Data.Monoid.Extra

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) <> z

(

Semigroup

law)

Concatenation

mconcat = foldr (<>) mempty

You can alternatively define mconcat instead of mempty, in which case the laws are:

Unit

mconcat (pure x) = x

Multiplication

mconcat (join xss) = mconcat (fmap mconcat xss)

Subclass

mconcat (toList xs) = sconcat xs

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. Data.Semigroup.Sum and Data.Semigroup.Product.

NOTE: Semigroup is a superclass of Monoid since base-4.11.0.0.

An associative operation.

Examples

>>> [1,2,3] <> [4,5,6][1,2,3,4,5,6]

>>> Just [1, 2, 3] <> Just [4, 5, 6]Just [1,2,3,4,5,6]

>>> putStr "Hello, " <> putStrLn "World!"Hello, World!

Boolean monoid under disjunction (||).

Any x <> Any y = Any (x || y)

Examples

>>> Any True <> mempty <> Any FalseAny {getAny = True}

>>> mconcat (map (\x -> Any (even x)) [2,4,6,7,8])Any {getAny = True}

>>> Any False <> memptyAny {getAny = False}

This data type witnesses the lifting of a Monoid into an Applicative pointwise.

Examples

>>> Ap (Just [1, 2, 3]) <> Ap NothingAp {getAp = Nothing}

>>> Ap [Sum 10, Sum 20] <> Ap [Sum 1, Sum 2]Ap {getAp = [Sum {getSum = 11},Sum {getSum = 12},Sum {getSum = 21},Sum {getSum = 22}]}

Maybe monoid returning the leftmost non-Nothing value.

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

Beware that Data.Monoid.First is different from Data.Semigroup.First. The former returns the first non-Nothing, so Data.Monoid.First Nothing <> x = x. The latter simply returns the first value, thus Data.Semigroup.First Nothing <> x = Data.Semigroup.First Nothing.

Examples

>>> First (Just "hello") <> First Nothing <> First (Just "world")First {getFirst = Just "hello"}

>>> First Nothing <> memptyFirst {getFirst = Nothing}

Maybe monoid returning the rightmost non-Nothing value.

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

Data.Semigroup.Last. The former returns the last non-Nothing, so x <> Data.Monoid.Last Nothing = x. The latter simply returns the last value, thus x <> Data.Semigroup.Last Nothing = Data.Semigroup.Last Nothing.

Examples

>>> Last (Just "hello") <> Last Nothing <> Last (Just "world")Last {getLast = Just "world"}

>>> Last Nothing <> memptyLast {getLast = Nothing}

Boolean monoid under conjunction (&&).

All x <> All y = All (x && y)

Examples

>>> All True <> mempty <> All False)All {getAll = False}

>>> mconcat (map (\x -> All (even x)) [2,4,6,7,8])All {getAll = False}

>>> All True <> memptyAll {getAll = True}

Monoid under <|>.

Alt l <> Alt r == Alt (l <|> r)

Examples

>>> Alt (Just 12) <> Alt (Just 24)Alt {getAlt = Just 12}

>>> Alt Nothing <> Alt (Just 24)Alt {getAlt = Just 24}

The dual of a Monoid, obtained by swapping the arguments of (<>).

Dual a <> Dual b == Dual (b <> a)

Examples

>>> Dual "Hello" <> Dual "World"Dual {getDual = "WorldHello"}

>>> Dual (Dual "Hello") <> Dual (Dual "World")Dual {getDual = Dual {getDual = "HelloWorld"}}

The monoid of endomorphisms under composition.

Endo f <> Endo g == Endo (f . g)

Examples

>>> let computation = Endo ("Hello, " ++) <> Endo (++ "!")>>> appEndo computation "Haskell""Hello, Haskell!"

>>> let computation = Endo (*3) <> Endo (+1)>>> appEndo computation 16

Monoid under multiplication.

Product x <> Product y == Product (x * y)

Examples

>>> Product 3 <> Product 4 <> memptyProduct {getProduct = 12}

>>> mconcat [ Product n | n <- [2 .. 10]]Product {getProduct = 3628800}

Monoid under addition.

Sum a <> Sum b = Sum (a + b)

Examples

>>> Sum 1 <> Sum 2 <> memptySum {getSum = 3}

>>> mconcat [ Sum n | n <- [3 .. 9]]Sum {getSum = 42}

Like when, but operating on a Monoid. If the first argument is True returns the second, otherwise returns mempty.

mwhen True  "test" == "test"
mwhen False "test" == ""