semigroups-0.14: Anything that associates

Portabilityportable
Stabilityprovisional
MaintainerEdward Kmett <ekmett@gmail.com>
Safe HaskellTrustworthy

Data.Semigroup

Contents

Description

In mathematics, a semigroup is an algebraic structure consisting of a set together with an associative binary operation. A semigroup generalizes a monoid in that there might not exist an identity element. It also (originally) generalized a group (a monoid with all inverses) to a type where every element did not have to have an inverse, thus the name semigroup.

The use of (<>) in this module conflicts with an operator with the same name that is being exported by Data.Monoid. However, this package re-exports (most of) the contents of Data.Monoid, so to use semigroups and monoids in the same package just

 import Data.Semigroup

Synopsis

Documentation

class Semigroup a whereSource

Methods

(<>) :: a -> a -> aSource

An associative operation.

 (a <> b) <> c = a <> (b <> c)

If a is also a Monoid we further require

 (<>) = mappend

sconcat :: NonEmpty a -> aSource

Reduce a non-empty list with <>

The default definition should be sufficient, but this can be overridden for efficiency.

times1p :: Whole n => n -> a -> aSource

Repeat a value (n + 1) times.

 times1p n a = a <> a <> ... <> a  -- using <> n times

The default definition uses peasant multiplication, exploiting associativity to only require O(log n) uses of <>.

See also timesN.

Semigroups

newtype Min a Source

Constructors

Min 

Fields

getMin :: a
 

Instances

Monad Min 
Functor Min 
Typeable1 Min 
MonadFix Min 
Applicative Min 
Foldable Min 
Traversable Min 
Bounded a => Bounded (Min a) 
Enum a => Enum (Min a) 
Eq a => Eq (Min a) 
Data a => Data (Min a) 
Ord a => Ord (Min a) 
Read a => Read (Min a) 
Show a => Show (Min a) 
Generic (Min a) 
(Ord a, Bounded a) => Monoid (Min a) 
Hashable a => Hashable (Min a) 
Ord a => Semigroup (Min a) 

newtype Max a Source

Constructors

Max 

Fields

getMax :: a
 

Instances

Monad Max 
Functor Max 
Typeable1 Max 
MonadFix Max 
Applicative Max 
Foldable Max 
Traversable Max 
Bounded a => Bounded (Max a) 
Enum a => Enum (Max a) 
Eq a => Eq (Max a) 
Data a => Data (Max a) 
Ord a => Ord (Max a) 
Read a => Read (Max a) 
Show a => Show (Max a) 
Generic (Max a) 
(Ord a, Bounded a) => Monoid (Max a) 
Hashable a => Hashable (Max a) 
Ord a => Semigroup (Max a) 

newtype First a Source

Use Option (First a) to get the behavior of First from Data.Monoid.

Constructors

First 

Fields

getFirst :: a
 

newtype Last a Source

Use Option (Last a) to get the behavior of Last from Data.Monoid

Constructors

Last 

Fields

getLast :: a
 

Instances

newtype WrappedMonoid m Source

Provide a Semigroup for an arbitrary Monoid.

Constructors

WrapMonoid 

Fields

unwrapMonoid :: m
 

timesN :: (Whole n, Monoid a) => n -> a -> aSource

Repeat a value n times.

 timesN n a = a <> a <> ... <> a  -- using <> (n-1) times

Implemented using times1p.

Re-exported monoids from Data.Monoid

class Monoid a where

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 = foldr mappend mempty

The method names refer to the monoid of lists under concatenation, but there are many other instances.

Minimal complete definition: mempty and mappend.

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

mempty :: a

Identity of mappend

mappend :: a -> a -> a

An associative operation

mconcat :: [a] -> a

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 
Monoid () 
Monoid All 
Monoid Any 
Monoid ByteString 
Monoid ByteString 
Monoid IntSet 
Monoid Text 
Monoid Text 
Monoid [a] 
Monoid a => Monoid (Dual a) 
Monoid (Endo a) 
Num a => Monoid (Sum a) 
Num a => Monoid (Product a) 
Monoid (First a) 
Monoid (Last a) 
Monoid 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 there is no "Semigroup" typeclass providing just mappend, we use Monoid instead.

Monoid (Seq a) 
Monoid (IntMap a) 
Ord a => Monoid (Set a) 
(Hashable a, Eq a) => Monoid (HashSet a) 
Semigroup a => Monoid (Option a) 
Monoid m => Monoid (WrappedMonoid m) 
(Ord a, Bounded a) => Monoid (Max a) 
(Ord a, Bounded a) => Monoid (Min a) 
Monoid b => Monoid (a -> b) 
(Monoid a, Monoid b) => Monoid (a, b) 
Ord k => Monoid (Map k v) 
(Eq k, Hashable k) => Monoid (HashMap k v) 
(Monoid a, Monoid b, Monoid c) => Monoid (a, b, c) 
(Monoid a, Monoid b, Monoid c, Monoid d) => Monoid (a, b, c, d) 
(Monoid a, Monoid b, Monoid c, Monoid d, Monoid e) => Monoid (a, b, c, d, e) 

newtype Dual a

The dual of a monoid, obtained by swapping the arguments of mappend.

Constructors

Dual 

Fields

getDual :: a
 

Instances

Bounded a => Bounded (Dual a) 
Eq a => Eq (Dual a) 
Ord a => Ord (Dual a) 
Read a => Read (Dual a) 
Show a => Show (Dual a) 
Monoid a => Monoid (Dual a) 
Semigroup a => Semigroup (Dual a) 

newtype Endo a

The monoid of endomorphisms under composition.

Constructors

Endo 

Fields

appEndo :: a -> a
 

Instances

newtype All

Boolean monoid under conjunction.

Constructors

All 

Fields

getAll :: Bool
 

newtype Any

Boolean monoid under disjunction.

Constructors

Any 

Fields

getAny :: Bool
 

newtype Sum a

Monoid under addition.

Constructors

Sum 

Fields

getSum :: a
 

Instances

Bounded a => Bounded (Sum a) 
Eq a => Eq (Sum a) 
Ord a => Ord (Sum a) 
Read a => Read (Sum a) 
Show a => Show (Sum a) 
Num a => Monoid (Sum a) 
Num a => Semigroup (Sum a) 

newtype Product a

Monoid under multiplication.

Constructors

Product 

Fields

getProduct :: a
 

Instances

Bounded a => Bounded (Product a) 
Eq a => Eq (Product a) 
Ord a => Ord (Product a) 
Read a => Read (Product a) 
Show a => Show (Product a) 
Num a => Monoid (Product a) 
Num a => Semigroup (Product a) 

A better monoid for Maybe

newtype Option a Source

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

getOption :: Maybe a
 

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

Fold an Option case-wise, just like maybe.

Difference lists of a semigroup

diff :: Semigroup m => m -> Endo mSource

This lets you use a difference list of a Semigroup as a Monoid.

cycle1 :: Semigroup m => m -> mSource

A generalization of cycle to an arbitrary Semigroup. May fail to terminate for some values in some semigroups.