alt * f a) Source # | Since: 4.8.0.0 |
| type Rep1 k (Alt k f) Source # | |
| type Rep (Alt k f a) Source # | |